Chapter Notes: Our Home: Earth, a Unique Life-Sustaining Planet

What would Earth be like if it had never known life at all?

Our planet is not just another ball of rock in space. It is the only known place in the universe that supports a vast variety of life — from towering mountains to deep oceans and green forests. With the help of Earth observation satellites such as those operated by ISRO, scientists study the special features that make Earth a life‑sustaining planet.

Image by Earth Observation Satellite (ISRO)

In this chapter you will explore the conditions and systems that make Earth uniquely fit for life. You will review Earth’s physical structure, its place in the Solar System, how living and non‑living things interact, and the challenges that threaten life on Earth.

About the Image:

• The image was captured by an ISRO Earth Observation Satellite.
• It is a mosaic made by combining nearly 3,000 smaller images.
• This is a false‑colour image — special colours are used to highlight specific information.
• Such images help scientists study land, water, plant growth, and environmental changes more clearly.

Why Is Earth a Unique Planet?

Out of the billions of planets in the universe, Earth is the only one we know that has life in so many forms — plants, animals, people, and microorganisms.

Earth’s Crust is like the thin skin of an apple

• All living things exist on a very thin layer on Earth’s surface called the crust.
• If Earth were the size of an apple, the crust where life exists would be as thin as the apple’s skin.
• Below the crust there are other layers: upper mantle, lower mantle, outer core, and inner core, but life is restricted mostly to the crust and the near‑surface parts of the mantle and atmosphere.

Activity 13.1: What Makes Earth Special?

Think about and list interesting features of Earth that matter for our lives. For example:

Here is a completed table you can discuss and add to:

• Although the crust is thin, it provides everything needed for life: air, water, soil, and resources.
• Earth supplies air to breathe, water for drinking and agriculture, and soil for growing food.
• Materials such as rocks, timber, and metals are available for building homes, roads and tools.

Planets of Our Solar System

The Solar System has eight planets that orbit the Sun in nearly circular paths.

Planets in order from the Sun:

1. Mercury
2. Venus
3. Earth
4. Mars
5. Jupiter
6. Saturn
7. Uranus
8. Neptune

Types of planets:

• Mercury, Venus, Earth, Mars: Smaller, rocky (terrestrial) planets.
• Jupiter, Saturn, Uranus, Neptune: Larger, mostly made of gases and ices (gas giants and ice giants).

Activity 13.2: Comparing Planets

Collect and compare information about:

• The average temperature of each planet.
• Relative size (radius) compared to Earth.
• Whether it has an atmosphere.

Observations:

• All planets receive energy from the Sun.
• Planets closer to the Sun are generally hotter; those further away are colder.

Why Is Venus the Hottest Planet?

• Venus is not the closest planet to the Sun, but it is the hottest.
• Its atmosphere is very thick and is made mainly of carbon dioxide (CO2).
• Carbon dioxide traps heat through the greenhouse effect, so Venus retains large amounts of heat.
• On Earth, the greenhouse effect also warms the surface, but it is much weaker and essential for maintaining temperatures suitable for life.

Difference between planetary greenhouse and a plant greenhouse:

• Planetary greenhouse: Gases in the atmosphere trap infrared radiation (heat) emitted from the surface.
• Plant greenhouse: Glass walls reduce air circulation and trap warm air physically.
• Both keep places warm, but the mechanisms differ.

What Makes the Earth Suitable for Life to Exist?

Several physical and chemical properties of Earth combine to make it habitable. These include its position in the Solar System, size and atmosphere, and its magnetic field.

1. Position of Earth — The 

Habitable Zone

Earth’s distance from the Sun is just right — neither too close nor too far.

• This distance keeps temperatures such that water mostly exists as a liquid, which is essential for all known life.
• If Earth were closer to the Sun, it would be too hot and water would evaporate.
• If Earth were further from the Sun, it would be too cold and water would freeze.
• The region around a star where liquid water can exist is called the habitable zone or the Goldilocks zone.Habitable Zone
• Over 70% of Earth’s surface is covered with water, giving Earth the name the Blue Planet.

Blue Planet

Mars and the Possibility of Life

Mars

• Mars lies near the edge of the Sun’s habitable zone.
• Many spacecraft and rovers have studied Mars; no proof of current life has been found yet.
• Evidence suggests Mars may once have had liquid water and possibly simple life. Future missions may provide new information.

2. Size of Earth and Its Atmosphere

Earth’s nearly circular orbit helps keep sunlight and temperatures fairly steady throughout the year.

• If Earth were smaller, its gravity might be too weak to hold a dense atmosphere; gases could escape to space (as on Mercury and Mars).
• If Earth were much larger, very strong gravity might make conditions unsuitable for life as we know it.
• The atmosphere supplies oxygen for breathing and carbon dioxide for plants.
• Some oxygen in the upper atmosphere forms ozone, creating the ozone layer that shields life from harmful ultraviolet (UV) radiation.
• The atmosphere also contributes to the greenhouse effect, which traps enough heat to keep Earth warm but not too hot.

Our Scientific Heritage: Exploring Mars

Mangalyaan

• Mangalyaan (Mars Orbiter Mission), launched by ISRO in 2013, is an important Indian mission to study Mars.
• The mission studied Mars’ atmosphere and surface and searched for signs of past water and conditions that could have supported life.
• Mangalyaan demonstrated how effective, low‑cost space missions can deliver valuable scientific results.

3. Magnetic Field of the Earth

Along with Earth’s right position, size, and atmosphere, the magnetic field is another key factor that helps make Earth safe for life.

• Earth behaves like a giant magnet; a freely suspended magnet (a compass needle) points toward the magnetic north and south.
• The region around a magnet where its influence is felt is called the magnetic field.
• Scientists believe the motion of molten iron in the Earth’s outer core generates the magnetic field.

Why Earth’s Magnetic Field Is Important

Earth is constantly hit by tiny, high‑energy particles from space:

• Cosmic rays from distant space.
• Solar wind, composed of charged particles emitted by the Sun.
• These particles can damage the atmosphere, reduce the ozone layer, increase harmful UV radiation and harm living organisms.
• The magnetic field acts as a shield by deflecting many of these charged particles away from Earth.
• This protection helps preserve the atmosphere and makes Earth safer for life.

What Allows Life to Be Sustained on Earth?

It is the connections between living (biotic) and non‑living (abiotic) parts of Earth that enable life to thrive.

Air, Water and Sunlight

• The atmosphere provides oxygen for respiration and contains carbon dioxide used by plants for photosynthesis.
• Sunlight supplies energy for photosynthesis and warms Earth’s surface. Part of this heat is trapped by the atmosphere (greenhouse effect), keeping temperatures suitable for liquid water.
• Water covers nearly 70% of Earth and forms the hydrosphere(oceans, seas, rivers, lakes, groundwater). Water:
• dissolves and transports nutrients,
• helps animals regulate body temperature and digestion,
• is essential for photosynthesis and all cellular processes.
• Freshwater is required for agriculture and human consumption.
• Water vapour forms clouds that produce rain and snow, which refill rivers, lakes, and groundwater.
• Air movement (wind) shapes weather and rainfall patterns; this influences farming and water supply.

Soil, Rocks and Minerals

• The geosphere (Earth’s crust) contains rocks, soil and minerals that support life.
• Soil provides a medium for plant growth and contains nutrients such as nitrogen and potassium, released by weathering of rocks and decomposition of dead organisms.
• Minerals in rocks and soil are resources for salt, coal, oil, iron, copper and many other materials essential for human life and technology.
• Geodiversity — a variety of landforms, rocks and soils — creates different habitats and supports biodiversity.
• Non‑living parts of nature actively shape ecosystems and biological communities.

Plants, Animals and Microorganisms

The biosphere is the zone where life exists — including land, water and the lower atmosphere. It contains all living organisms: trees, grasses, herbs, animals, insects and microscopic organisms such as bacteria and fungi.

• Plants perform photosynthesis, using sunlight, water and carbon dioxide to make food and release oxygen.
• Animals depend on plants and other animals for food and energy.
• Microorganisms (decomposers) break down dead matter and recycle nutrients into the soil for use by plants.
• These interactions form complex food chains and food webs that maintain ecological balance — the essence of ecology.

The Importance of Balance

• Earth is an interconnected system: land, air, water and living organisms interact constantly.
• Small changes — such as deforestation — can alter rainfall, soil quality, air composition and wildlife populations.
• Maintaining balance in these systems is essential for a healthy, habitable planet.
• Protecting clean air, water, soil and biodiversity helps secure Earth’s future.

What Keeps Life from Disappearing?

If organisms did not reproduce, species would eventually disappear. Reproduction ensures continuity of life across generations.

What is Reproduction?

• Reproduction is the biological process by which organisms produce new individuals of the same kind.
• Parents pass down instructions called genes (genetic material) that determine the form and functioning of an organism.
• Genes are found in cells and guide the development of body parts and biological processes.

Why is Reproduction Important?

• It keeps each species going generation after generation.
• It allows variation — small changes in genes — that can help organisms adapt to changing environments.
• Accumulation of such changes over many generations can lead to evolution of new traits or species.
• Examples: Camels developed humps to survive desert conditions; microbes can evolve antibiotic resistance.

How Can Offspring Be Similar Yet Different?

Reproduction can produce offspring that resemble their parents yet display differences.

There are two main types of reproduction:

1. Asexual reproduction: New individuals arise from a single parent and are genetically almost identical to it.
2. Sexual reproduction: Two parents contribute genetic material, producing offspring with mixed traits from both parents.

Asexual Reproduction

In asexual reproduction, one parent produces new individuals that are genetic copies of the parent.

Example: Many plants can reproduce by vegetative propagation — planting a part of the plant (leaf, stem, root) and growing a new plant.

Activity 13.3: Vegetative Propagation in Plants

• Plant stem cuttings (e.g. money plant), potato eyes, or pieces of ginger in moist soil.
• Provide water, air and sunlight.
• Observe daily when roots, shoots and new leaves appear.
• This demonstrates how some plants grow new individuals from a part of the original plant.
• Other examples of asexual reproduction:
  • Bacteria, amoeba: Binary fission — one cell divides into two identical cells.
  • Hydra: Budding — a new individual grows from the body and detaches.
  • Planaria, some algae: Regeneration — they can regrow from fragments.

Sexual Reproduction

Sexual reproduction is the process in which two parents (usually male and female) produce offspring.

This is common in most animals and flowering plants. Some microorganisms also have mating types that act like parents.

Special Cells for Reproduction: Gametes

Both parents produce special reproductive cells called gametes.

• Male gametes: sperm in animals; pollen in flowering plants.
• Female gametes: egg in animals; ovule in plants.
• Gametes carry half the genetic information of each parent. When a male and female gamete fuse (fertilisation), they form a zygote with a full set of genes.

Why Don’t Babies Look Exactly Like Their Parents?

• Offspring receive a unique mix of genes from both parents.
• That is why they resemble their parents but are not exact copies.
• Siblings may look different because each receives a different combination of genes.
• Mixing of genes increases variation, which is useful for adaptation and evolution.

Sexual Reproduction in Plants

Flowering plants have male and female parts:

• Anther (male) produces pollen grains (male gametes).
• Ovule (female), inside the ovary, contains the female gamete.
• Pollination: Transfer of pollen from anther to stigma, often by wind, insects or animals.

Fertilisation: Pollen reaches the ovule and male and female gametes fuse to form a zygote.

• The zygote develops into a seed.
• The ovule becomes the seed and the ovary develops into fruit.

Seed dispersal: Fruits or seeds are carried away by animals, wind or water. Seeds that fall in suitable places germinate using stored food to grow roots and shoots.

Sexual Reproduction in Animals

Animals have two types of reproductive cells: sperm (male) and egg (female).

• Fertilisation occurs when sperm and egg unite to form a zygote.
• In fish and amphibians (e.g. frogs), fertilisation often takes place externally in water, with parents releasing eggs and sperm into water.
• In birds and most mammals, fertilisation occurs inside the female’s body. Sperm swim to meet the egg.

After fertilisation:

• Birds lay eggs; the embryo develops using food stored in the egg until hatching.
• Mammals (most) give birth to live young; the embryo develops inside the mother who supplies food and oxygen.
• Main difference:
  • Egg‑laying animals provide food for the embryo inside the egg.
  • Mammals provide food to the embryo inside the mother’s body.

What Are the Threats to Life on Earth?

Earth’s life depends on a delicate balance between living things (plants, animals, microbes) and non‑living things (air, water, soil, sunlight). Human activities are disturbing this balance, leading to major environmental problems.

The three main global challenges today are:

1. Climate change
2. Biodiversity loss
3. Pollution

1. Climate Change

• Burning fossil fuels (coal, oil, gas) releases greenhouse gases like carbon dioxide (CO2) and methane.
• These gases trap more heat in the atmosphere, causing global warming.
• Normally, CO2 is absorbed by plants, trees and plankton in oceans, but excess CO2 from burning fossil fuels adds more heat than the Earth can absorb quickly.
• Even a small temperature rise can:
  • melt ice caps and raise sea levels, causing coastal flooding,
  • increase extreme weather events such as heavy rainfall, storms, droughts and heat waves,
  • cause extinction of plants and animals that cannot adapt quickly.
• Long‑term changes in temperature, rainfall and weather patterns are collectively known as climate change.

2. Biodiversity Loss

• Destroying habitats (forests, grasslands, wetlands) causes plants and animals to vanish.
• This upsets food chains and ecosystems:
  • If grasses disappear, herbivores lose food.
  • Without herbivores, predators cannot survive.
• Each species plays a role; losing species weakens nature’s capacity to support life.

3. Pollution

Air pollution

• Comes from factories, vehicles and burning fuels.
• Harms human health (respiratory illness), damages crops and causes smog and acid rain.

Water and soil pollution

• Caused by industrial effluents, agricultural chemicals and plastic waste.
• Harms aquatic life, makes water unsafe, and reduces crop yields.
• Polluted soil can introduce toxins into the food chain.

All these problems affect people, animals, plants and ecosystems.

The Importance of Global and Local Action

• Small changes in global temperature, atmospheric composition or ozone can endanger life.
• Earth’s systems — hydrosphere (water), biosphere (living things), atmosphere (air) and geosphere (rocks and soil) — are connected; harm to one affects the others.

International Agreements and Efforts

Countries have agreed on treaties to protect Earth:

• Montreal Protocol (1987): Reduced chemicals (CFCs) that damaged the ozone layer; helped ozone recovery.
• Earth Summit (1992): A global meeting that promoted cooperation on environment and development.
• Kyoto Protocol (entered into force 2005): Set binding emission reduction targets for some countries.
• Paris Agreement (2015): Countries committed to limit global warming; the goal is to keep warming well below 1.5°C above pre‑industrial levels.
• As of 2025, the world has not yet achieved the 1.5°C goal — stronger action is still needed.

How Can We Help? — Individual and Local Actions

• Cut down on pollution: Avoid burning waste; reduce vehicle emissions.
• Switch to cleaner energy: Use solar, wind and other renewable sources instead of coal and oil.
• Use energy and water carefully: Turn off lights, repair leaks, and use public transport or cycle where possible.
• Reduce, reuse and recycle: Repair items instead of throwing away; recycle paper, plastic, glass and metal.
• Practice sustainable farming and waste management: Use organic methods, reduce chemical use and manage sewage properly.
• Protect biodiversity: Conserve habitats, plant native trees and support protected areas.
• Community action: Local communities managing natural resources wisely can make a large positive difference.

Final Summary

• Earth is uniquely suited for life because of its right distance from the Sun, suitable size and gravity, atmosphere and magnetic field.
• Life depends on continuous interactions among air, water, soil, rocks and living organisms.
• Reproduction — both asexual and sexual — ensures continuation and variation of life.
• Human activities have introduced serious threats: climate change, biodiversity loss and pollution.
• Global treaties and local actions together can help protect Earth; everyone can contribute through small, practical steps.

Chapter Notes: How Nature Works in Harmony

How does nature keep everything in balance—and what happens when that harmony is disturbed?

• From forests and rivers to animals, people, and the land itself, every part of nature is interconnected. 
• Sometimes, when forests are cut down or rainfall changes, animals like elephants lose their homes and food, forcing them to move into farms or villages, leading to new problems. 
  Elephants moving in search of food and shelter
• Even small changes in one part of nature can affect everything else around it.

In this chapter, you’ll explore how water, sunlight, plants, animals, and even humans are all linked in a web of relationships. You’ll discover why balance in nature is so important, how living things depend on each other, and how human actions can impact the whole system—for better or worse. Let’s get started!

How Do We Experience and Interpret Our Surroundings?

Different habitats have different kinds of plants and animals

Habitat:

• A habitat is the place where an organism lives.
• It provides the surroundings and conditions an organism needs to survive.
• Habitats can be small (like tree bark) or large (like a pond or forest).

Diversity in Habitats:

• Different habitats have different kinds of plants and animals (living beings).
• Organisms adapt to survive in their specific habitats.

Activity: Explore two nearby habitats and identify both the living organisms and the non-living components in each.

• Select two nearby habitats (e.g., a pond and a forest).
• List the living and non-living components in each habitat.

Common Characteristics of Habitats

Both habitats (like a pond and a forest) have:

• Living beings (biotic components): plants, animals, and other organisms.
• Non-living things (abiotic components): air, water, sunlight, soil, temperature, stones, etc.

Similarities:

• Both have biotic and abiotic components.

Differences:

• The types of living beings vary (e.g., fish in a pond, trees in a forest).
• The types of non-living components also differ (e.g., more water in a pond, more soil and air in a forest).

Biotic and Abiotic Components

• Biotic components: All living things in a habitat (plants, animals, microbes).
• Abiotic components: All non-living things in a habitat (sunlight, air, water, soil, temperature).

Why do some organisms live on land and others in water?

• Every organism needs certain conditions to survive, such as food, water, oxygen, shelter, and space.
• Example: Fish live in ponds because they get food, oxygen, and shelter there. Pond water provides both biotic needs (food from plants/animals) and abiotic needs (oxygen from water).
• Many other creatures (frogs, turtles, snakes, insects, birds, plants) share the pond. Each interacts with others and with the non-living parts of the habitat.

Coexistence and Harmony in Habitats

• Each habitat has its own unique mix of living (biotic) and non-living (abiotic) components, like air, sunlight, water, soil, and temperature.
• Different species in the same habitat might use resources differently: For example, in a forest, a snake may be active at night, while a rodent is active during the day—helping both survive in the same place but at different times.

Who All Live Together in Nature?

Population

• Definition: A population is a group of the same kind of organisms (same species) living together in a specific habitat at a given time.
• Example: All the fish of the same species in a pond form a fish population.

How Do We Measure Population? 

To find the population of a certain plant or animal, you:

• Mark a fixed area (for example, 1m × 1m in your school garden).
• Count the number of each type of organism (plants or animals) present in that area.
• Record this information as the population for that type of organism in that particular area and time. Population means the number of individuals of a particular kind (species) living in a defined area at a certain time.

Community

• A community is formed by different populations (different types of plants, animals, and microorganisms) living together and interacting in the same habitat.
• It includes all living (biotic) components of the habitat.

Habitat

• Definition: A habitat is the physical place or environment where an organism lives.
• If there is only one type of organism in a habitat, there will be competition for resources like food, water, and space. This can cause shortages and make it hard for other creatures to survive.
• Diversity in a habitat (many types of organisms living together) helps maintain a balance and supports survival by providing different roles and interactions.

Pollination

• Flower Structure: Flowers have sepals, petals, stamens (male part), and carpels (female part).
• Pollination: The process of transferring pollen grains from the stamen of one flower to the carpel of the same or another flower.
• How it happens: Wind, water, insects, bats, and birds help carry pollen.
• Why it matters: Pollination is essential for fruits and seeds to form.

Try yourself:

What is a habitat?

• A.A type of animal
• B.A process of transferring pollen
• C.A group of organisms of the same species
• D.A physical place where an organism lives

Does Every Organism in a Community Matter?

Yes! Every organism in a community plays an important role, helping maintain balance and supporting the survival of other organisms. Here’s how the activities and scientific studies help us understand this:

Explanation Using the Pond Example:

• Pond A (with fish): There are fewer dragonflies because fish eat the dragonfly larvae in the pond.
• Pond B (without fish): There are more dragonflies since nothing eats their larvae.
  

What happens next?

• Dragonflies are predators of bees, butterflies, and other insects that help pollinate flowers.
• Fewer dragonflies in Pond A means more bees and butterflies can survive and pollinate the flowers.
• More pollinators = more pollination, which leads to more flowers producing seeds and more plants.

Scientific studies confirm this: researchers found that plants around ponds with fish were better pollinated than those around ponds without fish, mainly due to this chain of effects.

What Does This Show?

• Organisms are interconnected. The presence or absence of one type (like fish) can affect many others (like pollinators and plants).
• Every member of a community has a role (niche): Fish control dragonfly populations, dragonflies affect pollinators, pollinators are essential for plant reproduction, and plants provide food and shelter for all.
• This is known as a food web or ecological balance, where changes in one group lead to effects (sometimes called a “cascade”) throughout the community.

Impact of Overfishing by Humans

• Overfishing removes too many fish from ponds or oceans.
• If too many fish are taken away, dragonfly numbers might rise because their predators have gone. This could lower the number of pollinators, resulting in less pollination for nearby plants.
• Removing key species like fish upsets the delicate balance of the community, affecting not just other animals but also plants and even the non-living (abiotic) environment.

What Are the Different Types of Interactions Among Organisms and their Surroundings?

Organisms do not live alone. They constantly interact with both living (biotic) and non-living (abiotic) components in their environment.

1. Interactions Between Biotic and Abiotic Components

Definition: Interactions between living things and non-living things in their environment.

Examples:

• Plants need sunlight (abiotic) for photosynthesis, water and soil for growth, and air for respiration.
• Earthworms live in moist soil.
• Fish lay eggs in water (abiotic).
• Soil provides nutrients for plants.

2. Interactions Among Abiotic Components

Definition: Interactions between non-living things, which affect the conditions of a habitat.

Examples:

• Sunlight warms up the day, increasing the temperature.
• Water evaporation happens faster in strong sunlight.
• Air currents create gentle waves on the water.

3. Interactions Among Biotic Components

Definition: These are the relationships between living organisms in a community.

Examples:

• Frogs eat insects (food chain).
• Water snakes eat fish.
• Frogs and fish compete for larvae.
• Many microbes (tiny living beings) interact in the pond, breaking down dead material.
• Plants provide food and shelter for animals.
• Mushrooms (fungi) decompose dead plants and animals.

The Concept of Ecosystem

An ecosystem is made up of all the living (biotic) and non-living (abiotic) things in a particular area and all the interactions among them. 

• Aquatic (water-based) ecosystems include ponds, rivers, and lakes
• Terrestrial (land-based) ecosystems include forests, grasslands, and farmlands.
• Ecosystems can overlap—for example, a river running through a forest.

Types of Consumers and Producers

• Producers (Autotrophs): Make their own food (usually plants via photosynthesis).
• Consumers (Heterotrophs): Depend on others for food.
  – Herbivores: Eat only plants (deer, horse).
  – Carnivores: Eat only animals (vulture, shikra).
  – Omnivores: Eat both plants and animals (fox, mouse).
• Decomposers: Organisms like mushrooms and bacteria that feed on dead plants and animals, recycling nutrients back into the ecosystem.

Who Eats Whom?

Food Chain: A food chain is a simple sequence showing “who eats whom” in an ecosystem.

Example (Grassland Ecosystem):
1. Grass → Hare →TigerGrass is eaten by the hare; hare is eaten by the tiger.
2. Grass → Grasshopper → Frog → Snake → Eagle
Here, each organism is eaten by the next one in the chain.

Trophic Levels

Each organism in a food chain occupies a specific position, called a trophic level:

• First trophic level: Producers (plants, e.g., grass, millet)
• Second trophic level: Herbivores (organisms that eat plants, e.g., hare, mouse)
• Third trophic level: Small carnivores (those that eat herbivores, e.g., frogs)
• Fourth trophic level (and above): Large carnivores or top predators (e.g., eagle, hawk, fox)

Trophic Level Pyramid

Ecological Pyramid

• When you count the number of organisms at each level (e.g., many grasses, fewer mice, only one eagle), and arrange these numbers with the highest at the base and lowest at the top, you get a pyramid shape.
• Producers are always the base, and top predators are at the top, indicating energy loss at each step.

Food Web

In reality, feeding relationships are not simple chains—they’re much more complex, forming a food web.

• The grass can be eaten by rabbit or mouse.
• Grasshoppers can be eaten by the bird or frog.
• Owls can eat mice or frogs. 

Because each organism may be eaten by two or more types of organisms, the food chains overlap and link together, making a web.
Food Web

Try yourself:

What is the main topic of the text?

• A.Food chains
• B.Weather patterns
• C.Animal habitats
• D.Plant growth

What Happens to Waste in Nature?

What are decomposers?

• Definition: Decomposers are organisms that break down dead plants, animals, and animal waste into simpler substances, returning nutrients to the soil.
• Examples: Mushrooms (a type of fungi), bacteria, beetles, and flies.
  Examples of Decomposers

How Decomposition Works

• When plants, animals, or their waste die, decomposers feed on them.
• Fungi (like mushrooms) and bacteria break down complex substances in these dead materials into simpler forms.
• Beetles and flies often feed on things like animal dung (e.g., elephant dung), breaking it down further.
• This entire process is called decomposition.

Importance of Decomposers

• Nutrient Recycling: Decomposition returns important nutrients to the soil, which plants use to grow.
• Balance in Nature: Decomposers prevent the buildup of dead materials and waste in the environment.
• No Waste in Nature: Nothing truly goes to waste in nature—everything is reused in one form or another thanks to decomposers.
• Saprotrophs: Another name for decomposers. Sapro means “rotten,” and troph means “food.”

What are migratory birds?

• Birds that travel thousands of kilometers between different habitats and countries to avoid harsh climates or find food.
• Example: Demoiselle Cranes visit Khichan village in Rajasthan every winter.

Roles of Migratory Birds

• Enhance Beauty: Add color and vibrancy to habitats.
• Ecosystem Balance: Act as pollinators (helping flowers reproduce) and seed dispersers, linking different habitats.
• Pest Control: Feed on insect pests, helping farmers by reducing crop damage.

How Does One Change Lead to Another?

If plants in a pond die (e.g., due to pollution), then:

• Less oxygen is produced in the water.
• Fish population declines, as they need oxygen.
• Fewer fish means more insects (fish usually eat them).
• Extra insects spread to nearby farms and harm crops.
• Farmers use more pesticides, which further harms the environment.
• One small change (like plant death) causes a chain reaction or “cascading effects” through the ecosystem.

Effects of Human Intervention: The Frog Leg Export Story

• In the 1980s, India exported a huge number of Indian bullfrogs.
• This reduced frog populations.
• Fewer frogs meant more insects, including pests in farms.
• Farmers then used more pesticides, harming the environment, water, soil, and health of living beings.
• The Government banned frog-leg exports to help restore balance.

Ecosystem Balance

• Interactions between organisms and their environment keep populations and resources stable—this is called ecological balance.
• This balance is always changing (dynamic), but large disruptions (such as overuse, pollution, loss of species) can harm it.

How Do Interactions Maintain Balance in Ecosystems?

Besides feeding relationships, organisms also compete for common resources like food, water, physical space, or sunlight. This competition helps control population size and keeps the ecosystem balanced. Without it, one species could multiply too much causing an imbalance in the ecosystem

Other Relationships:

• Mutualism: Both organisms benefit (e.g., bees and flowers).
• Commensalism: One benefits, the other is unaffected (e.g., orchids on trees).
• Parasitism: One benefits, the other is harmed (e.g., ticks on dogs).

Benefits of an Ecosystem

• Forests: Provide clean air, fertile soil, food, timber, medicines, and beauty.
• Water bodies: Give water and food.
• All ecosystems: Offer aesthetic value, recreation, and support human well-being.
• Mangroves (e.g., Sundarbans): Protect against floods and storms, absorb carbon dioxide, support unique wildlife; recognized as a World Heritage Site.

Threats to Ecosystems

• Deforestation, pollution, unsustainable land use, and illegal hunting are harming all types of habitats.
• The Sundarbans mangroves are under threat from wood cutting, pollution, and resource overuse.
• Such activities disrupt natural cycles and reduce biodiversity.

How To Protect Ecosystems

• Protected Areas: Such as national parks, biosphere reserves, and sanctuaries conserve habitats and wildlife. Examples: Jim Corbett, Manas, Nilgiri Biosphere Reserve, Chilika Lake.
• Community Action: People must work together to conserve resources, avoid pollution, and preserve natural areas.

Human-Made (Artificial) Ecosystems

• Examples: Farms, fish ponds, parks.
• When managed well, they can help reduce pollution, support biodiversity, and provide recreational spaces.
• Need: Continuous care and human management.

Try yourself:

What is the main purpose of interactions?

• A.To create chaos
• B.To maintain balance
• C.To confuse others
• D.To avoid communication

What Are the Benefits of an Ecosystem?

Ecosystems consist of biotic (living) and abiotic (non-living) components that depend on each other to support life processes.

Humans benefit from ecosystems in many ways:

• Forests provide fresh air, fertile soil, food, fibres, timber, and medicines.
• Aquatic ecosystems provide water and food.
• Ecosystems also offer aesthetic (beauty) and recreational (enjoyment) value.
• This supports human well-being and shows the close connection between nature and humans.
• However, overusing or misusing natural resources disturbs the balance in nature.

 Real-Life Example: The Sundarbans – A Threatened Ecosystem

• The Sundarbans have the largest mangrove forests in the world.
• Located where the Ganges and Brahmaputra Rivers meet, between India and Bangladesh.
• Home to various flora (plants) and fauna (animals), many of which are endangered.
• Protects against storms and floods by slowing down strong winds and waves.
• Trees absorb carbon dioxide from the air and release oxygen.
• Declared a World Heritage Site by UNESCO (United Nations Educational, Scientific and Cultural Organization) in 1987 due to its importance.

Threats 

• Mangrove trees are cut for fuelwood and farming.
• Illegal hunting and overuse of forest resources threaten wildlife.
• Pollution from industrial waste and untreated sewage damages water and habitat.
• These human activities disrupt the natural functioning of ecosystems.

Other Threatened Ecosystems in India
Ecosystems across India (forests, rivers, scrublands, wetlands, grasslands, coastal areas) are under threat.

Problems:

• Deforestation (cutting trees).
• Overuse of natural resources.
• Spread of invasive species (non-native plants/animals that harm locals).
• Unsustainable land use.
• Pollution.

Call to Action: Think about actions you and your community can take to protect forests, rivers, and wetlands to stop damaging them.

Protected Areas for Conservation

• Definition: Protected areas are parts of land or water set aside to conserve wildlife and their habitats.
• India has many protected areas: national parks, wildlife sanctuaries, biosphere reserves, and community conserved areas.
• Benefits: They protect entire habitats, including endangered animals, birds, and rare plants.

Famous Examples:

• Jim Corbett National Park (Uttarakhand).
• Manas National Park (Assam).
• Nilgiri Biosphere Reserve (Western Ghats).
• Chilika Lake (Odisha).
• Eaglenest Wildlife Sanctuary (Arunachal Pradesh).
• Hemis National Park (Leh).
• Keibul Lamjao National Park (Manipur).
• Pirotan Island Marine National Park (Gujarat).
• Protected areas play a big role in saving nature for future generations.

Our Scientific Heritage

• The ancient text Vrikshayurveda emphasizes soil health and nourishment.
• It advocates for continuous soil nourishment through organic manure like Kunapa Jala (a liquid fertilizer made from animal and plant waste by fermentation, which breaks complex substances into simpler ones) and other composted materials.

Human-Made Ecosystems

• Humans create artificial ecosystems like fish ponds, farms, and parks to meet their needs.
• When well-designed, they reduce pollution, support biodiversity (variety of life), and provide recreational spaces.
• Unlike natural ecosystems, these need human care and management.

How Do Healthy Ecosystems Serve Our Farms?

• Farming is a major livelihood in India but can become unsustainable without environment-friendly practices.
• Humans have practiced farming for thousands of years to grow food.
• As population grew, dependence on agriculture increased.
• Between 1950 and 1965, India faced a food crisis due to low crop production.
• In the mid-20th century, the Green Revolution used tractors, machines, synthetic fertilizers, and pesticides to increase food production.

However, these methods are now seen as unsustainable due to:

• Overuse of synthetic chemicals.
• Excessive groundwater extraction.
• Growing only one type of crop (monoculture) for commercial gain.

Harms to Environment and Health:

• Overusing pesticides and monoculture lead to soil degradation (loss of quality).
• Reduces soil fertility by decreasing friendly microorganisms and organic matter (humus), which binds soil particles.
• Without humus, soil erodes easily.
• Reduces natural predators, increasing pest populations 
• Heavy irrigation and repeated ploughing disturb soil organisms like earthworms and snails, important for ecological balance.
• Pests may develop resistance to pesticides, making control harder.
• Monoculture reduces crop diversity and affects pollinators (e.g., bees), crucial for food production.
• Understanding ecosystems helps adopt sustainable farming.
• Some farmers explore organic and natural methods to reduce synthetic fertilizers and minimize interference in natural ecosystems.

Key terms to remember

• Habitat: A place that provides the right conditions for an organism to live and grow.
• Components of habitats: Biotic components (plants, animals, microbes) and abiotic components (air, water, soil, temperature).
• Ecosystem: The interaction of biotic and abiotic components in an area, forming a balanced system.
• Types of ecosystems: Terrestrial (forests, grasslands, deserts) and aquatic (ponds, lakes, seas).
• Classification of organisms: Producers (plants), consumers (herbivores, carnivores, omnivores), and decomposers (bacteria, fungi).
• Producers: Organisms that make their own food.
• Consumers: Organisms that obtain energy by eating plants or other animals.
• Decomposers: Organisms that break down dead matter and recycle nutrients.
• Food chains: Sequences showing who eats whom.
• Food webs: Interconnected food chains that show complex feeding relationships.
• Trophic levels: Positions of organisms in a food chain indicating their role in energy transfer.
• Mutualism: A relationship where both organisms benefit.
• Commensalism: A relationship where one organism benefits and the other is unaffected.
• Parasitism: A relationship where one organism benefits and the other is harmed.
• Benefits of ecosystems: Clean air, water, food, medicines, climate regulation, recreation and well‑being.
• Threats to ecosystems: Pollution, deforestation, habitat destruction, climate change, invasive species and overexploitation of resources.
• Conservation of ecosystems: Efforts such as protected areas, community conservation and sustainable practices to preserve habitats and biodiversity.

Chapter Notes: Keeping Time with the Skie

Introduction

The Moon can sometimes be seen during the daytime. This made Meera curious when she noticed the Moon in the sky on Makar Sankranti. This chapter explains why the Moon changes its appearance and how people have used observations of the sky to measure time.

Kites in the Sky

How Does the Moon’s Appearance Change and Why?

Phases of the Moon

The changing shapes of the Moon’s illuminated (bright) portion, as seen from Earth, are known as the phases of the Moon.

Waning Period (Krishna Paksha)

• After a full Moon, the bright portion decreases from a full circle to a half circle in about a week.
• The bright portion continues to shrink, disappearing completely in another week.
• This two‑week shrinking period is called the waning period or Krishna Paksha in India.

New Moon (Amavasya)

• The day when the Moon is not visible at all is called the new Moon or Amavasya.

Waxing Period (Shukla Paksha)

• After the new Moon, the bright side grows to a half circle in about a week and becomes a full circle (full Moon) in another week.
• This two‑week growing period is called the waxing period or Shukla Paksha in India.
• The Moon’s waxing and waning occurs in a cyclical (repeating) pattern each month.
• The full cycle from one full Moon to the next takes about one month (about 29.5 days).

Important Terms

• Full Moon (Purnima): The day when the Moon appears as a full bright circle.
• New Moon (Amavasya): The day when the Moon is not visible at all.
• Waxing: The period when the bright part of the Moon increases (from new Moon to full Moon).
• Waning: The period when the bright part of the Moon decreases (from full Moon to new Moon).
• Gibbous: More than half but not fully illuminated Moon.
• Crescent: Less than half illuminated Moon.

Locating the Moon

The Moon’s position in the sky changes each day, even at the same clock time.

On full Moon day:

• The Moon is nearly opposite the Sun.
• When the Sun rises in the east, the Moon is almost setting in the west.

After full Moon:

• Each morning at sunrise, the bright part of the Moon gets smaller and the Moon appears closer to the Sun’s position in the sky.
• When the Moon looks like a half circle, it is overhead at sunrise.
• A few days after, the crescent Moon appears even closer to the Sun.

The phase (shape) of the Moon and whether it is waxing or waning help you know where and when to look for it in the sky.

• Waxing Moon: Best seen at sunset.
• Waning Moon: Best seen at sunrise.
• The moonrise time becomes about 50 minutes later each day, which explains why the Moon can sometimes be seen during daylight (for example, in the afternoon between 2:00–4:00 p.m.).
• After moonrise, wait about 30 minutes for the Moon to climb higher into the sky for an easier view.

Making Sense of Our Observations – The Moon’s Changing Appearance

The Moon’s Shape:

• The actual shape of the Moon does not change; what changes is how much of the illuminated part we can see from Earth.

The Moon’s Light:

• The Moon does not produce its own light. It appears bright because it reflects sunlight that falls on it.

Sunlit and Dark Halves:

• At any moment, half of the Moon faces the Sun and is illuminated by sunlight.
• The other half, facing away from the Sun, remains in darkness (non‑illuminated).

Appearance from Earth:

• As the Moon revolves around the Earth, the position and viewing angle change.
• Although always one half of the Moon is sunlit, that half is not always fully visible from Earth.
• The portion of the Moon we see from Earth may be fully illuminated, partly illuminated, or not illuminated at all — producing the phases.

Full Moon and New Moon:

• Full Moon: When the entire sunlit half faces Earth, we see the Moon as a whole bright circle.
• New Moon: When the non‑illuminated half faces Earth, we cannot see the Moon at all.

Reason for Changing Appearance:

• The Moon seems to change shape because its position relative to Earth and Sun is constantly changing as it orbits Earth.
• We only see the illuminated part that faces toward us; the visible fraction changes with the Moon’s orbit.

The Moon’s Phases with a Simple Model

Model Demonstration:

• Use a small ball on a stick to represent the Moon, a torch or lamp as the Sun, and your head as the Earth.
• Hold the ball slightly above your head and shine the torch toward the ball to represent sunlight.
• As you turn in a circle, the ball (“Moon”) shows a changing illuminated portion to your eyes, similar to lunar phases.

What the Model Shows:

• Full Moon: When the ball is held opposite the lamp (behind you compared to the Sun), the side facing you is fully lit — like a full Moon.
• New Moon: When the ball is held between your head and the lamp (towards the Sun), you see only the dark side — like a new Moon.
• Crescent and Gibbous Phases: Turning the ball slowly changes the visible portion: sometimes crescent (less than half lit), sometimes gibbous (more than half lit).
• The line between the bright and dark parts is always curved — this matches what is seen on the real Moon.

The Science Behind the Phases:

• At every moment, half the Moon is lit by sunlight and half is dark.
• As the Moon revolves around the Earth, the angle between Earth, Moon and Sun changes, so the part of the Moon we see bright changes accordingly.

Phase Names:

• Crescent: Less than half illuminated.
• Gibbous: More than half illuminated.
• Full Moon: Whole face illuminated.
• New Moon: No illuminated part visible.

Why Do Moon Phases Occur?

• Incorrect Idea: Moon phases are not caused by Earth’s shadow falling on the Moon.
• Correct Reason: The phases of the Moon occur because of the changing relative positions (orientation) of the Sun, Moon and Earth as the Moon revolves around Earth.

Earth’s Shadow and Lunar Eclipse:

• The only time Earth’s shadow actually falls on the Moon is during a lunar eclipse.
• Lunar eclipses can only happen on a full Moon day.
• Solar eclipses can only happen on a new Moon day.

Why Don’t Eclipses Happen Every Month?

• Eclipses do not occur every month even though there is a full Moon and new Moon monthly.
• This is because the Moon’s orbit is slightly tilted relative to Earth’s orbit around the Sun.
• Most months, the Sun, Earth and Moon do not line up perfectly for the Earth’s shadow to cover the Moon (lunar eclipse) or for the Moon’s shadow to fall on Earth (solar eclipse).

How Did Calendars Come into Existence?

Natural Cycles and Time Measurement

• The apparent daily motion of the Sun (rising in the east, setting in the west) is due to Earth’s rotation on its axis.
• This natural cycle forms the basis of the day — the primary unit of timekeeping.
• Mean Solar Day: The time between one “highest Sun position” (shortest shadow at noon) to the next is about 24 hours, known as the mean solar day.

Shadow Tracking and the Day

• The shortest shadow during the day marks the Sun’s highest point in the sky (noon).
• Measuring from one day’s noon to the next gives the length of a day.
• The average solar day is about 24 hours.

The Month and the Moon

• The phases of the Moon create another natural cycle — one complete phase cycle (from full Moon to next full Moon) takes about 29.5 days (approximately one month).
• This lunar cycle is the basis for measuring a month.

The Year and the Seasons

• One year is the time Earth takes to make a full revolution around the Sun, which is about 365¼ days.
• The repetition of seasons (spring, summer, autumn, winter) marks the annual cycle.

Try yourself:

What is the primary unit of timekeeping based on Earth’s rotation?

• A.Month
• B.Year
• C.Day
• D.Hour

Types of Calendars

1. Lunar Calendars

• Based on the phases of the Moon.
• Each lunar month is about 29.5 days and 12 lunar months make a year of about 354 days.
• This lunar year is shorter than a solar year, so the months shift with respect to the seasons over time.

2. Solar Calendars

• Based on Earth’s revolution around the Sun and the arrival of seasons.
• The Gregorian calendar is a solar calendar (widely used) with months adjusted so a regular year has 365 days.
• To correct the extra quarter day, a leap day is added every 4 years (February 29).
• Leap year rule: Years divisible by 4 are leap years, with additional adjustments: skip leap years every 100 years, but add them back every 400 years.

3. Luni‑Solar Calendars

• Combine lunar months with corrections to keep in sync with the solar year and seasons.
• In India and elsewhere, an extra month (Adhika Maasa) is added every 2–3 years to adjust the calendar with the solar year.
• The names of months in Indian luni‑solar calendars include: Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha (Agrahayan), Pausha, Magha, and Phalguna.
• Amant calendars: A month begins after a new Moon and ends on the next new Moon.
• Purnimant calendars: A month begins after a full Moon and ends at the next full Moon.

Observations and Heritage

• Ancient observers noticed 12 cycles of Moon phases fit in one yearly cycle of seasons.
• The Sun’s position at sunrise changes through the year (northward in summer, southward in winter) due to Earth’s tilt.
• This movement is described in Indian tradition as Uttarayana (northward, roughly December–June) and Dakshinayana (southward, June–December).
• Solstices and equinoxes are important points for tracking the Sun’s yearly journey and for adjusting calendars.

The Indian National Calendar

The Indian National Calendar, also known as the Saka Calendar, is a solar calendar officially used by the Government of India along with the Gregorian calendar.

• It consists of 365 days in a year in a normal year.
• The year begins on 22 March (the day after the spring equinox). In leap years it begins on 21 March.
• Each month is named after traditional Indian months: Chaitra, Vaisakha, Jyestha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Agrahayana, Pausha, Magha, and Phalguna.
• Months have 30 or 31 days: In a regular year, months 2–6 have 31 days; the rest have 30 days.
• In a leap year, a day is added to Chaitra (the first month), making it start on 21 March.
• The Indian National Calendar was introduced in 1956, based on recommendations by the Calendar Reform Committee (CRC) headed by the astrophysicist Meghnad Saha.
• The calendar follows principles that relate to ancient Indian astronomical works such as the Surya Siddhanta.
• This calendar helps unify civil timekeeping across India.

Are Festivals Related to Astronomical Phenomena?

Many Indian festivals are linked to the phases of the Moon and thus are based on lunar or luni‑solar calendars.

Examples

• Diwali: Celebrated on the new Moon of Kartika.
• Holi: Celebrated on the full Moon of Phalguna.
• Buddha Purnima: Occurs on the full Moon of Vaisakha.
• Eid‑ul‑Fitr: Celebrated after sighting the crescent Moon at the end of Ramadan.
• Dussehra: Occurs on the tenth day of Ashwina.
• These festivals appear on different dates in the Gregorian calendar each year because the lunar year and solar year are not the same length.
• Luni‑solar calendars add an extra (intercalary) month every few years to align lunar months with the solar year, causing date shifts of less than a month relative to the Gregorian calendar.
• Pure lunar calendars do not make this adjustment; so festivals like Eid‑ul‑Fitr move through the Gregorian calendar months over the years.

Solar Festivals

• Some Indian festivals follow a solar sidereal calendar, occurring nearly on the same Gregorian date each year.
• Examples: Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh, and Puthandu.
• These were originally linked to solstices or equinoxes, but a slight difference between sidereal and tropical years causes slow drift of dates over centuries (for example, Makar Sankranti shifts by one day about every 71 years).

Variations in Festival Dates

• The exact lunar phase at sunrise can vary for eastern and western parts of India, causing festival dates to differ by a day in different regions even in the same year.
• To standardise dates, the Rashtriya Panchang (national almanac) is published by the Positional Astronomy Centre, Government of India, giving advanced calculations for official festival dates.

Cultural Connection

• The Moon and Sun inspire Indian classical art forms:
• Music: Ragas such as Chandrakauns and Chandranandan are inspired by the Moon.
• Dance: Gestures (mudras) such as Chandrakala and Ardhachandran in Bharatanatyam and other dances invoke lunar imagery.
• Visual Arts: Traditional painting styles (e.g., Madhubani, Warli), sculpture and pottery frequently depict the Moon and Sun, reflecting their cultural significance.

Why Do We Launch Artificial Satellites in Space?

Natural vs Artificial Satellites

• The Moon is Earth’s natural satellite, revolving around our planet.
• Artificial satellites are man‑made objects placed in orbit around Earth by various countries and organisations.

Artificial Satellites: Purpose and Functions

• Appearance: Artificial satellites can appear as small, bright, continuously moving dots across the sky.
• Typical Low Earth Orbit: Many satellites orbit at heights of a few hundred kilometres (for example around 800 km) and complete an orbit in roughly 100 minutes.
• Key Uses:
  • Communication (TV, telephone, internet)
  • Navigation (GPS, map services)
  • Weather monitoring
  • Disaster management (detecting floods, cyclones, etc.)
  • Scientific research (studying space, atmosphere, and Earth)

Try yourself:

What is one reason we launch artificial satellites into space?

• A.To communicate with animals
• B.To study weather
• C.To explore oceans
• D.To grow plants

Satellites and Missions by ISRO (Indian Space Research Organisation)

• Cartosat Series: High‑resolution imaging satellites for mapping, city planning, and disaster response. Platforms such as Bhuvan use Cartosat images to analyse soil, land use, vegetation, and terrain.
• AstroSat: Observatory satellite for studying stars and other celestial objects.
• Chandrayaan 1, 2, 3: Moon missions for exploration and scientific study.
• Aditya‑L1: Satellite for studying the Sun.
• Mangalyaan: Mars Orbiter Mission.
• Student Satellites: ISRO encourages students to build and launch small satellites, such as AzaadiSat, InspireSat‑1, and Jugnu.

Observing Artificial Satellites

• How to Spot: Look for a small, bright, continuously moving dot in the sky, typically visible just after sunset or before sunrise.
• Satellites can often be seen without a telescope.
• Mobile apps and websites for satellite tracking show which satellites are visible at your location and time.

Space Debris (Space Junk)

• When artificial satellites and rocket parts become old and stop working, they turn into space debris.
• Risks: Collisions with functional satellites and cluttering of useful orbits.

Disposal and Mitigation

• Small debris usually burns up while entering the atmosphere.
• Larger fragments can survive re‑entry and fall to Earth.
• Countries and agencies collaborate on solutions to minimise and remove space debris (for example, controlled re‑entry, moving satellites to graveyard orbits, and debris‑removal research).

Key Figures in the Indian Space Programme

Vikram Sarabhai: Pioneer of India’s space programme, often called the “Father of the Indian Space Programme”.

• The Vikram Sarabhai Space Centre (VSSC) in Thiruvananthapuram is named in his honour.
• VSSC focuses on rocket and launch vehicle technology.

Key Points to Remember

• Phases of the Moon: The changing shapes of the illuminated part of the Moon observed from day to day (new Moon, crescent, half, gibbous, full Moon).
• Cause of Moon Phases: Phases occur because we see varying portions of the Moon’s sunlit side as it orbits Earth.
• Cycle of Moon Phases: A complete sequence of the Moon’s phases takes about a month (≈29.5 days).
• Calendars: Systems created using natural cycles — day (Earth’s rotation), month (Moon’s phases), and year (Earth’s revolution around the Sun).
• Lunar Calendar: Follows the cycle of the Moon’s phases and has about 354 days in 12 months.
• Solar Calendar: Follows the cycle of seasons determined by Earth’s orbit; the Gregorian calendar is an example.
• Luni‑solar Calendar: Adapts lunar months to the solar year by adding an extra month occasionally (as in many Indian traditional calendars).
• Artificial Satellites: Human‑made objects launched into orbit to provide communication, navigation, weather and scientific data.

Chapter Notes: Light: Mirrors and Lenses

Have you ever wondered why the warning “Objects in mirror are closer than they appear” is written on the side-view mirrors of cars?  Or why reading glasses sometimes have a curved line on their lenses?
Let’s explore these questions with Meena! On a sunny afternoon during her summer holidays, Meena visited a science centre. Among all the amazing displays, something unusual caught her eye—a row of curved mirrors. 
MirrorsTo her surprise, when she looked into one, her face seemed comically large, while her brother, just a little farther away, looked upside down! At another mirror, she saw a tiny version of herself staring back.

Why do mirrors behave this way? Through the world of spherical mirrors and lenses, get ready to discover the secrets of light and learn how mirrors can make images appear bigger, smaller, or even flipped around!

What Are Spherical Mirrors?

Spoon as a Mirror—A Simple Observation

• A shiny metallic spoon can act like a mirror. You can see your face in it if you hold it close. 
  
• If you look at the inner (curved inward) side of the spoon, your image appears inverted (upside down).
• If you look at the outer (bulging outward) side, your image appears erect (upright) but much smaller than your real face.
• This difference occurs because each side of the spoon curves in a different way, mimicking different types of curved mirrors.

Curved Mirrors and Spherical Mirrors

• Mirrors like your spoon can be specially made as curved mirrors for scientific and everyday use.
• The most common type of curved mirror is the spherical mirror.

Definition of Spherical Mirrors:
Spherical mirrors are mirrors whose reflecting surfaces are shaped like a part of a hollow glass sphere. Reflecting surfaces of spherical mirrors can curve either inwards or outwards.

Types of Spherical Mirrors

1. Concave Mirror

• A mirror whose reflecting surface is curved inwards, like the inside of a spoon or a bowl..
• Edge bulges out, center dips in — think “cave.”
• The outline of this mirror is part of a circle when viewed from the front.

2. Convex Mirror

• Reflecting surface curves outwards (like the outer side of a spoon, or the back of a bowl).
• Center bulges out, edge curves back.

How Spherical Mirrors Are Made?

• Shape: Spherical mirrors have a shape as if they are parts of an imaginary hollow sphere.
• Manufacture: Despite their shape, these mirrors are not made by slicing a real hollow glass sphere.
• Process: They are actually made by grinding and polishing a flat piece of glass into a curved surface. Then, a reflective coating (like a thin layer of aluminum) is added.

Placement of Reflective Coating:

• If the coating is placed on the outer curved surface, the result is a concave mirror.
• If the coating is placed on the inner curved surface, the result is a convex mirror.

What Are the Characteristics of Images Formed by Spherical Mirrors?

Concave Mirror:

• When object is close (small distance): Image is erect but larger (enlarged) than the object.
• When object moves farther: Image becomes inverted. It starts enlarged but gets smaller as distance increases.

Convex Mirror:

• At any distance: Image is always erect and smaller (diminished) than the object.
• As object moves farther: Image size decreases slightly.

Common in Both: Lateral inversion (left-right reversal) is observed in the images.

Comparison to Plane Mirrors:

• Spherical mirrors differ from plane mirrors.
• Plane mirrors always form an erect image of the same size as the object.
• In concave and convex mirrors, image size changes with object distance.
• In concave mirrors, images also invert when the object is moved away.

Distinguishing Spherical Mirrors

Idea: Identify if a mirror is plane, concave, or convex by observing object images.

• Concave: Enlarged erect image close up, inverted when far.
• Convex: Always erect and diminished.
• Plane: Always erect and same size.

Real-Life Uses of Spherical Mirrors

 Concave and convex mirrors are used in everyday surroundings.

Concave Mirrors:

• Reflectors in torches, car headlights, and scooters (concave shape).
• Dental mirrors used by dentists: Provide enlarged view of teeth when held close inside the mouth.

Convex Mirrors:

• Side-view mirrors on vehicles: Form erect, smaller images of traffic behind; curved outward for wider road view.
• Installed at road intersections or sharp bends: Provide visibility from both sides to prevent collisions.
• Used in big stores: Monitor large areas to deter thefts.

Telescopes

• Most modern telescopes are reflecting telescopes using curved mirrors.
• The main mirror is a large concave mirror.

What Are the Laws of Reflection?

Reflection is the bouncing back of light from a surface, like a mirror. The laws of reflection explain how light behaves when it strikes any mirror—plane (flat), concave (curved inward), or convex (curved outward).

The Two Laws of Reflection

1. First Law of Reflection: Angle of Incidence Equals Angle of Reflection

Definition: The angle at which the incoming light ray hits the mirror (angle of incidence, i) is equal to the angle at which it bounces off (angle of reflection, r). In symbols: i = r.

First Law of Reflection

Key Concepts from Setup:

• Use a plane mirror with stand, torch, comb (with black paper to make a thin slit), paper clip for holding, white paper sheet, and black paper strip.
• Spread white paper on a table, place mirror upright, and shine a thin beam through the slit onto the mirror.
• Adjust the beam to hit at different angles; the reflected beam shifts accordingly.
  

Terms to Remember:

• Incident Ray: The incoming light ray that strikes the mirror.
• Reflected Ray: The outgoing light ray that bounces back from the mirror.
• Normal: An imaginary line drawn at 90° (right angle) to the mirror at the point of incidence.
• Angle of Incidence (i): The angle between the incident ray and the normal.
• Angle of Reflection (r): The angle between the reflected ray and the normal.
• Light is represented as straight rays (lines with arrows) because light travels in straight lines.

How to Prove First Law of Reflection (Observation Process):

• Draw the mirror line, incident ray, reflected ray, and normal at point O.
• Measure i and r for different incident angles; record in Table.
  
• Special Case: If the incident ray is along the normal, both i and r = 0 (light bounces straight back).
• Inference (First Law): Measurements show i always equals r, no matter the incoming angle—this is the first law of reflection.
• Example: Shine a torch beam on a plane mirror at various angles—the bounce angle matches the incoming angle exactly.

2. Second Law of Reflection: All in the Same Plane 

Definition: The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane. 

Key Concepts from Setup:

• Use the same materials as in activity above but add a stiff chart paper sheet extending beyond the table edge.
• Shine a beam on the mirror; see the reflected beam on the extended flat paper (Figure (a)).
• Bend the extended part down along the table edge; the reflected beam disappears (Figure (b)).
  
• Flatten the paper; the beam reappears.
• Inference (Second Law): Bending creates a new plane, breaking alignment—the law ensures the rays stay “flat” together for predictable reflection.
• Step Further: Even if incident rays come from different directions but hit the same point, the normal remains the same, and all (incident ray, normal, reflected ray) stay in one plane.
• Example: On flat paper, you see the full path; bending hides it because the plane changes.

How Laws Apply to Spherical Mirrors 

The Laws Are Universal: Both laws (i = r and same plane) apply to all mirrors, including spherical ones.

Key Concepts from Setup:

• Use plane, concave, and convex mirrors with stands, torch, comb (multiple slits uncovered for parallel beams, and paper clip.
• Shine parallel beams on each mirror one by one.

Observations:

• Plane Mirror: Reflected beams stay parallel
• Concave Mirror: Reflected beams come together (converge)
• Convex Mirror: Reflected beams spread out (diverge).

• Inference: Each ray obeys the laws, but the mirror’s curve causes parallel rays to converge (concave) or diverge (convex)—this explains focusing or widening effects.

Concentrating Light with Concave Mirrors 

Never look at the Sun or into the mirror reflecting sunlight—it can damage eyes. Focus light only on paper, not on faces or people.

Key Concepts from Setup:

• Use a concave mirror and thin paper (e.g., newspaper).
• Hold the mirror facing the Sun; direct reflected light onto the paper.
• Adjust paper distance for a sharp bright spot.
• Keep steady for a few minutes.
  
• Observation: The paper starts burning and produces smoke.
• Inference: Concave mirrors converge sunlight to a small point, creating intense heat that can ignite paper—this shows the power of focused reflection.
• Step Further (Solar Concentrators): Devices using mirrors/lenses to focus sunlight for heating liquids, making steam for electricity, large-scale cooking, or solar furnaces (even melting steel). Recall electric furnaces from an earlier chapter.

What Is a Lens?

Imagine looking through a flat transparent glass window pane—all objects look the same size and shape. But if the surface of the transparent material is curved, objects may not look the same.

How a Water Drop Acts Like a Lens 

Materials: A flat strip of glass or clear plastic (e.g., flat scale), few drops of oil, dropper, water, and a paper or book with printed text.

Key Concepts from Setup:

• Spread a few drops of oil (or wax) on the glass/plastic strip and rub to make a thin coating (helps water form a round drop).
• Use a dropper or finger to place a small water drop on the oiled/waxed spot.

Observations as Concepts:

• The water drop’s surface is curved outward (not flat or curved inward).
• Place printed text under the strip so it’s directly below the drop.
  
• Look down through the drop: Letters below appear different—often larger (enlarged) than nearby letters.
• Inference: The curved surface of the water drop changes the text’s size, acting like a simple lens.

Definition and Types of Lenses

• A magnifying glass is a lens that enlarges small print, making letters look bigger.
  
• Lens Definition: A piece of transparent material (usually glass or plastic) with curved surfaces.
• Lenses can be convex or concave, like mirrors.
• Convex Lens: Thicker at the middle than at the edges.
  
• Concave Lens: Thicker at the edges than at the middle.
  
• Unlike mirrors, lenses allow light to pass through; we see things through a lens, not reflected in it.

How Objects Look Through Lenses 

Materials: A convex lens, a concave lens, a lens holder, and a small object.

Setup:

• Place the lens upright in the holder.
• Put the object behind the lens (raise it to lens level if needed).
• Look through the lens from the other side.
• Move the object farther and observe changes; repeat for both lenses.

Observations :

Convex Lens:

• At small distance: Object appears erect and enlarged (larger).
• As distance increases: Object appears inverted; starts enlarged but gets smaller (diminishes).

Concave Lens:

• At any distance: Object always appears erect and diminished (smaller).
• Size changes (gets even smaller) as distance increases.
• Inference: Distance from the lens affects image size and orientation. Convex lenses can enlarge and invert; concave always diminish and keep erect.

Do Lenses Converge or Diverge Light? 

Materials: A thin transparent glass plate, a convex lens, a concave lens, a torch and comb (for multiple parallel beams), paper clip to hold comb, two identical books, and white paper sheets.

Setup:

• Use books to hold the glass plate or lens upright between them .
• Spread paper on both books.
• Shine multiple parallel beams on the glass plate, convex lens, and concave lens one by one 

Observations:

• Thin glass plate: Parallel beams pass through unchanged.
• Convex lens: Beams come together (converge).
• Concave lens: Beams spread out (diverge).
• Inference: Convex lenses are converging lenses (focus light); concave are diverging lenses (spread light). (Diagrams show rays passing through each.)

 Drawing Light Through Lenses

• Drawings of Activity show rays passing through: unchanged in glass plate, converging in convex lens, diverging in concave lens.
  

Can a Convex Lens Burn Paper?

• Setup: Use a convex lens instead of a concave mirror in the path of sunrays.
  
• Observation: Yes, you can burn the paper—the lens converges sunlight to a hot point, like a concave mirror.
• Inference: Convex lenses focus light to create heat, similar to converging mirrors.

Real-Life Uses of Lenses

• Lenses are important and used everywhere.
• Eyeglasses: Help people see clearly.
• Cameras, telescopes, and microscopes: Use lenses to capture or magnify images.
• Human eye: Has a convex lens that changes shape to focus on near (e.g., reading) or far objects.

Key Points to Remember

• Concave Mirror Images: A concave mirror forms images that can be bigger (enlarged), smaller (diminished), or the same size as the object. The image can be upright (erect) or upside down (inverted), all depending on how far the object is from the mirror.
• Convex Mirror Images: A convex mirror always forms images that are upright (erect) and smaller (diminished) than the object, no matter the distance.
• Laws of Reflection: These are two key rules for how light bounces off mirrors: (1) The angle where the light hits (angle of incidence) is equal to the angle where it bounces back (angle of reflection). (2) The incoming light ray, the normal line (straight up from the mirror at the hit point), and the bouncing ray all stay in the same flat surface (plane).
• Validity of Reflection Laws: The laws of reflection work the same way for every type of mirror, whether it’s flat (plane), curved inward (concave), or curved outward (convex).
• Behavior of Mirrors with Light: A concave mirror brings light rays together (converges them), like focusing sunlight to make heat. A convex mirror spreads light rays apart (diverges them), giving a wider view.
• Convex Lens Images: A convex lens forms images that can be bigger (enlarged), smaller (diminished), or the same size as the object. The image can be upright (erect) or upside down (inverted), depending on how far the object is from the lens.
• Concave Lens Images: A concave lens always forms images that are upright (erect) and smaller (diminished) than the object, no matter the distance.
• Behavior of Lenses with Light: A convex lens brings light rays together (converges them), like in a magnifying glass. A concave lens spreads light rays apart (diverges them), like in some eyeglasses.

Have you ever wondered why sugar completely disappears when you stir it in water, but sand does not?

Welcome to The Amazing World of Solutes, Solvents, and Solutions! In this chapter, you’ll explore what happens at the microscopic level when different substances mix, why some mixtures turn clear while others stay cloudy, and how countless solutions around us—from tasty juices to salty seas—are a big part of daily life. Let’s dive in and discover the science behind the solutions you encounter every day!

What Are Solute, Solvent, and Solution?

What Is a Solution?

• A solution is a uniform (homogeneous) mixture where components are evenly distributed and cannot be seen separately.
• Example: Mixing salt or sugar with water forms a clear solution 
• Sand or sawdust in water settles or separates, not forming a solution 
• Basic Formula: Solute + Solvent = Solution

Key Definitions

• Solute: The substance that dissolves in the solvent to form a solution. It is usually the component present in a smaller amount. In solid-liquid solutions: The solid (e.g., salt or sugar) is the solute.
• Solvent: The substance that dissolves the solute. It is usually the component present in a larger amount and determines the state of the solution. In solid-liquid solutions: The liquid (e.g., water) is the solvent.
• Solution Formation: The solute mixes completely with the solvent, creating a uniform mixture where particles are too small to see or separate easily.

Types of Solutions and Examples

Solid in Liquid:

• Solute: Solid (e.g., salt, sugar).
• Solvent: Liquid (e.g., water).
• Example: Saltwater or sugar water—uniform and clear.

Liquid in Liquid:

• Not always clear which dissolves which.
• Rule: Substance in smaller amount = solute; larger amount = solvent.
• Example: Vinegar (acetic acid in water)—acetic acid (smaller) is solute, water (larger) is solvent.

Gas in Gas (Gaseous Solutions):

• Gases can form solutions too. Example:Air is a gaseous solution.
• Solvent: Nitrogen (largest amount, about 78%).
• Solutes: Oxygen, argon, carbon dioxide, and other gases (smaller amounts).

Air is a mixture of Gases

Special Cases and Interesting Facts

• When Solute is More Than Solvent: Even if the solute amount is larger, the liquid is still the solvent.
• In Chashni (sugar syrup for Gulab Jamun), a large amount of sugar (solid solute) dissolves in a small amount of water (liquid solvent). Water remains the solvent despite being less.

How Much Solute Can a Fixed Amount of Solvent Dissolve?

Capacity of Water to Dissolve Solutes

The experiment/steps below demonstrate the limits of dissolution by gradually adding salt to water and observing when it stops dissolving. It illustrates that solvents have a maximum capacity for solutes, leading to concepts of saturation and solubility.

• Take a clean glass tumbler half-filled with water.
• Add one spoonful of salt and stir until it dissolves completely.
• Continue adding spoonfuls of salt, stirring each time, and observe dissolution.

Observations and Inferences:

• Initially, salt dissolves completely, forming a clear solution.
• After a few spoons (varies, but typically 3–5 depending on water volume), added salt does not dissolve and settles at the bottom.
• This indicates water reaches a limit and cannot dissolve more salt.

Concepts:

• Typically 3–4 spoons before undissolved salt remains (exact number depends on water amount and temperature).
• Water has a finite ability to dissolve salt; adding more beyond this limit results in undissolved solute settling.
• If more salt is added to a given amount of water, it remains undissolved, showing the solvent’s dissolution limit.

Important Definitions:

• Unsaturated Solution: A solution where more solute can be dissolved at a given temperature (e.g., initial stages with 1–2 spoons of salt).
• Saturated Solution: A solution where no more solute can dissolve at a particular temperature, and excess solute settles at the bottom.
• Concentration: The amount of solute present in a fixed quantity of solution (or solvent); it determines how “strong” the solution is.
• Dilute Solution: A solution with a relatively small amount of solute in the solvent (e.g., 1 spoon of salt in water is dilute compared to 2+ spoons).
• Concentrated Solution: A solution with a relatively large amount of solute in the solvent (e.g., more spoons of salt make it more concentrated).
• Relative Terms: Dilute and concentrated are comparative; for example, 2 spoons of salt in 100 mL water is less concentrated than 4 spoons in 50 mL water.
• Solubility: The maximum amount of solute that can dissolve in a fixed quantity of solvent at a given temperature; it represents the solvent’s dissolving capacity.

Try yourself:

What happens when you add more salt to water after reaching its limit?

• A.It changes color.
• B.It forms bubbles.
• C.It settles at the bottom.
• D.It dissolves completely.

View Solution

Effect of Temperature on Solubility 

The following experiment/ steps show how heating affects a solute’s dissolution, which shows that solubility generally increases with temperature. 

Steps:

• Take 50 mL of water in a glass beaker and measure its temperature (e.g., 20°C) using a laboratory thermometer.
• Add a spoonful of baking soda (sodium hydrogen carbonate) and stir until it dissolves; continue adding small amounts while stirring until some remains undissolved at the bottom.
• Heat the mixture to 50°C while stirring.
• Observe the undissolved baking soda dissolving.
• Add more baking soda until undissolved solid remains again.
• Heat further to 70°C while stirring and observe again.

Observations and Inferences:

• At 20°C: Limited baking soda dissolves; excess remains undissolved.
• At 50°C: Previously undissolved baking soda dissolves; more can be added before saturation.
• At 70°C: Even more baking soda dissolves, showing increased capacity.
• Inference: Water at 70°C dissolves more baking soda than at 50°C, and much more than at 20°C.

Important Definitions:

• Temperature Effect on Solubility: For most substances, solubility increases with rising temperature (e.g., a saturated solution at lower temperature becomes unsaturated when heated, allowing more solute to dissolve).
• Practical Implication: Heating can turn a saturated solution into an unsaturated one, enabling more dissolution.

Our Scientific Heritage: Solvents in Traditional Indian Medicine

• Water is primarily used as a solvent for preparing medicinal formulations in systems like Ayurveda, Siddha, and other traditional Indian medicines.
• Hydro-alcoholic extracts: Herbs are mixed with water-alcohol solutions to create drug formulations.
• Other solvents mentioned: Oils, ghee, milk, and similar substances are used to enhance therapeutic benefits of drugs.

Be a Scientist: Asima Chatterjee’s Contributions

Inspiration: Asima Chatterjee was inspired to work on medicinal plants, focusing on extracting and isolating important compounds using solvents and solutions.

Asima Chatterjee

Achievements:

• Renowned for developing anti-epileptic and anti-malarial drugs from plants.
• Earned a Doctorate of Science (second Indian woman after Janaki Ammal).
• First woman to receive the Shanti Swarup Bhatnagar Award in chemical science.
• Honored with the Padma Bhushan for her contributions.

Solubility of Gases

• Many gases, including oxygen, can dissolve in water.
• Oxygen dissolves only to a small extent (minute quantities), but this is crucial for life.
• Importance for Aquatic Life: The dissolved oxygen sustains all aquatic organisms, such as plants, fishes, and other life forms—even in small amounts.

Nature of Gas-Liquid Mixtures

• Uniform or Non-Uniform?: The mixture of gases (like oxygen) in water is a uniform mixture (homogeneous).
• Reason: Gases dissolve evenly throughout the water, forming a solution where no separate layers or particles are visible.

Effect of Temperature on Gas Solubility

• Does Temperature Affect It?: Yes, temperature impacts the solubility of gases in liquids.
• How It Affects: Solubility of gases generally decreases as temperature increases.
• Examples: 
  Cold Water: More oxygen can dissolve, providing sufficient oxygen for aquatic life.
  Warm Water: Solubility of oxygen decreases, resulting in less dissolved oxygen.
• Key Inference: Unlike solids (where solubility often increases with temperature), gases behave oppositely—higher temperatures make it harder for them to stay dissolved.

Why Do Objects Float or Sink in Water?

• You must have observed that some objects float while others sink in water 
• You may have noticed that, while washing rice, husk particles present in the rice float on the surface of water while rice sinks to the bottom of the container. Question: Why does this happen?
• Another Example: If you add oil to water, it floats on water.
• Generally, it is believed that objects that float in a liquid are lighter and others that sink are heavier than the liquid.
• A wooden stick and an iron rod may be of the same size, yet the iron rod feels much heavier.
• Explanation of Heaviness: When we say that iron is heavier than wood, we are referring to a special property known as density, which describes the heaviness of an object.
• Note: However, the density of a substance is not the only factor that decides whether it will float or sink in a particular liquid.

What is Density?

• Imagine a crowded bus where many people are packed together—this is an example of high density. The same bus with only a few people is an example of low density.
• Similarly, a forest where trees grow close to each other is called a dense forest, but if the trees are far apart, it is considered less dense.
  
• Question: How do scientists define density? Let us find out.
• Recall: We have learnt that matter is anything that possesses mass and occupies space (volume).

Definition and Formula of Density

• Density Definition: Density is defined as the mass present in a unit volume of that substance.
• Mathematical Formula: Density = Mass / Volume.

• The density of a substance is independent of its shape or size.
• However, it is dependent on temperature and pressure.
• Pressure primarily affects the density of gases, while effect of pressure on the density of solids and liquids is negligible.

Oil Packet Example

• Have you noticed that some packets of ghee or oil are labelled with a volume of 1 litre but a weight of only say 910 grams.
• What This Tells Us: The density of the oil is less than that of water. 
• A 1-litre packet of oil weighing 910 grams indicates a density of 0.91 g/cm³ (since 1 litre = 1000 cm³, and 910 g ÷ 1000 cm³ = 0.91 g/cm³). Water has a density of 1 g/cm³. Since 0.91 g/cm³ is less than 1 g/cm³, the oil is less dense than water.

Units of Density

• The units in which density is expressed will depend upon the units of mass and volume taken.
• SI Units: The SI units of mass and volume are kilogram (kg) and cubic metre (m³), respectively. Therefore, the SI unit of density is kilogram per cubic metre, abbreviated as kg/m³.
• Units for Liquids: In case of liquids, other units of density are also used for convenience, such as gram per millilitre, abbreviated as g/mL and gram per cubic centimetre, abbreviated as g/cm³.
• Conversion Factor for Density 
  1 kg/m³ = 1000 g/m³ = 1000 g/1000 L = 1 g/L = 1 g/1000 mL = 1 g/1000 cm³ 
• The mass of 1 mL of water is close to 1 g at room temperature.
• For the measurement of the mass of water, we generally consider the volume in mL and its mass in g.
• Hence, 10 mL of water would be approximately 10 g. Similarly, 100 mL of water would be approximately 100 g.

Relative Density Example and Definition

• Suppose the mass of an aluminium block is 27 g and its volume is 10 cm³, its density is 2.7 g/cm³.
• From this, it can be said that aluminium is 2.7 times denser than water.
• We express this fact by saying that the relative density of aluminium with respect to water is 2.7. It is a number without any units.
• Formula for Relative Density: Relative density of any substance with respect to water = Density of that substance / Density of water at that temperature.

Determination of Density

The density of an object can be determined by measuring its mass and volume.

How to Measure Mass?

• Mass Definition: Mass is the quantity of matter present in any object.
• Instrument: The instrument used to measure the mass of an object is known as a balance.
• Concept of Using a Digital Balance: A digital weighing balance shows mass readings directly.
• Zero Reading Concept: The balance should show a zero reading initially. If not, press the tare or reset button to set it to zero.
• Taring Concept: Place a dry and clean watch glass or butter paper on the pan, note the reading, then reset to zero using the tare button. This ignores the container’s mass.
  
• Measuring Solid Objects: Carefully place the solid object (e.g., a stone) on the watch glass and note the reading, which gives the mass. You may use any other type of balance available in your school.
• Note on Liquids: The mass of a liquid may be measured by replacing the watch glass with a beaker and pouring the desired amount of liquid into it.

Mass vs. Weight

• The words ‘mass’ and ‘weight’ are often used interchangeably in everyday language. But they have different meanings in science, which can sometimes cause confusion.
• Mass: Is the quantity of matter present in an object or a substance. Its units are gram (g) and kilogram (kg).
• Weight: Is the force by which the Earth attracts an object or a substance towards itself, and it is measured in newtons (N).
• Most balances actually measure weight, but their scales are marked in mass units, so they show values in grams or kilograms.

How to Measure Volume?

• Volume is the space occupied by an object.
• You also know that the SI unit of volume is cubic metres, written as m³. It is the volume of a cube whose each side is one metre in length.
• Volume of smaller objects is conveniently expressed in a decimetre cube (dm³) or centimetre cube (cm³). One centimetre cube is also written as one cc.
• Volume of liquids is expressed in litres (L) which is equivalent to 1 dm³.
• A commonly used submultiple of a litre is millilitre (mL) which is equivalent to 1 cm³.
• One of the common apparatuses used to measure the volume of liquids is a measuring cylinder. It is a narrow transparent cylindrical container with one side open and the other side closed.
  
• There are markings on the transparent body of the cylinder that indicate the volume of liquid in the measuring cylinder.
• We can use it to measure the desired amount of a liquid.
• Measuring cylinders are available in different sizes to measure volume—5 mL, 10 mL, 25 mL, 50 mL, 100 mL, 250 mL, etc.
  

Observing and Calculating with Measuring Cylinder

• Concept of Capacity: The measuring cylinder can measure up to its marked maximum volume (e.g., 100 mL).
• Smallest Volume Concept: The smallest volume it can measure is found by checking divisions.
• Division Calculation: Volume difference between bigger marks (e.g., 10 mL to 20 mL) is 10 mL. If there are 10 smaller divisions, each is 1 mL (10 ÷ 10 = 1 mL).
• Accuracy Depends on Size: Smaller cylinders (e.g., 10 mL or 25 mL) measure 0.1 mL accurately. 
• Choosing the Right Cylinder: For 70 mL, a 100 mL cylinder is best (accurate, one step). 50 mL needs two steps (less convenient). Larger ones (250 mL or 500 mL) are less accurate for small amounts.

Key Concepts: Measuring 50 mL of Water

• Setup Concept: Place a clean, dry measuring cylinder on a flat surface and pour water slowly to the mark.
• Adjustment Concept: Use a dropper to add or remove water for exact level.
• Meniscus Concept: Water forms a curved surface called the meniscus. Read the bottom of the meniscus for water or colorless liquids.
  
• Reading Concept: Keep eyes at level with the bottom of the meniscus for accurate reading.
• Transfer Concept: Once at 50 mL, pour into the required container.
• For Colored Liquids: Read the top of the meniscus.

Determining Volume of Solid Objects with Regular Shapes 

• Concept: For cuboid shapes (e.g., notebook, shoe box, dice), measure length (l), width (w), height (h) with a scale.
• Formula: Volume = l × w × h.
• Example: Notebook: l = 25 cm, w = 18 cm, h = 2 cm → Volume = 25 × 18 × 2 = 900 cm³.

Determining Volume of Objects with Irregular Shapes

For irregular objects like a stone, volume is hard to measure directly. Use water displacement in a measuring cylinder.

• Fill cylinder with water (e.g., 50 mL initial;) and record.
• Tie object (e.g., stone) with thread and lower slowly into water.
• Water level rises (e.g., to 55 mL)—this is displacement.
• Volume of object = Final volume – Initial volume (e.g., 5 mL).
• Units: mL for liquids = cm³ for solids.

Calculating Density

• Use formula: Density = Mass / Volume.
• Question: An object has a mass of 16.400 g and a volume of 5 cm³. Calculate its density using the formula Density = Mass / Volume. (Show your working and express the answer in g/cm³.)Solution:
  Density = 16.400 g / 5 cm³ = 3.28 g/cm³.

Did you know?

• Our planet, Earth, is composed of several layers, such as crust, upper mantle, lower mantle, outer core, and inner core, each with its particular range of density. 
  
  
• The outermost layer, called the crust, is the lightest and the density of the different layers increases as we move towards the centre.
• As one moves deeper into the Earth, both the pressure and the temperature rise significantly, making the materials heavier and more compact.

Bamboo and Wooden Logs

• In ancient times, before large ships were invented, people used bamboo and wooden logs to travel across rivers and seas.
• Bamboo was used because it is light, hollow, and floats easily on water.
  
• People tied bamboo poles together to make rafts and small boats for fishing, trading, and crossing water bodies.
• Wooden logs, especially from strong trees were either hollowed out to make boats or used as rafts.
• These simple boats, made from locally available materials, were important for moving around and connecting different places.
• Even today, similar traditional boats made of bamboo or wood are used in some regions—not just for transport, but also as tourist attractions.

Effect of Temperature on Density

Generally, the density of a substance decreases with heating and increases with cooling.

• As temperature increases, the particles of a substance whether, solid, liquid, or gas, tend to move away and spread.
• This results in an increase in volume but there is no change in mass.
• Since the Density = Mass/Volume, upon heating mass remains same, the volume increases and the density decreases.
• This explains why hot air moves up as it is less dense than the cool air around it.
  
• The hot air balloon works on the same principle.

Effect of Pressure on Density

Pressure affects density differently depending on the state of matter.

• Gases: For gases, increasing pressure causes the particles to move closer together. As a result, the volume of the gas decreases and its density increases.
• Liquids: In the case of liquids, pressure has a small effect because they are nearly incompressible.
• Solids: The particles in solids are very close to each other. Solids are even less affected by pressure than liquids, and changes in their density are usually negligible.

Why Does Ice Float on Water?

• Ice floats on water because it is lighter than liquid water.
  
• Water has a special property that its density is highest at 4 °C. It means water is heaviest at 4 °C.
• As the temperature drops, and water turns into ice at 0 °C, it undergoes a change in structure—the particles arrange themselves in a way that takes up more space.
• This process is called expansion.
• Because the same amount of water now occupies a larger volume, its density decreases.
• As a result, ice becomes lighter than liquid water and floats on its surface.
• This is important for animals living in lakes and oceans because ice floats, it forms a layer on top, keeping the water underneath warm enough for fish and other creatures to survive, even in extremely cold weather.

Egg Experiment

• Take a glass tumbler and fill it with tap water. Carefully place a raw whole egg into the water and observe what happens. You will notice that the egg sinks to the bottom.

Key Terms to Remember

• Solution: A uniform mixture formed when two or more substances mix evenly together.
• Solute (in Solid-Liquid Solution): The solid component that dissolves in a liquid to form a solution.
• Solvent (in Solid-Liquid Solution): The liquid component in which a solid dissolves to form a solution.
• Solute (in Liquid-Liquid Solution): The component present in lesser quantity when two liquids mix to form a solution.
• Solvent (in Liquid-Liquid Solution): The component present in greater quantity when two liquids mix to form a solution.
• Solvent and Solutes in Air: In air, nitrogen acts as the solvent (major component), while oxygen, argon, carbon dioxide, and other gases are the solutes (minor components).
• Saturated Solution: A solution where the maximum amount of solute is dissolved, and no more can be added at that temperature.
• Unsaturated Solution: A solution where additional solute can still be dissolved at a given temperature.
• Solubility: The maximum amount of solute that can dissolve in a fixed quantity (100 mL) of solution or solvent at a specific temperature.
• Temperature Effect on Solubility: In liquids, the solubility of solids generally increases with rising temperature, while the solubility of gases decreases.
• Mass: The amount of matter present in an object.
• Volume: The space occupied by an object or substance.
• Devices for Measurement: A weighing balance measures mass, and a measuring cylinder measures volume.
• Density: The mass per unit volume of a substance, calculated as Density = Mass / Volume.
• Temperature and Pressure Effects on Density: Density generally decreases with increasing temperature; pressure affects density differently by state of matter (e.g., increases gas density by compressing volume).

Introduction

What is everything around you really made of?

• From the air you breathe to your food, your clothes, and even your school books—almost everything is matter made of tiny particles! 
• But most things aren’t made from just one pure substance; instead, they’re created by combining different kinds of particles in various ways. 
• In this chapter, you’ll explore how the world is built from elements, compounds, and mixtures, and discover how new combinations can even help solve big problems like cleaning our air.

What Are Mixtures?

A mixture is formed when two or more substances are combined such that each substance keeps its own properties and does not react chemically with the others. The individual substances in a mixture are called its components.

Types of Mixtures

1. Non-uniform (Heterogeneous) Mixtures:
   • You can see and often separate each component easily.
   • Examples: Poha, sprout salad, mixture of sand and salt, trail mix, and the components in vegetable soup.
     
2. Uniform (Homogeneous) Mixtures:
   • Components are so well mixed you cannot see them separately—even under a microscope.
   • Examples: Sugar dissolved in water, saltwater, air, or alloys like stainless steel.

• Mixtures are common in daily life: Most things we use—food, air, beverages—are actually mixtures.
• Properties: The properties of a mixture depend on the proportion of its components, which can vary.

Scientific Heritage: Alloys

• Alloy (Mishraloha): An ancient Indian mixture of two or more metals with distinct properties.
• Examples: Bronze (copper + tin), Brass (copper + zinc), and Stainless Steel (iron + nickel + chromium + carbon).
• Alloys are typically uniform (homogeneous) mixtures.

Is Air a Mixture?Air is a uniform mixture of gases: mainly nitrogen, oxygen, argon, carbon dioxide, and water vapour.
Components of AirWhy is air a mixture?

• The gases in air are not chemically bonded; each gas keeps its own properties.
• Air is uniform throughout (homogeneous mixture).
• You cannot see or separate the gases by sight.

Testing for Carbon Dioxide in Air
Add calcium oxide (quick lime) to water to get lime water (solution of calcium hydroxide).
Expose lime water to air:

• After some time, the clear solution turns milky.
• This happens because carbon dioxide in the air reacts with calcium hydroxide to produce insoluble calcium carbonate (which looks milky)
• Conclusion: The air around us contains carbon dioxide.

 Mixtures Can Have Other Components (Dust, Pollutants): Dust Particles in Air

• Leave a clean black sheet of paper by an open window or outside for a few hours.
• Observe dust particles settling on its surface (examine with a magnifying glass for detail).
• Conclusion: Dust and other particulates are present in air, though their amount varies with location and time. These are considered pollutants.
• Major air pollutants include particulate matter (dust, soot) and gases (carbon monoxide, ozone, NO₂, SO₂). Air quality is described using the Air Quality Index (AQI).

Types of MixturesMixtures can be made by mixing substances in any physical state (solid, liquid, gas).

Table of Mixture Types (with examples):

Why separate mixtures?

• In daily life: To get a component you want (e.g., seeds from fruit, stones from rice).
• In science: To obtain pure substances for further study or use.

What Are Pure Substances?

Everyday Meaning:

• When you see “pure” on a packet of milk, ghee, or spices, it usually means the product is unadulterated, i.e., it does not have harmful or cheaper substances mixed in.
• Adulteration is the illegal act of mixing lower-quality or harmful substances into foods or products to increase quantity or cut costs, but this reduces quality and can be dangerous to health.

Scientific Definition of a Pure Substance

• Contains only one kind of matter (one type of particle) throughout.
• Examples: sugar, distilled water, baking soda (chemically pure), pure iron, pure gold (24K).
• A pure substance cannot be separated into other substances by any physical process (like filtering, sieving, or evaporation).
• If something is made of more than one kind of substance (like milk, soil, packed juice), science classifies it as a mixture, not a pure substance.

Q. Classify the following as mixture or pure substance: Milk, Packaged fruit juice, Baking soda, Sugar, Soil.

• Milk: Mixture (contains water, fats, proteins, etc.)
• Packaged fruit juice: Mixture (water, sugars, flavors, vitamins, etc.)
• Baking soda: Pure substance (if chemically pure—only sodium bicarbonate)
• Sugar: Pure substance (if only sucrose)
• Soil: Mixture (sand, clay, minerals, organic matter, water, air)

In science, “pure” means only one substance present—the same kind of particle everywhere in the sample.

What Are the Types of Pure Substances?

Pure substances can be divided mainly into: Elements & Compounds

Physical Changes Do Not Change the Type of Pure Substance

• When water changes from ice to liquid to vapor and back, it’s still water—the basic particles remain the same.
• A physical change (like melting or boiling) only changes state, not the type of substance.

Passing Electricity Through Water

• Electrolysis of water produces two different gases (not water vapor): one that makes a “pop” sound with a flame (hydrogen), the other that makes a flame glow brighter (oxygen).
• Water breaks down (chemically) into hydrogen and oxygen—proving it is a compound made of two elements.

Elements

Elements are pure substances made of only one type of atom.

• Cannot be broken into simpler substances by chemical means.
• Each element has its own type of particles (atoms).
• Examples: Hydrogen, oxygen, gold, silver, sulfur, carbon.

Most elements exist as molecules (atoms grouped together):

• H₂ (hydrogen), O₂ (oxygen).
• Metals (gold, iron), non-metals (oxygen, sulfur), and metalloids (silicon).

Classification of Elements:

• Metals: E.g., gold, silver, magnesium, iron, aluminum (good conductors, shiny, malleable).
• Non-metals: E.g., carbon, sulfur, hydrogen, oxygen (poor conductors, brittle).
• Metalloids: Elements with properties between metals and non-metals, e.g., silicon, boron (learned in higher grades).

Key Facts:

• Total known elements: 118.
• Most are solids at room temperature.
• 11 are gases (all non-metals, e.g., oxygen, helium, nitrogen).
• 2 are liquids: Mercury (metal) and bromine (non-metal).
• Special cases: Gallium and caesium (solids) melt to liquids around 30°C (303 K).

Compounds

Compounds are pure substances made by the chemical combination of two or more elements in a fixed ratio. 

Key Properties:

• Formed in fixed ratios (e.g., water: 2 hydrogen atoms to 1 oxygen atom, ratio 2:1).
• New substance with unique properties (e.g., hydrogen is a fuel, oxygen supports burning, but water extinguishes fire).

Examples:

• Water: Compound of hydrogen and oxygen.
• Sodium Chloride (Common Salt): Compound of sodium (soft metal) and chlorine (hazardous gas) in 1:1 ratio. Harmless and used for taste; can be separated from water by evaporation (physical), but sodium and chlorine require chemical separation.
• Sugar: Compound of carbon, hydrogen, and oxygen (decomposes to carbon and water on heating).

Intersting Fact: Mobile phones use over 45 elements like aluminum, copper, silicon, cobalt, lithium, gold, silver for screens, batteries, etc.

Mixture vs. Compound – Iron and Sulfur

In this experiment, we explore how iron and sulfur behave as a mixture before heating and form a compound after heating. This highlights key differences between mixtures (where substances stay separate) and compounds (where they combine chemically into something new). Let’s break it down step by step, starting with the initial mixture.

Before Heating: Forming Sample A (The Mixture)

To begin, mix iron filings and sulfur powder together to create Sample A. This is a classic example of a mixture, where the two substances are simply combined without any chemical change.

• Appearance: You can clearly see both components as separate substances—black iron filings and yellow sulfur powder—making it look non-uniform.
• Magnet Test: When you bring a magnet near Sample A, it attracts only the iron filings, leaving the sulfur behind. This shows the components retain their individual properties.
• Acid Test: Add dilute hydrochloric acid to Sample A. The iron reacts to produce hydrogen gas (which makes a “pop” sound when ignited), while the sulfur does not react and remains as a yellow solid.

These tests confirm that in a mixture, substances can be separated easily and keep their original traits. Now, let’s see what happens when we apply heat to transform this mixture.

After Heating: Forming Sample B (The Compound)

Next, gently heat Sample A while stirring. This causes a chemical reaction, resulting in a new black mass called iron sulfide (Sample B). The transformation shows how elements combine to form a compound with entirely new properties.

• Appearance: The black mass looks uniform throughout—no separate iron or sulfur visible anymore.
• Magnet Test: Unlike Sample A, a magnet has no effect on Sample B. The iron is now chemically bound and doesn’t respond magnetically.
• Acid Test: Add dilute hydrochloric acid to Sample B. It produces hydrogen sulfide gas, which has a distinct rotten egg smell—completely different from the odorless hydrogen gas in Sample A.

At this point, iron and sulfur can no longer be separated by physical methods like magnets or simple filtering. A compound has formed, with fixed ratios and unique characteristics that differ from the original elements.

Differences Between Mixtures and Compounds

How Do We Use Elements, Compounds, and Mixtures?

Elements are basic building blocks, compounds create new substances with unique traits, and mixtures combine properties for practical uses. This knowledge drives innovation in health, food, construction, and environmental protection—showing how science improves daily life.

Everyday Presence of Elements, Compounds, and Mixtures

• Elements, compounds, and mixtures surround us: They form the basis of everything we see and use.
• Air: A mixture of gases like oxygen, nitrogen, and carbon dioxide—essential for breathing.
• Water: A compound made from elements hydrogen and oxygen—vital for life.
• Metals for construction: Elements like iron and aluminum are used to build bridges, buildings, and vehicles due to their strength and durability.

Role in Innovation and Science

• Key to new discoveries: Understanding how elements combine into compounds helps create innovative products.
• Medicines and vaccines: Chemists study element combinations to develop life-saving drugs that fight diseases.
• Fertilizers for agriculture: Knowledge of compounds enhances crop production, helping feed the growing global population.
• Engineering and materials: Engineers use compounds and mixtures to design stronger materials. Example: Alloys like stainless steel (a mixture) are stronger and more durable than pure iron.

Building Materials as Mixtures

• Common construction items are often mixtures: Wood, steel, and concrete—all mixtures used for their combined properties like strength and flexibility.
• Minerals and metals: Many metals (elements) are extracted from minerals, which are naturally occurring mixtures or compounds.

Special Example: Graphene Aerogel (A ‘Wonder’ Material)
Graphene Aerogel

• What is it?: A lightweight material made from carbon (an element); known as the lightest material on Earth—so light that even grass can hold it.
• Properties: Highly porous (full of tiny holes), giving it excellent absorbing capacity.
• Uses: 
  Environmental cleanup: Absorbs oil spills in seas and on land, acting as a cleaner. 
  Energy and construction: Useful for energy-saving devices and special coatings for buildings to improve efficiency.

What Are Minerals?

Minerals are Naturally occurring substances found in rocks; most rocks are a mixture of different minerals.

• Viewing Minerals: Can be seen with the naked eye, a magnifying glass, or a microscope.
• Building Blocks: Minerals are either pure elements or compounds made of more than one element.
• Relation to Matter: Elements and compounds in minerals are the basic building blocks of matter—anything that has mass and takes up space.

Types of Minerals: Native and Compound

1. Native Minerals: Pure elements (not compounds); occur naturally in their elemental form.

• Metals: Examples include gold, silver, copper.
• Non-metals: Examples include sulfur, carbon.

2. Compound Minerals: Most common type; made up of two or more elements combined chemically.

• Examples: Quartz, calcite, mica, pyroxene, olivine, talc.

Everyday Uses of Minerals

• Minerals (or elements extracted from them) are used in many daily items due to their properties.
• Examples:
  • Cement: Made from minerals like calcite, quartz, alumina, and iron oxide (or obtained from minerals); used in construction.
  • Talcum Powder: Made from the mineral talc; used for personal care.
• Minerals provide raw materials for buildings, tools, and products, showing how elements and compounds from nature support human needs.

What Is Matter? (Concluding Ideas)

• Matter: Everything that has mass and takes up space; built from elements and compounds (e.g., rocks, water, air). Examples of Matter: Materials we see and use daily, like minerals, metals, and mixtures.
• Non-Matter: Not everything is matter—some things lack mass or volume. Examples: Light, heat, electricity, thoughts, and emotions; these are important but not made of particles like elements or compounds.
• Why It Matters: Understanding matter vs. non-matter helps us better grasp the world, from science to innovation.

Why can you stack a pile of sand, but not a puddle of water? This chapter will take you on a journey into the tiny building blocks that make up everything around us. You’ll find out what matter really is? Why solids, liquids, and gases behave so differently, and just how small the basic particles of matter can be!

What Is Matter Composed of?

All matter is made up of tiny units called constituent particles or basic building blocks.

Matter is made up of tiny particles

Example 1: Breaking Down Chalk

• Breaking a piece of chalk into two… then into smaller and smaller pieces… and finally grinding it into fine powder.
• Even the finest chalk powder you can make still looks like chalk under a magnifying glass.

• This shows that no matter how small the pieces become, each is still chalk. Breaking or grinding chalk does not change it into something else; it is still the same substance.
• Grinding is a physical change—no new substance is formed; only the size of each piece changes.
• If you could keep breaking the chalk smaller and smaller (far beyond what you can see), you’d eventually reach the constituent particles that can’t be broken down further by normal means.
• Constituent Particle: The constituent particle is the basic unit that makes up a substance.

Example 2: Dissolving sugar in water

• When sugar is added to water (without stirring), the top layer does not taste sweet.
• After stirring, the whole solution tastes sweet—even though you cannot see any sugar grains.
  
• The sugar has disappeared from sight, but you can taste it—sugar particles are still present, just too small to be seen.
• The sugar has separated into its constituent particles, which spread out among the water particles and occupy the available spaces (called interparticle spaces).
  

Conclusion:

Both chalk and sugar can be broken down to pieces made of their basic particles, and these are so small they are invisible to the naked eye or even ordinary microscopes.

What Decides Different States of Matter?

The constituent particles of matter are held together through forces which are attractive in nature. These forces are called interparticle attractions

Interparticle Attractions in Matter

• The strength of these inter attractions depends on how close the particles are and the nature of the substance.
• The physical state (solid, liquid, or gas) depends on:
  – The strength of the interparticle attractions.
  – The distance between the particles.

Solid State

In solids, particles are packed tightly together. The interparticle attractions are very strong, so particles cannot move freely—they just vibrate about their positions.

Why are solids hard and keep their shape?
When you try to hammer objects like an iron nail, piece of wood, or rock salt, they all:

• Have a definite shape and volume. This is due to the fact that in solids, the particles are tightly packed and the interparticle attractions are very strong. 
• These strong forces of attraction keep the particles in solids in fixed positions, giving solids a definite shape and making them hard to break apart.

When Solids Melt:

• When solids are heated, their particles vibrate more vigorously. 
• A stage is reached when these vibrations become so vigorous that the particles start leaving their position. 
  
• The interparticle forces of attraction get weakened and the solid gets converted into the liquid state
• The temperature at which this happens is the melting point of the solid.  

Melting Point: The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. Some examples of solids and their melting points are:

Liquid State

In liquids, particles are less tightly packed than in solids. They are still close together, but interparticle attractions are weaker than in solids.

Why do liquids flow and take the shape of containers? 
When water is poured from one container to another:

• The water always takes the shape of the container.
• The volume stays the same.
• Liquid particles can move past each other, so liquids do not have fixed shapes but have a fixed volume.
• You can move your finger through water because its particles are not fixed—they are free to move, but only within the volume of the liquid.
  

Heating Liquids

• When a liquid is heated, its particles gain energy, leading to increased movement.
• Eventually, a stage is reached where the liquid starts boiling, marking the rapid conversion to vapor.

Key Definitions

• Boiling Point: The specific temperature at which a liquid boils and turns into vapor under atmospheric pressure. At this point, particle movement becomes extremely vigorous, causing particles to move apart and weakening the interparticle forces of attraction.
• Boiling: The fast process of vapor formation that occurs at the boiling point, happening not only at the liquid’s surface but also within the liquid itself. This is visible as bubble formation, as constituent particles escape the liquid state and convert it into vapor (gaseous state).
• Evaporation: The slower process of vapor formation that occurs at all temperatures, even below the boiling point. Unlike boiling, it happens only at the liquid’s surface and proceeds gradually without bubble formation.

Particle Behavior During Heating

• As heating continues, particles move more vigorously and separate from each other.
• This separation results in a decrease in interparticle forces of attraction, allowing particles to escape the liquid and form vapor.
• The overall transformation: The liquid converts into its gaseous state (vapor), with boiling being the rapid, internal version and evaporation the slower, surface-only version.

Gaseous State

In gases, particles are very far apart and interparticle attractions are almost zero. Gas particles can move freely in all directions.

Why do gases have no fixed shape or volume?

• When you trap smoke in a glass jar and then allow it to move into a second jar:
• The smoke spreads and fills the entire space of the new jar.
  
• Gas particles always spread out to completely fill any container.
• Gases do not have a fixed shape or a fixed volume—they are compressible and can expand to fill any space.
• This is seen with both smoke and iodine vapour (or any gas)—particles are always in rapid, random motion.

Summary of the Section:

• Solids: Particles are packed tightly due to strong attractions—giving definite shape and volume.
• Liquids: Particles can move past each other; weaker attractions let them flow and take the container’s shape, but not compress much.
• Gases: Particles are far apart with negligible attractions; no definite shape or volume, can fill any container, highly compressible.

How Does the Interparticle Spacing Differ in the Three States of Matter?

The spaces between constituent particles—called interparticle spacing—play a crucial role in determining whether a substance is a solid, liquid, or gas.

• Smaller interparticle spacing: Stronger attractions; less movement; substance is more rigid (solid).
• Larger interparticle spacing: Weaker attractions; more movement; substance is fluid or gaseous.

Understanding Compression in Fluids (Gas and Liquid) using a Syringe

• When you pull the plunger of a syringe, air fills the inside. Put your thumb over the open end and push the plunger in—the volume of air decreases.
  
• Gas particles have large gaps between them, so gases are easily compressible. When compressed, particles are forced closer together.
• Repeat the experiment with water: Try pushing the plunger when the syringe is filled with water (thumb on end). The plunger cannot move much.
• Conclusion: Liquids have much smaller gaps between particles, so they are almost incompressible.

Dissolving Sugar—Evidencing Interparticle Spaces in Liquids

Add sugar to water in a glass vessel:

• Water level first rises (as sugar is added).
• After stirring, sugar dissolves; the final liquid level (C) is less than the expected sum of water plus sugar.
• There are empty spaces between water particles (interparticle spaces). When sugar dissolves, its particles fill these spaces.
• If you add an insoluble solid (like sand), the water level stays high or rises, because sand does not dissolve or fill the interparticle spaces—the particles of sand just add their own volume.

What About Solids?

• In solids, strong attractions hold the particles tightly together in a fixed arrangement.
• Interparticle spacing is very small—the particles are closely packed with little movement.
• Even though they’re tightly packed, a tiny amount of space still exists between the particles, but it’s much less than in liquids or gases.
• These spaces contain nothing (not even air).

Scientific Meaning of “Particle”

• In science, “particle” can mean the constituent unit that builds matter (atoms or molecules)—these are much, much smaller than dust or sand grains.
• Other contexts (like air pollution) use “particle” for small dust bits, but even those are made of atoms/molecules.

Summary of the section

• Gases: Can be compressed easily—lots of space between particles.
• Liquids: Nearly incompressible—small interparticle gaps; dissolved substances can fit into these spaces.
• Solids: Fixed, small gaps—particles are packed tightly, hardly any compressibility.

Try yourself:

What happens to gas particles when they are compressed?

• A.They move farther apart.
• B.They stay in the same place.
• C.They are forced closer together.
• D.They dissolve in liquid.

View Solution

How Particles Move in Different States of Matter

Particles Are Always in Motion: The movement of particles (tiny basic units) is a key reason for differences in the behavior of solids, liquids, and gases.

Drop potassium permanganate in water:

• At first, you see colored streaks spreading out.
• Soon, the whole glass of water is evenly colored.
  
• Reason: The water particles are constantly moving. They pull potassium permanganate particles from the grain, and then both kinds of particles keep moving and mixing until the color is uniform.
• Key Concept: Particles of liquids are always moving, pulling apart and mixing other particles—this is why substances can dissolve and diffuse in water.

How Temperature Affects Movement:

• The color spreads faster in hot water, slower at room temperature, slowest in ice-cold water.
• More heat energy = faster movement of particles.

Movement in Gases (Incense Stick Experiment)

• Burn an incense stick in one corner of a room.
• Soon, the fragrance spreads throughout the room, even to people far from the incense stick.
  
• Reason: Air particles are always moving rapidly in all directions, colliding with and spreading the fragrance particles.
• Key Concept: Particles of gases are in constant, fast motion, filling all available space and carrying other particles with them (diffusion).

Why Some Substances Don’t Dissolve

Some solids (like sand) don’t dissolve in water. Their particles are held together too tightly and aren’t pulled apart or mixed by the moving water particles.

Other Real-Life Examples of Movement of Gases:

• Cooking Smells: When someone cooks food in the kitchen, you quickly smell it in other rooms, even before you see the food. Smell particles move through the air into every part of the house.
• Gas Leak Detection: If there’s a gas leak or someone breaks open a bottle of ammonia, the sharp smell rapidly spreads, even to people far away.
• Smoke or Fog: If someone smokes or a candle burns, the smoke travels and fills the space, showing how gas particles (and suspended particles) move.
• Air Fresheners: Plug-in or spray air fresheners quickly make a room smell pleasant because their tiny scent molecules travel via moving air particles.

How Soap Cleans Oily Stains (Particulate Nature in Action):

• Soap molecules have two ends: One end sticks to oily stains (which water alone can’t remove).
• The other end mixes with water.
  
• When soap is added to stained clothes, the soap particles “grab” the oil and let water wash it away, demonstrating how tiny particles (of soap, oil, and water) interact and move.
• This microscopic movement is why washing with soap and water lifts away dirt and oil from clothes and skin.

Thermal Energy and Particle Movement

The thermal (heat) energy of particles decides how much they move and how far apart they are:

• Solids: Lowest thermal energy; particles vibrate in place.
• Liquids: More energy; particles move around each other, but not too far.
• Gases: Highest energy; particles move freely in all directions, far apart.

Summary Table: 

Particle nature of the three states of matter–

Key Terms to Remember

• Matter: Anything composed of extremely small particles that occupy space and have mass.
• Constituent Particles: The tiny, basic building blocks that make up matter; they are extremely small and cannot be seen with the naked eye.
• Interparticle Forces of Attraction: The attractive forces that hold particles together; strongest in solids, weaker in liquids, and weakest (negligible) in gases.
• Interparticle Spacing: The space or distance between particles; minimum in solids, slightly more in liquids, and maximum in gases.
• Solid State: A state of matter with strong interparticle attractions, minimum spacing, and no free particle movement, resulting in fixed shape and fixed volume (size).
• Liquid State: A state of matter with slightly weaker interparticle attractions than solids, allowing limited particle movement and more spacing; has a definite (fixed) volume but no fixed shape.
• Gaseous State: A state of matter with negligible interparticle attractions, free particle movement, and maximum spacing; has no fixed shape or volume.
• Fixed Shape: A property where matter maintains its form (seen in solids due to strong attractions and minimal spacing); liquids and gases lack this.
• Fixed Volume: A property where matter occupies a definite amount of space (seen in solids and liquids); gases do not have this as they expand to fill available space.
• Particle Movement: The motion of particles; none (only vibrations) in solids, limited in liquids, and completely free in gases, influenced by interparticle attractions and spacing.

Introduction                                                           Why can a gentle breeze one day become a raging storm the next?

• Every time wind slams a door, swirls leaves, or bends trees, it’s showing us the invisible force of air pressure at work. But what causes pressure, and how do moving air and pressure changes lead to powerful winds, storms, and cyclons? 
• In this chapter, you’ll discover how pressure and air movements shape the weather—from gentle winds to destructive cyclones—and why these forces matter in our daily lives.

What is Pressure?

• Pressure is defined as the force applied per unit area.
• The formula for pressure is:
• Only the force acting perpendicular to the surface (normal force) is considered for calculating pressure.

Everyday Examples of Pressure

Example 1: When Megha and Pawan go for a picnic, both carry bags of equal weight. Pawan is uncomfortable because his bag has narrow straps while Megha’s has broad straps.
Narrower straps = same force acts on a smaller area ⇒ higher pressure, which hurts more.
Broader straps spread the force over a larger area ⇒ lower pressure, which feels more comfortable.

Example 2: When lifting a heavy water bucket, a broad handle is easier on your hand than a narrow one for the same reason.

Example 3: When people carry loads on their heads, they often place a round piece of cloth under the load. This increases the area, reducing pressure and making it more comfortable.
Porter carrying load

Example 4: Using the pointed end of a nail is easier because the small area of the point increases pressure, allowing the nail to pierce the wood with less effort. When you hammer a nail into a wall to hang a picture, the sharp tip cuts through easily due to high pressure.

Example 5: Using the sharp edge of a knife cuts the apple easily because the thin edge has a smaller area, creating higher pressure to slice through. Chopping vegetables like carrots or potatoes with a sharp knife is easier than using a blunt one, as the sharp edge applies more pressure.

Example 6: Overhead water tanks are placed high to increase water pressure in pipes, as the height (gravity) adds force to push water through taps. In your house, water flows faster from a tap connected to a rooftop tank compared to a ground-level tank because the height increases pressure.

Units of Pressure

• SI unit of force = newton (N)
• SI unit of area = metre² (m²)
• Therefore, SI unit of pressure = newton/metre² (N/m²)
  This is also known as a pascal (Pa).

Example 1: Caclulate pressure exerted If 100 N force is applied on 2 m² area:
Ans: If a force of 100 N is applied on a cardboard of area 2m² , then the pressure applied on the cardboard will be:

Example 2:  An elephant stands on four feet. If the area covered by one foot is 0.25 m², calculate the pressure exerted by the elephant on the ground if its weight is 20000 N.
Ans: 
Total area for four feet: 4 × 0.25 m² = 1 m².
Pressure = Force / Area = 20000 N / 1 m² = 20000 Pa.

Pressure Exerted by Liquids: Water Column Activity

When two pipes of different diameters each have a balloon tied at the bottom and are filled equally with water, both balloons bulge the same amount—even though the amount (weight) of water is different in each.

Concept Learned:

• The important thing isn’t the weight or diameter, but the height of the water column.
• Liquid pressure at any point depends only on how high the liquid stands above that point.
• If you pour in even more water, making the column taller, the balloon bulges more—meaning more pressure at the bottom.
  
• That’s why overhead tanks are placed high up: the greater the height, the more pressure water has as it flows out the tap.

Pressure Exerted by Liquids: On Walls of ContainerIf you make small holes near the bottom of a plastic bottle and fill the bottle with water, water squirts out of all the holes sideways.

Liquid exerts pressure on walls of container

Concept Learned:

• This shows that liquids exert pressure not just at the bottom but also on the sides of containers.
• The pressure acts in all directions (downwards, sideways).
• Examples in life: Water spurts from holes or cracks in pipes, and the design of dams (broadest at the base) helps them withstand the very strong sideways (horizontal) water pressure at depth.

Why Are Dams Broad at the Base?

• Water at the bottom of a dam is under higher pressure due to the height of the water column.
• Dam bases are built broad and strong to withstand the large horizontal water pressure near the bottom.
• The deeper the water, the more pressure pushes sideways on the dam walls.

Try yourself:

Why are dam bases built broad and strong?

• A.To save space
• B.To look bigger
• C.To withstand high pressure
• D.To hold more water

View SolutionPressure Exerted by Air

What Is Atmospheric Pressure?

• The envelope of air that surrounds Earth is called the atmosphere.
• The atmosphere contains various gases (mainly nitrogen, oxygen, argon, carbon dioxide) and extends several kilometres above the Earth.
• Atmospheric pressure is the pressure exerted by air molecules on all objects, in all directions.

Atmospheric Pressure

Does Air Exert Pressure?

When you cover an inverted paper plate (attached to a stick) first with a folded chart paper (small area) and then with an unfolded chart paper (large area) and try lifting the plate by the stick:

• It’s much harder to lift the plate when the larger sheet is used.
• Main Reason: The larger the surface area covered, the greater the force needed to lift the plate.
• Key Scientific Explanation: Air applies a force (pushes down) on the chart paper; the force increases as the area increases, showing that air all around us is constantly exerting pressure.

Air Pressure in Action: Balloons

• When you blow air into a balloon, it inflates because the air inside pushes outwards in all directions.
• This happens because air exerts pressure in every direction, not just one.
  
• If you open the mouth of an inflated balloon, the air inside escapes quickly—this is because the air inside is at a higher pressure than the outside, and escapes until the pressure equalizes.

Air Pressure in Action: Using a Rubber Sucker

When you firmly press a rubber sucker onto a smooth surface and remove most of the air underneath:

• The outside air pressure is much higher than the pressure inside the cup, causing the sucker to stick.
• When you try to pull the sucker off, you have to overcome the pressure difference.
• This proves that atmospheric pressure is powerful—it presses objects with enough force to hold them tightly unless balanced by another pressure.

How Strong Is Atmospheric Pressure?

• The force exerted by the air column on an area of 15 cm × 15 cm is about 2,250 N—the same as the weight of a 225 kg object!
• Why aren’t we crushed? The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure. 
• This balances the pressure exerted from outside. The pressure inside our body is caused by the movement of fluids and gases in tissues and organs of the body.

Units of Air Pressure

The SI unit of pressure is newton per square metre (N/m²), also called a pascal (Pa).

In weather reports and practical measurements, air pressure is often given in:

• Millibar (mb): 1 mb = 100 Pa
• Hectopascal (hPa): 1 hPa = 100 Pa
  (So, 1 mb = 1 hPa; both are commonly used and mean the same thing.)
• Example: A typical atmospheric pressure at sea level is about 1,013 mb or 1,013 hPa.
• Pascal (Pa) is the SI unit, but millibar (mb) and hectopascal (hPa) are often used when talking about air pressure.

Formation of Wind

Wind blows strongly some days and is calm on others. Sometimes, strong winds can even cause damage. The movement of air (wind) is linked to differences in air pressure. Air moves from a region of higher pressure to a region of lower pressure.

Movement of Air

Everyday examples:

• If you release the mouth of an inflated balloon, air rushes out until the balloon pressure equals outside pressure.
• When a cycle tube is punctured, air escapes from inside (high pressure) to outside (lower pressure).

Key Concept:

• Air always flows from high pressure to low pressure.
• The flow stops when pressures are equal.

How Pressure Differences Create Winds 

Sea Breeze (Daytime):

• Land heats up faster than water.
• Warm air above the land rises (creating low pressure).
• Cooler, high-pressure air from the sea moves in to replace it—this moving air is the sea breeze.
  Sea Breeze

Land Breeze (Nighttime):

• Water is warmer than land at night.
• Warm air above the sea rises (creating low pressure over the sea).
• Cooler, higher-pressure air from the land moves toward the sea—this is the land breeze.
  Land Breeze

Conclusion:

• Winds are created because of pressure differences in the atmosphere.
• The greater the difference in pressure, the stronger the wind speed.

High-Speed Winds Result in Lowering of Air Pressure

High-speed winds create areas of low air pressure. Surrounding higher-pressure air then pushes toward these low-pressure zones.

• Hang two inflated balloons on strings, leaving some space between them.
• Blow air directly between the balloons:
  The balloons move toward each other—not away!
  Blowing harder (faster air) makes them move even closer, more quickly.
• Blowing between the balloons creates a zone of fast-moving air (high wind speed).
• This fast-moving air makes the air pressure between the balloons drop.
• The higher outside air pressure on the opposite sides pushes the balloons together.
• The faster you blow, the lower the air pressure between the balloons and the stronger the effect.
• High-speed wind reduces air pressure in the area where it flows rapidly.

Real-Life Application: Wind and Roofs

• During a storm, when strong winds blow over the roof of a house, a zone of low pressure forms above the roof.
  
• The air inside the house still has higher pressure.
• The greater pressure from inside can push the roof upwards (sometimes blowing it off) if the difference is strong enough and the roof is weak.
• Safety Tip: Keeping doors and windows open during storms allows air to flow freely, helping equalize the pressure inside and outside, which can prevent roofs from being blown away.

Try yourself:

What happens to air pressure during high-speed winds?

• A.It increases
• B.It decreases
• C.It stays the same
• D.It fluctuates

View SolutionStorms, Thunderstorms, and Lightning

Formation of Storms

Storm

• Land heating: Sun heats the land, causing the air above it to get warm and moist.
• Air rises: Warm, moist air being lighter rises, creating a low-pressure area.
• Air circulation: Cooler air from high-pressure areas moves in to replace it; this air gets heated and rises too.
• Cloud formation: Rising air cools and moisture condenses to form water droplets.
• Rain: Water droplets join, get heavier, and fall as rain, hail, or snow.
• Strong winds + rain = Storm: Strong moving air, often with heavy rain, is called a storm.
• Storms are frequent in: Hot, humid, tropical regions (like India).

How Thunderstorms and Lightning Happen

1. Updrafts and Downdrafts:
   • In some storms, air rises to such great heights that the low temperature turns water droplets into ice particles.
   • Strong winds push water droplets and ice particles up and down inside the clouds.
2. Charging of Clouds:
   • As water droplets and ice particles rub against each other, they get electrically charged (just like when you rub two objects together).
   • Lighter, positively charged ice particles move to the top of the cloud.
   • Heavier, negatively charged water droplets gather at the bottom.
3. Separation of Charge:
   • The cloud ends up with opposite charges at top and bottom.
   • When the negatively charged bottom of the cloud comes close to the ground, it induces a positive charge on the ground and nearby objects like trees and buildings.
4. Lightning Formation:
   • Normally, air acts as an insulator and prevents charges from meeting.
   • If charge buildup is very high, air’s resistance breaks down, causing a sudden flow of charges—this is seen as a bright flash of light called lightning.
   • Lightning can happen:
     • Within a cloud
     • Between clouds
     • Between clouds and the ground
5. Thunder: Lightning heats the air rapidly, causing it to expand and produce a loud sound called thunder.
6. Thunderstorm: A storm that has thunder and lightning is called a thunderstorm.

Regional Names and Importance of thunderstorms: 

Local thunderstorms in India have different names:

• Kalboishakhi: West Bengal, Bihar, Jharkhand
• Bordoisila: Assam
• Mango showers: Kerala, Karnataka, Tamil Nadu
• They often help in crop growth or fruit ripening (like mangoes and coffee).

Protect yourself during lightning:
Lightning is dangerous—it can start fires, damage buildings, and injure or kill people and animals.

• Stay away from tall objects (trees, poles).
• Find a low, open area and crouch; do NOT lie flat on the ground.
• Don’t use umbrellas with a metallic rod.
• If in water, get out immediately.
• If inside a car or bus, stay inside—you are comparatively safer.

Lightning Conductor: How Buildings Are Protected

• Lightning conductor: A pointed, metallic rod installed along the wall of a building.
• The pointed end is higher than the roof.
• The lower end is buried deep in the ground.
• Purpose: Provides a safe path for electric charges from lightning to flow directly into the ground, preventing damage to the building.
  

Cyclone

A cyclone is a large, spinning storm system that forms over warm ocean waters. It involves high-speed winds, heavy precipitation, and very low pressure at its center.

How Do Cyclones Form?

1. Heating and Rising Air:
   • Warm ocean water heats the air above it, making the air moist and light.
   • This warm, moist air rises up, leaving a low-pressure area below.
2. Condensation and Further Warming:
   • As the air rises, water vapour in it condenses to form raindrops.
   • Condensation releases heat, which warms the air even more, causing it to rise further and pressure to drop more.
3. Intensification and Spinning:
   • More surrounding air rushes in to fill the low-pressure area and also rises.
   • The rotation of the Earth causes the moving air to spin.
   • This cycle continues, making the low-pressure center even stronger.
4. Cyclonic Structure:
   • The result is a rapidly-spinning system of clouds, winds, and rain—this is a cyclone.
   • At the center is the eye of the cyclone—a calm zone of lowest pressure.
   • Around the eye are high-speed winds and heavy rain.
5. On Landfall:
   • When the cyclone reaches land, it loses its source of warm, moist air and weakens.
   • Even after weakening, cyclones can leave behind massive destruction.

Why Are Cyclones Dangerous?

Cyclones can uproot trees, blow off roofs, break power lines, and destroy buildings. 

• Storm Surge: Strong winds push ocean water toward land, creating a high wall of water (3–12 meters tall). This surge can flood coastal and even distant inland areas.
• Heavy Rainfall: Causes rivers to overflow, leading to floods and landslides.
• Seawater Damage: Saltwater can contaminate drinking water. Seawater can flood and make fields less fertile, damaging crops.
• Blocked Roads & Power Outages: Fallen trees and debris can block help from reaching affected areas. Long-lasting power outages disrupt emergency response and daily life.

Staying Safe During a Cyclone

• Stay informed: Regularly listen to radio/TV for weather warnings (India Meteorological Department, IMD).
• Be prepared: Keep an emergency kit ready (Food, water, medicines, flashlight, batteries).
• Shelter: Move quickly to a nearby cyclone shelter or safe building if advised.
• Weather monitoring: Satellites now help track and predict cyclones, reducing loss to life and property.
• Authorities: Many organizations work together to respond to cyclone disasters nationally and internationally.

Let’s Wrap up!

Key Terms to Remember

1. Pressure: Pressure is defined as the force exerted per unit area.
2. SI Unit of Pressure: The SI unit of pressure is the newton per square metre (N/m²), also called the pascal (Pa).
3. Pressure by Liquids and Gases: Liquids and gases exert pressure on the walls of their container.
4. Atmospheric Pressure: The pressure exerted by the air around us is known as atmospheric pressure.
5. Cause of Winds: Differences in air pressure cause winds to blow.
6. Low and High Pressure: Warm air rises to create a low-pressure area; cooler air from high-pressure areas moves in to replace it.
7. Formation of Thunderstorms: Moisture and strong winds are important for the formation of thunderstorms.
8. Development of Electric Charges: Upward and downward strong winds rub ice particles with water droplets in clouds, creating electric charges.
9. Lightning Formation: Lightning is caused by the collision of electric charges within a cloud, between different clouds, or between a cloud and the ground.
10. Effects of Lightning: Lightning strikes can cause destruction to life and property.
11. Lightning Conductors: Lightning conductors are devices that protect buildings from the damaging effects of lightning.
12. IMD (India Meteorological Department): The IMD constantly monitors cyclones and thunderstorms in India.

Introduction

Have you ever wondered why cycling uphill feels like a much tougher workout, but when you’re coming downhill, your cycle seems to almost zoom forward on its own?  These are all questions about something invisible but powerful—forces acting on us and the things around us.

Imagine Sonali and Ragini, two friends setting out for an adventure on their bicycles during the summer holidays. As they cycle against a strong wind, struggle up rough and hilly paths, and then speed down effortlessly without pedaling, they are experiencing different kinds of forces. Even nature seems to join in—with wind pushing them, the ground offering resistance, and finally, gravity pulling them down the hill.

What are these forces? What makes it hard or easy to move in different situations?
Let us get ready to explore and find out what’s really happening when you push, pull, slip, slide or speed up and slow down.

What Is a Force?

Understanding Force through Activity

• Take a large cardboard box.
• Try to move the box in as many different ways as possible—push it, pull it, slide it, drag it, or roll it.
• Notice: In every method you use, you are either pushing or pulling the box.

a) Pushing b) Pulling and c) Lifting

Definition: In science, a force is simply a push or a pull applied to an object. Anytime you move, stop, or change the direction or shape of an object, you are using a force.

• Force can be a push: For example, when you push a swing away from you.
• Force can be a pull: For example, when you draw water from a well using a rope.
• No matter how you chose to move the box (from the activity)—by dragging, sliding, rolling, lifting, etc.—some form of push or pull is always involved.

Try yourself:

What is a force in science?

• A.A swing or a rope
• B.A box or an object
• C.A move or stop
• D.A push or a pull

View SolutionWhat Can a Force Do to the Bodies on Which It Is Applied?We experience forces all the time, often without noticing. Let’s explore real-life situations to see what a force can do:

• Holding a moving bicycle from behind to stop: A pull force stops or slows down the bicycle.
• Hitting a moving ball with a bat: A push force changes the direction of the moving ball.
• Pressing an inflated balloon: A push force changes the shape of the balloon.
• Kicking a football: A push force moves the football from rest, starting its motion.
• Applying brakes on a moving cycle: A push force slows down or stops the cycle.
• Stretching a rubber band: A pull force changes the shape of the rubber band.
• Rolling a chapati: A push force changes the shape of the dough.
• Turning the steering handle of an autorickshaw: A push or pull force changes the direction of the autorickshaw.
• Opening a drawer: A pull force moves the drawer out from rest.
• Stopping a cricket ball (fielder catches it): A push force stops or slows down the ball.

Application of Force

From the above examples, we see that the application of force can:

• Start or move an object at rest (e.g. push a stationary object)
• Stop or slow down a moving object (e.g. catch a moving ball, apply brakes)
• Change the speed of a moving object (e.g. push a swing harder to go faster)
• Change the direction of a moving object (e.g. hit a cricket ball to send it in another direction)
• Change the shape of an object (e.g. squeeze, stretch, roll, twist)

In summary: A force can make an object move, stop, speed up, slow down, change direction, or change its shape

Are Forces an Interaction Between Two or More Objects?

Whenever you push or pull something, there are always two objects involved.

• Example 1: Your hand and the table when you push a table.
• Example 2: The bat and the moving ball.

Forces are the result of interactions between two or more objects.

A force is a push or pull on an object resulting from its interaction with another object. The SI unit (international scientific unit) for measuring force is the newton(symbol: N).

Is Force Needed for Change?

• Every change in speed, direction, or shape of an object means that a force has acted.
• If nothing is changing (object at rest, no change in speed/direction/shape), either there is no force, or the forces acting are balanced.
• If an object is at rest, it doesn’t mean no force is acting—it means forces are balanced.

SI Unit of Force

• The SI unit (international scientific unit) for measuring force is the newton(symbol: N).
• 1 newton is the amount of force needed to give a 1 kg object an acceleration of 1 meter per second squared.

Forces Always Work in Pairs

• When you apply a force on another object, you will feel a force back on yourself as well.
• Example: When pushing a table, your hand feels a resistance.
• As soon as the interaction stops, the force disappears for both.

Try yourself:

What happens when you push a table?

• A.The table moves only.
• B.The table gets heavier.
• C.You feel a force back.
• D.Nothing happens.

View Solution

What Are the Different Types of Forces?

Forces can be divided into two main categories:

• Contact Forces
• Non-contact Forces

Contact ForcesForces which act only when there is physical contact between the objects are called contact forces.

1. Muscular Force

The force applied by the action of muscles in humans and animals. All movement we do—walking, lifting, pushing, pulling, chewing, even our heartbeat—uses muscular force.

Examples:

• Lifting a bag, kicking a football, chewing food, pumping blood (heart muscle).
• Animals and humans have used animal muscular force for tasks like pulling carts or turning wheels.
• This force only acts when a person or animal is in direct (or indirect, through a tool) contact with the object.

2. Friction (Force of Friction)

Friction is a force that opposes the motion of an object when it moves or tries to move over another surface. It acts in the opposite direction to the motion.

Frictional Force

• How friction works: Arises because of tiny irregularities in the surfaces—these irregularities “lock” together and resist motion.
• When you push a box on the table, it stops after a while because friction between the box and table slows it down and brings it to rest.
  
• Nature of Surfaces: Friction changes on different surfaces—rougher surfaces (like sandpaper or cloth) have more friction than smoother ones (like glass or ceramic tile). This is why a box stops more quickly on rough surfaces.
• Friction in air and water: Air and water also create friction called drag or air resistance (for airplanes, cars, boats). This is why these vehicles are designed to be streamlined—to reduce friction.
  
• Friction is a contact force because it only acts when two surfaces are in contact.

Non-contact Forces

There are forces whose effect can be experienced even if the objects are not in contact. These forces are called non-contact forces. 

1. Magnetic Force 

The force exerted by a magnet on magnetic materials (like iron) or another magnet, at a distance.

• If like poles of two ring magnets are brought close (north-north or south-south), they repel; if unlike, they attract. You can “float” one ring magnet above another if like poles are facing.
• This is a non-contact force since magnets don’t have to touch to attract or repel.

2. Electrostatic Force 

• When two objects of certain materials are rubbed together, electrical charges build up on their surfaces. These charges are called static charges as they do not move by themselves. 
• The object that acquires static charges is said to be a charged object

The force exerted by a charged body on another charged body or an uncharged body is called electrostatic force. It is a non-contact force.

Examples:

• Rubbing a plastic scale with polythene: The charged scale attracts paper bits.
• Rubbing two balloons with wool and hanging them: The similarly-charged balloons repel each other; a charged balloon and the wool attract each other.
• Explanation: When certain materials are rubbed, they gain static charges (positive or negative). Like charges repel; unlike charges attract.
• When the charges move, they constitute an electric current in an electrical circuit. It is the same current which makes a lamp glow or generates a heating effect or a magnetic effect.

Gravitational Force

The force with which the Earth attracts objects towards itself is called the gravitational force. The gravitational force exerted by the Earth is also called force of gravity or simply gravity. 

• If you throw a ball up, no matter how hard, it always comes down to the ground. This is gravity pulling it down.
• Always attractive (never repulsive—unlike magnetic or electrostatic).
• Acts between any objects with mass (not just Earth—you’ll learn more about universal gravitation in higher classes!).
• While going up, the speed of the object goes on decreasing till the object comes to a stop, its direction of motion changes and while coming down the speed goes on increasing. 
• We say that the object undergoes a vertical motion when it moves in a vertical direction under the influence of the gravitational force.
• Gravitational force works at a distance and needs no contact.

Weight and Its Measurement

Weight is the force with which the Earth pulls an object towards itself due to gravity.

• Weight measures how strongly Earth attracts an object.
• SI unit of weight: newton (N), the same as force.

Measuring Weight with a Spring

• When you suspend an object from a spring, the spring stretches due to the force with which the Earth pulls (the weight of the object). 
• If you hang different-mass objects one after another, each causes a different amount of stretching. 
• This proves that heavier objects are pulled with more force—the weight of an object depends on its mass; heavier objects have greater weight.

Spring Balance: Measuring Force (Weight) with a Device

• Spring balance: A tool with a spring fixed at one end and a hook at the other. The amount the spring stretches tells you the force (weight) applied (by the hanging object).
• Scale markings: Usually shown in newtons (N) for weight, and also in grams/kilograms for mass when used on the Earth.
  
• Observing Maximum Weight Measurable (Range): The range of a spring balance is the maximum weight it can measure. If a spring balance shows values from 0 to 10 N, its range is 0–10 N, meaning it can measure weights up to 10 N.
• Determining the Smallest Readable Value (Least Count) of a Spring Balance: The least count of a spring balance is the smallest difference in weight it can measure. It is calculated by dividing the difference between two major marks by the number of divisions between them.
  Example: If 1 N is divided into 5 small marks, each small mark is 0.2 N.
  This tells us the spring balance can measure changes as small as 0.2 N.
• Measuring Weight with a Spring Balance: To measure the weight of an object, hang it from the hook of a spring balance (without exceeding its maximum range). The pointer or reading on the scale shows the object’s weight in newtons. This method can be repeated for many objects, and results should be recorded in a table.
• Using Spring Balance to Measure Mass: The mass scale (in g/kg) on a spring balance assumes Earth’s gravity: 
  The mass reading is only correct on Earth where gravitational acceleration is standard. Some balances also allow comparing an unknown object to an object with known mass (using beam balance).

Mass vs. WeightMass is the amount of matter in an object.

• Measured in grams (g) or kilograms (kg).
• Stays the same at every place, whether on Earth, Moon, or any other planet.

Weight is the force with which Earth (or another planet) pulls an object.

• Measured in newtons (N).
• Calculated as:
  =×Weight = Mass × Acceleration due to gravity (9.8m/s² )
• Weight can change if the object is taken to another planet or if the gravitational force varies.

Variation of Weight on Different Planets:
(For an object with mass 1 kg)PlanetMass (kg)Weight (N)Earth110Moon11.6Mars13.8Venus19Jupiter125.4

Everyday vs Scientific Language:

• In daily life, units of mass (like “10 kg”) are often used to talk about weight (for example, “the weight of the wheat bag is 10 kg”).
• Scientifically, this is not correct. The proper unit for mass is kilogram (kg) and for weight is newton (N).
• It is important to use the correct terms and units, especially in scientific work.

Buoyant Force: Upthrust, Floating, and Sinking

Upthrust (Buoyant Force) is the force applied by a liquid on an object in the upward direction. All liquids exert this force on objects immersed in them. Example: When you push a tightly closed empty plastic bottle into a bucket full of water, you feel an upward push on your hand and the bottle bounces back up when released.

What’s Happening Physically:
When an object is placed in a liquid:

• Gravity pulls it downwards.
• The buoyant force (upthrust) pushes it upwards.

What decides whether an object floats or sinks?

• If the gravitational force (weight of the object) is greater than the buoyant force, the object sinks.
• If the two forces are equal, the object floats.
• The density of the liquid affects the buoyant force—a factor you will study in more detail in later chapters.

Interesting Fact:
Some rocks, like pumice (formed during volcanic eruptions), can actually float on water.
Pumice rocks are filled with holes and air pockets, making them less dense than water. That’s why they float!

Archimedes’ Principle

• Discovered by Archimedes, an ancient Greek scientist.
• Archimedes’ Principle: When an object is fully or partially immersed in a liquid, it experiences an upward force (buoyant force) that is equal to the weight of the liquid displaced by the object.

Consequences:

• If the weight of the displaced liquid is less than the object’s weight, the object sinks.
• If the weight of the displaced liquid is equal to the object’s weight, the object floats.

Key Points to Remember:

• Force: A force is a push or pull on an object resulting from its interaction with another object.
• SI Unit of Force: The SI unit of force is newton (N).
• Types of Force: Forces can act with or without contact between objects.
• Contact Forces: Examples of contact forces include muscular force (force by muscles) and frictional force (force between surfaces in contact).
• Non-contact Forces: Magnetic force, gravitational force, and electrostatic force are examples of non-contact forces (they act without physical contact).
• Effects of Force: A force can change an object’s speed, the direction of its motion, or both. Force can also change the shape of an object.
• Friction: The force which comes into play when an object moves or tries to move over another surface is called the force of friction. Friction always acts in a direction opposite to the direction of motion.
• Magnetic Force: The force exerted by a magnet on another magnet or magnetic material is called magnetic force.
• Electrostatic Force: The force exerted by a charged body on another charged or uncharged body is called electrostatic force.
• Gravitational Force: The force with which the Earth attracts objects towards itself is called gravitational force. This force is always attractive.
• Weight: The force with which the Earth pulls an object towards itself is called the weight of the object. The SI unit of weight is newton (N).
• Mass and Weight: The mass of an object remains unchanged wherever it is, while its weight may change from place to place.
• Upthrust/Buoyant Force: When an object is placed in a liquid, the force applied by the liquid upwards on the object is known as upthrust or buoyant force.

Introduction

Have you noticed how a bulb lights up when you switch it on? But what if you don’t have a bulb—how else could you tell that electricity is flowing? Imagine being at an exciting science exhibition, just like Mohini and Aakarsh, and discovering that electric current can do things even a magnet can—not just create light, but also attract metals and produce heat!
Electric Circuit

This chapter will help you explore the amazing things electricity can do beyond just lighting bulbs—like producing heat and creating magnetic forces. Let’s dive in!

Does an Electric Current Have a Magnetic Effect?

Magnetic Effect of Electric Current: When electric current flows through a conductor (like a wire), it produces a magnetic field around it. This is called the magnetic effect of electric current.

Electric Circuit and Magnetic Compass

• Magnetic Compass: The presence of this magnetic field can be detected using a magnetic compass. When current flows, the compass needle is deflected; when current stops, the needle returns to its original direction.
• Magnetic field: The area around a magnet or current-carrying wire where its magnetic effect is felt (for example, by the deflection of a compass needle).
  

Discovery of Magnetic Effect by Oersted

• Hans Christian Oersted (1777–1851) discovered in 1820 that electricity and magnetism are linked. He found that when an electric circuit is switched ON or OFF near a magnetic compass, the needle deflects, showing that electric current creates a magnetic field.
• This discovery laid the foundation for understanding the connection between electricity and magnetism.

​

Try yourself:

What happens to a compass needle when electric current flows through a wire?

• A.It deflects.
• B.It spins wildly.
• C.It glows.
• D.It stops working.

View Solution

Electromagnet: Definition and Features

A current-carrying coil of wire wrapped around an iron core (like a nail) behaves as a magnet. Such a setup is called an electromagnet.

• Electromagnet: A soft iron core wound with a wire coil that produces a magnetic field only when electric current flows through it.
• The magnetism of the electromagnet disappears when the current is switched off.

How to Make an Electromagnet (from Activities):

1. Wrap insulated wire around an iron nail to form a coil.
2. Connect both ends of the wire to a cell.
3. When current flows, the iron nail can attract iron objects.
4. Disconnecting the current causes the nail to stop acting as a magnet.

Strength and Polarity of Electromagnets

• Electromagnets have two poles (North and South), just like bar magnets. 
• The poles can be determined using a magnetic compass. 
• The polarity of the electromagnet reverses if the direction of the current is reversed.

The strength of an electromagnet increases with:

• Greater current (using more cells/battery).
• More turns of wire in the coil.
• Inserting an iron core inside the coil.

Lifting Electromagnets

Lifting electromagnets are strong electromagnets used in cranes for lifting/moving heavy iron or steel objects. They are widely used in factories and scrap yards to move, lift, and sort heavy metal items efficiently.

Lifting Electromagnets

• Turning the current ON activates the magnet (lifts the metal).
• Turning the current OFF drops/releases the metal.

Earth’s Magnetic Field

• Earth acts as a giant magnet due to the movement of liquid iron in its core, which creates electric currents and generates a magnetic field.
• Many migratory birds, fish, and animals use the Earth’s field to navigate.
• The Earth’s magnetic field also shields life on Earth from harmful particles from space.

Does a Current-Carrying Wire Get Hot?

• Observation: When electric current flows through a conductor (like a nichrome wire), the wire becomes warm or even hot.
• Reason: This happens because the conductor resists the flow of electric current. This resistance causes some electrical energy to turn into heat energy.
• Heating Effect of Electric Current: The warming up of a conductor due to passage of electric current is called the heating effect of electric current.

Heating Effects in a Wire

What is Resistance?
Resistance is the property of a material that opposes the flow of electric current. Different materials offer different resistance:

• Nichrome wire has high resistance and gets heated more than a copper wire of the same size.

The amount of heat produced depends on:

• Type of material.
• Thickness and length of the wire (longer and thinner wires heat up more).
• The strength of the electric current (more cells = more current = more heat).
• Time duration for which current flows.

Everyday Examples of Heating Effect of Electric Current

• Applications: Many appliances work on the heating effect of electric current because they contain a wire element made of nichrome or similar material.
• Examples: Electric room heaters, electric stoves, irons, immersion rods, kettles, hair dryers, and incandescent lamp filaments.
• In some devices, the heating element becomes red-hot and may even glow.

Safety Measures

• ​Wires and appliances can overheat if too much current flows, which may cause damage, melting, or even fires.
• ​Use wires, plugs, and sockets rated for the correct amount of current. Install safety devices in household wiring to prevent overheating.

Industrial Uses

• High-temperature electric furnaces are used in industries to melt and recycle steel.
• These furnaces rely on the heating effect of a large electric current flowing through special heating coils.

Advantages and Disadvantages of the heating effect of electric current

• Advantage: The heating effect is useful for many daily and industrial tasks.
• Disadvantage: It sometimes leads to energy loss and overheating of wires, which is wasteful or dangerous.

How Does a Battery Generate Electricity?

What is an Electric Cell?

• An electric cell is a device that produces electricity through a chemical reaction.
• Cells have two terminals (positive and negative), which can be connected in a circuit to supply electric current.

Voltaic Cell (Galvanic Cell)

​One of the earliest electric cells, also called a Voltic or Galvanic cell.

• It has two different metal plates (electrodes) partly immersed in an electrolyte (a liquid that can conduct electricity).
• The chemical reaction between the metals and the electrolyte releases energy in the form of electricity.
• Electric current flows from the positive terminal (one metal) to the negative terminal (other metal) through the circuit.
• After some time, the materials inside are used up; the cell stops working and is called a dead cell.

Origin of the Voltaic Cell

• Alessandro Volta and Luigi Galvani studied how electricity could be produced using metals and liquids.
• Volta demonstrated that electricity was generated from the combination of different metals and an electrolyte, not from living things.

 Making a Simple Cell (Lemon Battery Activity)
You can make a simple electric cell at home using a lemon, a copper strip/wire, and an iron nail.

• The lemon juice acts as an electrolyte.
• Copper and iron are electrodes.
• Connecting several lemon cells in series can light up an LED.
• If the LED does not glow at first, reverse its connections (polarity matters!).

Dry Cell

The dry cell is the most commonly used battery in daily life (like in torches, clocks, TV remotes).
It is called “dry” because its electrolyte is not a liquid, but a moist paste.
Structure:

• Zinc container (acts as the negative terminal)
• Carbon rod (positive terminal, at the center), surrounded by the paste
• A dry cell is usually single-use (not rechargeable)—once exhausted, it must be thrown away.

Rechargeable Batteries

​Rechargeable batteries can be recharged and used many times (for example: in phones, laptops, cameras, vehicles).

• They reduce waste and are more cost efficient over time.
• However, they also wear out after many cycles of charging and usage.
• Lithium-ion (Li-ion) batteries are the most common rechargeable batteries today—found in almost all modern devices.
• Scientists are working on solid-state batteries for the future, which will be safer, longer-lasting, and even more efficient.

Points to Remember: Electricity—Magnetic and Heating Effects

• Magnetic Effect of Electric Current: When electric current flows through a conductor (such as a wire), it produces a magnetic field around it. This is called the magnetic effect of electric current.
• Electromagnet: A coil of wire carrying electric current behaves like a magnet and is called an electromagnet. Most practical electromagnets have an iron core to increase their strength.
• Polarity: Electromagnets, like regular magnets, have two poles (North and South). The poles can be reversed by changing the direction of the current.
• Lifting Electromagnet: Powerful electromagnets are used in industries to lift and move heavy metal objects. They can be turned ON and OFF by controlling the flow of current.
• Heating Effect of Electric Current: When electric current flows through a conductor, it generates heat due to the resistance of the material. This is known as the heating effect of electric current.
• Heating Element: Devices such as electric heaters, irons, and kettles have a coil or rod made of high-resistance material (like nichrome) that acts as a heating element.
• Electric Cell and Battery: A cell or a battery generates electric current through chemical reactions occurring inside. Cells have a positive and a negative terminal.
• Dry Cell: A common type of battery used in daily life, containing a moist paste as electrolyte; generally single-use.
• Rechargeable Batteries: These batteries can be recharged and used multiple times, reducing waste. Examples include lithium-ion batteries used in phones and laptops.
• Electrolyte and Electrode: An electrolyte is a substance that conducts electricity by the movement of ions; electrodes are the metal terminals through which current enters or leaves a cell.
• Resistance: The opposition to the flow of electric current, leading to heating in wires or devices.
  

Electric current can create both magnetism and heat. Devices like electromagnets use the magnetic effect, while many household appliances use the heating effect. Batteries and cells are sources of electric current based on chemical reactions, and rechargeables allow repeated use, making them vital for modern life.