We see many changes happening all around us every day. Ice melts into water, flowers bloom from buds, fruits change color and smell, and cold water becomes warm over time. These changes can affect how things look, smell, feel, or taste. Using our five senses — sight, smell, touch, hearing, and taste — we can observe and understand these changes better.
Let’s explore some common changes around us and learn how to notice them carefully.
A Substance May Change in Appearance but Remain the Same
What is a Physical Change?
A physical change is when a substance or object changes in appearance (e.g., shape, size, state) but remains the same substance, with no new material formed.
Physical Changes
Key Features
Only physical properties like shape, size, or state change.
The substance’s chemical composition remains unchanged.
Examples include melting ice, boiling water, or folding clothes.
Examples
Paper Folding: Folding paper into shapes (e.g., a boat or plane) changes its form, but unfolding it returns the original paper.
Balloon Inflation: Inflating a balloon stretches it, but letting the air out returns it to its original shape. However, pricking an inflated balloon causes it to burst, and the original shape cannot be restored, though the rubber material remains the same.
Crushing Chalk: Crushing chalk into powder changes its form, but the material (chalk) stays the same, though the original piece cannot be reformed.
Water’s States: Water changing from solid (ice) to liquid (water) to gas (steam) is a physical change, as it remains water despite changing states.
Try yourself:
What is a physical change?
A.A change that creates a new substance.
B.A change in appearance without new material.
C.A change that cannot be undone.
D.A change that occurs only in nature.
View Solution
A Substance May Change in Appearance and Not Remain the Same
What is a Chemical Change?
When substances react chemically, they form new substances with different properties. This is called a chemical change. Unlike physical changes where only appearance or state changes, chemical changes create entirely new materials.
Chemical Changes
Examples:
1. Blowing Air into Lime Water
A common example is when carbon dioxide (CO₂) gas comes into contact with lime water (a solution of calcium hydroxide). Lime Water Turns Milky
The carbon dioxide reacts with the lime water to form calcium carbonate, which is a white solid that makes the solution look milky or cloudy.
This solid eventually settles at the bottom of the container.
Along with calcium carbonate, water is also formed during this reaction.
This reaction can be written as: Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
This shows that a new substance, calcium carbonate, is produced, proving that a chemical change has taken place.
2. Vinegar and Baking Soda
Another example is the reaction between vinegar (which contains acetic acid) and baking soda (sodium bicarbonate).
When these two substances mix, they react to produce carbon dioxide gas, which can be seen as bubbling or fizzing.
This gas, when passed through lime water, turns the lime water milky due to the formation of calcium carbonate.
This confirms that carbon dioxide gas was produced by the chemical reaction.
Try yourself:
What happens to lime water when carbon dioxide is blown into it?
A.It evaporates.
B.It turns milky.
C.It remains unchanged.
D.It becomes fizzy.
View Solution
Some Other Processes Involving Chemical Changes
Rusting
Rusting is a chemical change where iron reacts with air and moisture to form a new substance called rust (iron oxide).
This brown-colored substance forms on iron objects like nails and makes them weak and crumbly.
Since a new substance is formed, rusting is a chemical change.
Rusting of Iron
Combustion
Combustion is a chemical reaction where a substance reacts with oxygen, producing heat and/or light. Substances that burn are called combustible substances (e.g., wood, paper, cotton, kerosene).
An example is burning magnesium ribbon. When magnesium burns in air, it forms magnesium oxide, a white powder, and releases heat and light.
This shows that combustion involves forming new substances along with energy release.
The chemical reaction for burning magnesium can be written as: Magnesium + Oxygen → Magnesium oxide + Heat + Light (Ribbon) (Air) (White powder)
Substances that can catch fire and burn are called combustible substances. Examples include wood, paper, cotton, and kerosene.
Oxygen’s Role in Combustion
Oxygen from the air is essential for combustion.
If you cover a burning candle so it cannot get air, the flame goes out because there is no oxygen to support burning.Candle (a) burning (b) covered with a glass tumbler
During combustion, carbon in the fuel reacts with oxygen to form carbon dioxide gas.
Carbon dioxide can be detected by passing it through lime water, which turns milky due to the formation of calcium carbonate.
This confirms that oxygen supports burning and that combustion produces carbon dioxide.
What Else is Needed to Start Combustion?
Besides fuel (the combustible substance) and oxygen, heat is required to start combustion. This heat raises the fuel’s temperature to a point called the ignition temperature, at which it catches fire.
For example, paper will catch fire quickly if touched with a lighted matchstick because the matchstick’s temperature is above paper’s ignition temperature.
However, paper can also catch fire without a matchstick if focused sunlight heats it enough. Using a magnifying glass to concentrate sunlight on paper can increase its temperature until it starts burning.Paper catching fire
Three Requirements for Combustion
To start and maintain combustion, these three things are necessary:
Fuel (Combustible substance) — Something that can burn.
Oxygen — To support the burning process.
Heat — To raise the fuel to its ignition temperature.
Fire Triangle: Combustion requires fuel, oxygen, and heat (ignition temperature), represented as a triangle.Fire triangle
Fascinating Fact
Have you seen tiny glowing insects in gardens at night?They are called fireflies, and their light comes from a chemical change inside their bodies. This special kind of light, made without heat, is called bioluminescence.
Science and Society
What to do if someone’s clothes catch fire? Wrap them in a blanket or cloth to stop the air supply — this helps put out the fire.
Important: Never use synthetic blankets or clothes, as they can melt and stick to the skin, causing more harm.
Try yourself:
What substance is formed when iron rusts?
A.Water
B.Iron oxide
C.Carbon dioxide
D.Magnesium oxide
View Solution
Can Physical and Chemical Changes Occur in the Same Process?
Burning a Candle Physical Changes
Melting Wax: Heat from the flame melts solid wax into liquid, a physical change as no new substance forms.
Evaporation: Liquid wax moves up the wick and evaporates into vapor, another physical change.
Solidification: Wax dripping and cooling back into solid is also physical.
Burning a Candle Chemical Change:
Burning Wax Vapor: The wax vapor burns in the presence of oxygen, producing carbon dioxide, water, heat, and light, forming new substances, indicating a chemical change.
Conclusion
Burning a candle involves both physical changes (melting, evaporation) and a chemical change (burning), showing that some processes combine both types.
Know A Scientist: Michael Faraday
Scientist Michael Faraday studied candles in the 19th century, using them to explain physical and chemical processes like melting, vaporization, and combustion in his lectures, “Chemical History of a Candle.”
Are Changes Permanent?
Reversible Changes
Definition: Changes where the original substance or object can be restored.
Examples:
Melting Ice: Ice melts into water, but freezing the water reforms ice.
Boiling Water: Water turns to steam, but condensing the steam returns liquid water.
Folding Paper: Folding and unfolding paper restores its original shape.
Irreversible Changes
Definition: Changes where the original substance or object cannot be restored.
Examples:
Chopping Vegetables: Cut pieces cannot be reassembled into the whole vegetable.
Making Popcorn: Corn kernels transform into popcorn, which cannot revert to kernels.
Burning Wood: Wood turns to ash, which cannot become wood again.
Are All Changes Desirable?
Desirable Changes
Changes that are useful or beneficial.
Examples:
Milk to Curd: Curdling produces tasty curd.
Ripening Fruits: Fruits become sweet and edible.
Cooking Food: Raw ingredients become nutritious meals.
Cutting Fruits: Makes them easier to eat.
Undesirable Changes
Changes that are harmful or unwanted.
Examples:
Rusting of Iron: Damages tools and structures.
Food Decay: Spoils stored food, making it inedible.
Some changes can be good or bad depending on the situation
Decomposing food is bad if it rots in the kitchen, but good when it’s used to make compost for plants.
Environmental Impact
Human activities, like burning fuels in vehicles or drying paint, release carbon dioxide and pollutants, increasing atmospheric pollution over time.
These changes have long-term effects on the environment, like climate change.
Try yourself:
What happens to wax when it is melted by a candle flame?
A.It forms a new substance.
B.It evaporates into vapor.
C.It becomes liquid without a new substance.
D.It turns into ash.
View SolutionSome Slow Natural Changes Weathering of Rocks
Weathering is the process where rocks break down into smaller pieces (sediments) through physical and chemical changes, eventually forming soil.
1. Physical Weathering
Caused by temperature changes, tree roots growing into cracks, or water freezing in rock crevices, breaking rocks into smaller pieces.
Example: Sediments like sand, soil, and stones collect at the base of mountains or cliffs.
2. Chemical Weathering:
Occurs when water or chemicals in water react with rock components.
Example: Basalt rock, containing iron, reacts with water or air to form a red layer of iron oxide (similar to rust), changing its composition.
Together, these changes help break down rocks and form soil, which is important for plant growth and life on Earth.
Erosion
Erosion is the physical process where rocks, soil, and sediments are broken down and moved by natural forces like wind or flowing water.
Examples
You may have seen fine sand on riverbeds or lakes—this is created when larger rocks and soil are worn down and moved by rivers or wind.
River rocks become smooth over time because the flowing water keeps rubbing them, slowly wearing them down.
During landslides, erosion happens quickly, as large amounts of soil and rock are displaced. This is an example of a physical change.
When wind or water slows down (like in lakes or oceans), the carried sediments settle at the bottom.
Over thousands of years, these sediments get pressed and hardened to form new rocks.
These changes are slow, natural, and usually irreversible.
Points to Remember
Physical changes: Only shape, size, or state changes. No new substance is formed. Examples: Melting ice, boiling water, folding clothes.
Chemical changes: New substances are formed. Examples: Rusting, burning, lime water turning milky with carbon dioxide.
Rusting: Iron reacts with air and water to form rust — a new substance.
Combustion: Burning with oxygen produces heat and light. Needs fuel, oxygen, and enough heat (ignition temperature).
Fireflies: Glow due to a chemical change called bioluminescence, which gives light without heat.
Burning a candle:
Physical change: Wax melts and evaporates.
Chemical change: Wax vapour burns to form carbon dioxide and water.
Reversible changes: Can go back (e.g., melting ice). Irreversible changes: Cannot go back (e.g., chopping vegetables).
Desirable changes: Helpful (e.g., cooking food, making curd). Undesirable changes: Harmful (e.g., rusting, food spoilage), but some like decomposition can help make compost.
Human activities: Burning fuel and drying paint release gases that pollute the air and increase carbon dioxide.
Weathering: Rocks break into soil slowly due to temperature or water — both physical and chemical changes.
Erosion: Wind or water moves rocks and soil. It shapes the land but is often irreversible.
Michael Faraday: A scientist who used candles to explain physical and chemical changes in simple ways.
Try yourself:
What is weathering?
A.Creating new rocks from sediments.
B.A type of chemical reaction.
C.Breaking down rocks into smaller pieces.
D.The process of moving sediments.
View Solution
Difficult Words and Their Meanings
Physical Change: A change in a substance’s appearance (e.g., shape, size, state) without forming a new substance, like melting ice.
Chemical Change: A change where new substances are formed through a chemical reaction, like rusting or burning.
Chemical Reaction: A process where substances (reactants) transform into new substances (products), e.g., vinegar and baking soda forming carbon dioxide.
Rusting: A chemical change where iron reacts with air and water to form brown rust (iron oxide).
Combustion: A chemical reaction where a substance burns with oxygen, producing heat and/or light, e.g., burning magnesium.
Combustible Substance: A material that can burn, like wood, paper, or kerosene.
Ignition Temperature: The minimum temperature at which a substance catches fire, e.g., paper’s ignition temperature when heated by a matchstick.
Fire Triangle: The three requirements for combustion—fuel, oxygen, and heat (ignition temperature).
Reversible Change: A change where the original substance can be restored, e.g., freezing melted water.
Irreversible Change: A change where the original substance cannot be restored, e.g., burning wood to ash.
Desirable Change: A beneficial change, like cooking food or ripening fruits.
Undesirable Change: A harmful change, like rusting iron or food spoilage.
Weathering: The physical and chemical breakdown of rocks into soil, caused by temperature, water, or chemicals.
Erosion: The physical movement of rocks, soil, or sediments by wind or water, shaping landscapes.
Bioluminescence: Light produced by living organisms, like fireflies, through a chemical change without heat.
Yashwant and Anandi live in a village in Rajasthan. For their school project, they decided to learn about ironsmiths—people who make useful items from metals. With their grandfather’s help, they visit Sudarshan uncle, a local ironsmith, to see how he shapes iron into everyday tools like pans, buckets, and farming equipment.Curious and amazed, they begin exploring the fascinating world of metals and how they can be shaped and used.
Let’s explore the different ways metal can be used.
Properties of Materials
Properties are characteristics that help us identify and classify materials as metals or non-metals, such as their appearance, hardness, or ability to conduct heat.
Malleability
Malleability is the property of materials that allows them to be beaten into thin sheets without breaking. Beating an iron nail with a hammer
Malleability of Metals
1. Most metals, like copper, aluminium, and iron, are malleable. For example, copper and aluminium can be flattened into sheets, and iron is shaped into tools like axes.
2. Malleability is very useful in everyday life. For example:
Silver foil used to decorate sweets is made by hammering silver into extremely thin sheets.
Aluminium foil, which is commonly used for wrapping food, is produced due to aluminium’s malleability.
3. Some metals, like gold and silver, are exceptionally malleable, allowing them to be made into very thin sheets called gold leaf or silver leaf.
Some Exceptions: Soft Metals and Unique States
Not all metals are hard; some metals, like sodium and potassium, are very soft and can be easily cut using a knife. These metals are much softer than copper or iron.
Another special metal is mercury, which is liquid at room temperature, unlike most metals that are solids.
Mercury’s liquid state makes it unique and useful in devices like thermometers and barometers.
Q: How does malleability benefit everyday life? View Answer
Brittleness in Non-Metals
Unlike metals, non-metals such as coal and sulfur do not flatten when struck.
Instead, they tend to break or shatter into pieces.
This characteristic is called brittleness.
Brittle materials cannot be bent or shaped easily; they break under pressure or impact.
Behaviour of Wood
Wood behaves differently from both metals and brittle non-metals:
Wood does not flatten like metals when hit with a hammer.
At the same time, wood does not break easily like brittle non-metals.
Therefore, wood is considered neither malleable nor brittle, possessing some flexibility and toughness.
Some other properties of Metals and Non-Metals:
Metals such as copper, aluminium, and iron have a characteristic shiny appearance known as metallic lustre. This means they reflect light well, giving them a bright, polished look.
These metals are generally hard, meaning they resist scratching or denting under normal conditions.
In contrast, non-metals like coal, sulfur, and wood do not have this shiny appearance. Instead, they look dull or matte because they do not reflect light like metals.
Non-metals are usually softer compared to metals, meaning they can be scratched or broken more easily.
HOLISTIC LENS: The impact of iron on the progress of civilisation of India
In ancient India, early civilizations like the Harappans used metals such as copper and gold to make tools and jewellery. However, the widespread use of iron came much later. Iron became important because of its strength and durability. Iron tools, especially agricultural implements like ploughs, greatly helped improve farming and contributed to the progress of Indian civilization. Why do you think copper was discovered and used before iron?
Copper was discovered before iron because it occurs naturally in a pure form and has a lower melting point, making it easier to find and work with using early technology. Iron, found mostly as ore, requires higher temperatures and more advanced tools to extract and shape.
Ductility
Ductility is the property of materials that allows them to be drawn into thin wires.
Ductility of Metals
Most metals, like copper and aluminium, are ductile, used in electrical wires and ornaments like bangles and necklaces.
Gold is highly ductile, with one gram drawable into a 2-kilometer-long wire.
Everyday Uses of Ductility of Metals
Electrical Fittings: Metal wires like copper and aluminium are commonly used in electrical wiring because they conduct electricity well. You might have seen them in homes, appliances, and other electrical devices.
Jewellery and Ornaments: Many ornaments such as bangles, necklaces, and earrings are made from metal wires. These wires are shaped and twisted to create beautiful designs.
Musical Instruments: Metal wires are also used in many stringed musical instruments like the veena, sitar, violin, and guitar. The wires produce sound when plucked or bowed.
Dive Deeper: Steel Wires and Their Uses:
Steel is an alloy made from iron (metal) and carbon (non-metal). Steel wires are very strong and can support heavy loads. Due to their strength, steel wire ropes are used in important structures like suspension bridges. They are also used in cranes to lift heavy objects safely.
Q: What does ductility refer to in materials science? View Answer
Sonority
This ability of metals to produce ringing sounds is called sonority. Metals are described as sonorous materials because they can create loud and resonant sounds.
Metals: Metals like those in spoons or coins produce a ringing sound, making them sonorous. This is why school bells and ghungroos (dance bells) ring.
Non-metals: Coal and wood produce dull sounds when dropped, so they are not sonorous.
Everyday Examples of Sonority
The ringing sound of ghungroos (the small bells worn by dancers) is because of the sonorous nature of metals.
The school bell produces its loud ringing sound due to the sonority of the metal it is made from.
People can also use the difference in sound between hitting wood or metal to help navigate or identify objects, like using a stick to find their way by the sound it makes when it hits different materials.
Conduction of Heat
Conduction is the transfer of heat through a material from one point to another.
Metals: Metals like those used in cooking vessels (e.g., aluminium, copper, iron) are good conductors of heat, transferring heat quickly to cook food.
Non-metals: Wood is a poor conductor of heat, staying cooler than metal when placed in hot water, which is why vessel handles are made of wood or other poor conductors.
For example, if a metal spoon and a wooden spoon are both left in hot water for the same time, the metal spoon feels much hotter to the touch.
This is because the metal conducts heat faster from the hot water to your hand, while wood does not.
Metal and wooden spoons immersed in hot water
That’s why vessel handles are often made of wood or plastic—so that we can hold them safely without getting burned.
Conductors and Poor Conductors
The process of heat moving through a material from one point to another is called conduction.
Materials that allow heat to pass through them easily are known as conductors. Metals fall in this category.
On the other hand, materials like wood do not transfer heat well and are known as poor conductors or insulators.
Conduction of Electricity
Materials that allow electricity to flow easily are good conductors of electricity, while those that don’t are poor conductors.
Metals: Aluminium, iron, and copper are good conductors, making bulbs glow in a tester circuit, used for electrical wires.
Non-metals: Sulfur, coal, wood, stone, rubber, and nylon are poor conductors, not allowing bulbs to glow, used for insulating materials like screwdriver handles and electrician’s gloves.
Electrical Conductivity and Safety
For example, the handle of a screwdriver used by electricians is often made of plastic or rubber, which are poor conductors of electricity.
Electricians also wear rubber gloves and rubber-soled shoes while working. This is because rubber does not allow electricity to pass through easily, protecting them from electric shocks.
Conductors and Insulators
Materials that allow electricity to flow through them easily are called good conductors of electricity.
Metals such as aluminium, iron, and copper are excellent conductors and are widely used in electrical wiring and devices.
On the other hand, materials like sulfur, coal, wood, stone, rubber, and nylon do not allow electricity to flow freely and are called poor conductors of electricity or insulators.
Try yourself:
What is the property of materials that allows them to be drawn into thin wires?
A.Conductivity
B.Malleability
C.Ductility
D.Sonority
View Solution
Effect of Air and Water on Metals: Iron
Rusting of Iron:
When iron objects are left exposed to the environment, they often develop brown deposits on their surface. This phenomenon is called rusting.Rusting Iron
The brown deposits that form on iron are called rust.
Rusting is a chemical reaction where iron reacts with oxygen and moisture in the air.
This reaction causes the iron to deteriorate and weaken over time.
Conditions for Rusting:
Rusting happens only when iron comes into contact with both air and water (moist air).
Iron does not rust if it is exposed to dry air alone.
Iron also does not rust if it is submerged in water without exposure to air.
Therefore the process of rust formation, called rusting, requires both air and water, making moist air the cause of the brown deposits.
Impact
Rusting causes iron objects and structures to become weak and unsafe.
It leads to damage and decay of iron used in buildings, vehicles, bridges, and tools.
In many countries, including ours, a large amount of money is spent every year on repairing or replacing rusted iron.
Prevention of Iron from Rusting
Rusting can be prevented by several methods, such as:
Painting iron surfaces to keep air and moisture away.
Applying oil or grease to form a protective layer.
Galvanisation: Coating iron with a layer of zinc to protect it.
Q: What role do air and water play in the corrosion of iron? View Answer
Corrosion
Corrosion is the gradual deterioration of metal surfaces due to air, water, or other substances.
Fascinating Fact: The Iron Pillar of Delhi
The Iron Pillar of Delhi was built over 1600 years ago during Chandragupta II’s time. It is 8 metres tall and weighs more than 6000 kilograms. Despite facing rain, wind, and weather for centuries, it has hardly rusted. This shows how skilled ancient Indian metalworkers were in making strong and lasting iron.
Effect of Air and Water on Other Metals
1. Magnesium
Reaction with Air: When a magnesium ribbon is burned, it produces a dazzling white flame and turns into a white powder called magnesium oxide.
Nature of Oxide: Mixing magnesium oxide with warm water and testing with litmus shows it turns red litmus blue, indicating it is basic in nature.
General Rule: Oxides of metals are generally basic.
2. Sodium
Storage: Sodium is stored in kerosene to prevent it from reacting with oxygen and water in the air, as it reacts vigorously, producing a lot of heat.
Oxide Nature: Sodium oxide is basic, like other metal oxides.
Try yourself:
What causes rust to form on iron objects?
A.Exposure to sunlight
B.Exposure to water only
C.Exposure to both air and water
D.Exposure to air only
View Solution
Substances that Behave Differently from Metals in Air and Water
Certain substances, such as sulfur and phosphorus, behave very differently from metals when exposed to air and water.
Sulfur
Reaction with Air: When sulfur is burned, it produces sulfur dioxide gas, which, when dissolved in water, forms sulfurous acid.
Nature of Oxide: Testing the solution with litmus shows it is acidic, turning blue litmus red.
Reaction with Water: Sulfur does not react with water when mixed, unlike some metals.
Phosphorus
Storage: Phosphorus is stored in water because it catches fire when exposed to air.
Properties of Non-metals:
Usually soft and dull (non-lustrous) in appearance.
Not malleable or ductile, so they cannot be shaped into sheets or wires.
Not sonorous, producing dull sounds when struck.
Poor conductors of heat and electricity.
Oxides are acidic, unlike the basic oxides of metals.
Materials like plastic, glass, wood, rubber, and paper are not classified as metals or non-metals because they are not elements.
Dive Deeper: Elements (The Building Blocks of Matter)
Elements are pure substances that cannot be broken down into simpler substances by ordinary chemical methods.
Everything around us is made up of these elements.
There are currently 118 known elements.
Some elements occur naturally in the environment, such as oxygen, iron, and gold.
Others are artificially created in laboratories and do not exist naturally.
Metals and non-metals are two important sub-categories of elements.
You will learn more about elements and their properties in higher classes.
Are Non-metals Essential in Everyday Life?
While metals are very visible in daily life due to their shiny appearance, strength, and ability to conduct heat and electricity, non-metals play equally vital roles in our lives.
Importance of Non-metals
1. Oxygen is a non-metal that we breathe every day. Without oxygen, life on Earth would not survive. Other uses of oxygen include:
Used in hospitals to assist patients who have difficulty breathing.
Used in welding and combustion processes.
2. Carbon is essential because it is the building block of all life forms.
It is a key part of proteins, fats, and carbohydrates which provide energy and help in growth.
3. Nitrogen is a non-metal widely used in the manufacture of fertilizers and chemicals.
It is a vital nutrient that helps plants grow healthy and strong.
4. Chlorine is commonly used in water purification to make drinking water safe.
5. Iodine solution is applied on wounds as an antiseptic to prevent infections.
Science and Society
1. Many metals and alloys (mixtures of two or more metals or metals with non-metals) are used daily in utensils, tools, and machines.
2. Metals are essential in modern technology and almost every industry, including special fields like:
Atomic energy (e.g., zirconium)
Aerospace (e.g., titanium)
3. In India, recycling metals like iron and aluminium is common and helps to reduce waste and promote sustainability.
Try yourself:
What happens when sulfur is burned in air?
A.It becomes a metal.
B.It turns blue litmus red.
C.It produces sulfur dioxide gas.
D.It reacts with water.
View Solution
Points to Remember
Metals are shiny, hard, bendable (malleable), stretchable (ductile), make ringing sounds (sonorous), and conduct heat and electricity well.
Non-metals are usually dull, soft or brittle, and do not conduct heat or electricity well.
Malleability lets metals like copper and gold be made into thin sheets (foils, tools).
Ductility lets metals be drawn into wires for electrical use and jewelry.
Metals produce clear ringing sounds used in bells, while non-metals produce dull sounds.
Metals conduct heat, so they are used for cooking pots; non-metals like wood are used for handles because they don’t conduct heat.
Metals like copper and aluminium conduct electricity, while non-metals like rubber and plastic act as insulators for safety.
Iron rusts when exposed to both air and water, forming reddish-brown rust.
Copper and silver also get coated when exposed to air and moisture (green and black coatings).
The Iron Pillar of Delhi (1600+ years old) shows how ancient Indians made iron that doesn’t rust easily.
Magnesium burns in air forming a basic oxide; sodium is stored in kerosene to prevent reaction with air.
Sulfur burns to form sulfur dioxide, which forms acidic sulfurous acid when dissolved in water, but sulfur alone doesn’t react with water.
Non-metals like phosphorus are stored in water to stop them catching fire in air; their oxides are acidic.
Non-metals are important: oxygen for breathing, carbon for life, nitrogen for fertilizers, chlorine to clean water, iodine as medicine.
There are 118 elements, divided into metals and non-metals, some natural and some made in labs.
Metals and alloys are used in tools, machines, and industries; recycling metals like iron and aluminium helps protect the environment.
Q: What properties of metals allow them to be used in making electrical wires and tools? View Answer
Difficult Words and Their Meanings
Malleability: The ability of a material, like metal, to be beaten into thin sheets without breaking.
Ductility: The ability of a material, like metal, to be drawn into thin wires.
Sonority: The property of a material, like metal, to produce a ringing sound when struck.
Conduction: The transfer of heat or electricity through a material, like heat through a metal spoon.Metal Characteristics
Conductor: A material, like copper, that allows heat or electricity to flow easily.
Insulator: A material, like rubber, that does not allow heat or electricity to flow.
Rusting: The process where iron forms brown deposits (rust) when exposed to both air and water.
Corrosion: The gradual damage to a metal’s surface by air, water, or other substances, like rusting or green coating on copper.
Oxide: A substance formed when a metal or non-metal reacts with oxygen, like magnesium oxide or sulfur dioxide.
Element: A basic substance, like iron or oxygen, that cannot be broken into simpler substances, with 118 known types.
IntroductionIn this chapter, Nihal and his classmates are excited about their upcoming school trip to the Bhakra Nangal Dam, where they will learn how falling water is used to generate electricity at the hydroelectric power house. During the trip, they will also enjoy a scenic 13-kilometer train ride along the Sutlej river and through the Shivalik hills. Before the trip, their teacher assigns them a task to prepare a presentation on the uses of electricity. The students explore electricity’s applications in various settings—home, school, neighborhood, and city—and learn that electricity is generated from different sources, including wind, solar power, and falling water. They also realize that while electricity is essential in daily life, it must be handled carefully to avoid dangers. The chapter introduces the concept of portable sources of electricity, like batteries used in devices such as torchlights, wall clocks, and remotes.
Caution — The warning signs on electric poles and appliances remind us that electricity can be dangerous if not handled carefully. Never conduct experiments with the power supply at home or school. Even electricity from portable generators can pose a risk. For safe experimentation, use only batteries or cells, like those found in torchlights, wall clocks, radios, or remotes.
A Torchlight
What is a Torchlight?
A torchlight (also called a torch or flashlight) is a portable device that produces light, commonly used to see in the dark.
Components of a Torchlight:
Lamp: The part that produces light when the torch is turned on.
Switch: A control that turns the lamp “on” or “off”. Sliding the switch to one position makes the lamp glow, and sliding it back turns it off.
Electric Cells: Inside the torch, there are usually two or more electric cells that provide the energy to make the lamp glow.
How It Works
When the switch is in the “on” position, it connects the cells to the lamp, allowing electricity to flow and make the lamp glow. In the “off” position, the connection is broken, and the lamp stays off.
A Simple Electrical Circuit
What is an Electrical Circuit?
An electrical circuit is a complete path that allows electric current to flow from a power source (like a cell) through a device (like a lamp) and back, making the device work.
1. Electric Cell
An electric cell is a small, portable source of electrical energy, like those used in torchlights, clocks, or remotes.
1. Terminals: Every cell has two terminals:
Positive Terminal (+): Marked with a “+” sign, usually a metal cap.
Negative Terminal (-): Marked with a “-” sign, usually a flat metal disc.
2. Function: The cell provides the energy needed to make devices work by allowing current to flow from the positive to the negative terminal.
2. Battery
A battery is a combination of two or more electric cells connected together to provide more energy or last longer.
Connection: In a battery, the positive terminal of one cell is connected to the negative terminal of the next cell, forming a chain. This forms battery. Battery made up of (a) two cells (b) four cells
Example: In a torch, two cells are placed so the positive terminal of one touches the negative terminal of the other, making the lamp glow when connected properly.
Fascinating Fact
The term “battery” is often used for a single cell, like the one in a mobile phone, even though it’s technically one cell.
Electric Lamp
1. Incandescent Lamp:
Structure: An incandescent lamp has a glass bulb with a thin wire called a filament inside, supported by two thicker wires.
Terminals: The filament connects to two terminals—one at the metal case of the lamp’s base and one at the metal tip in the center.
How It Works: When electric current flows through the filament, it gets hot and glows, producing light.
Usage: Found in older torchlights, these lamps glow regardless of which terminal connects to the cell’s positive or negative terminal.
Fascinating Fact
If the filament breaks, the lamp “fuses,” stopping the current flow and preventing the lamp from glowing.
2. LED Lamp
Structure: A Light Emitting Diode (LED) lamp has no filament. It has two wires—one longer (positive terminal) and one shorter (negative terminal).An LED lamp for torch
How It Works: LEDs glow only when connected correctly, with the positive terminal (longer wire) to the battery’s positive terminal and the negative terminal (shorter wire) to the negative terminal. Current flows in one direction only.LEDs of different colours
Usage: Many modern torchlights use LEDs because they are efficient and long-lasting.
Making an Electric Lamp Glow Using an Electric Cell or Battery:
To make an electric lamp glow, we can use a simple circuit with an electric cell (or battery), an incandescent lamp, a cell holder, and some electric wires. Here’s how it works:
Electric Cell: This provides the electrical energy required to make the lamp glow. The energy comes from the chemical reaction inside the cell.
Incandescent Lamp: The lamp is a device that uses electricity to produce light. It has a filament that glows when electricity passes through it.
Cell Holder: A holder is used to securely place the electric cell in the circuit. It ensures the proper connection of the battery’s terminals to the wires.
Electric Wires: Wires are used to connect the components in the circuit. The wires allow the flow of electric current from the cell to the lamp.
How the Circuit Works:
Connection: The two terminals of the electric cell are connected to the lamp through the electric wires.
Current Flow: When the circuit is complete, the electric current flows from the negative terminal of the cell, through the wires, and into the lamp. This causes the filament inside the lamp to heat up and produce light.
Prediction of Lamp Glow: Depending on how the circuit is set up, the lamp may or may not glow. If the circuit is properly connected (with all components in place), the lamp will glow. Otherwise, it will not light up.
Electrical Circuit
A circuit is a closed loop that allows current to flow from the positive terminal of a cell, through a device (like a lamp), and back to the negative terminal.
Current Flow: Electric current is considered to flow from the positive to the negative terminal of the cell.
Incandescent Lamp: Glows when the circuit is complete, regardless of terminal connections.
LED: Glows only when terminals are correctly aligned with the battery’s terminals.
Electric Switch
A switch is a device that controls the flow of current by completing (closing) or breaking (opening) a circuit.
How It Works:
1. ON Position: The switch closes the circuit, allowing current to flow, making the lamp glow.
2. OFF Position: The switch opens the circuit, stopping the current, so the lamp doesn’t glow.
Placement: A switch can be placed anywhere in the circuit and still control the current flow.
Example: In a torch, sliding the switch to “on” completes the circuit, and sliding it to “off” breaks it.
Real-Life Switches: Home switches for lights work similarly but are designed differently for safety and convenience.
Circuit Diagrams
What is a Circuit Diagram?
A circuit diagram is a drawing that uses standard symbols to represent the components of an electrical circuit, making it easy to understand how the circuit is built.
Symbols for Components:
Electrical components and their symbols
Purpose Circuit diagrams simplify complex circuits, making them easier to draw and understand.
Dive Deeper – Standardization
International organizations like the International Electrotechnical Commission (IEC), American National Standards Institute (ANSI), and the Institute of Electrical and Electronics Engineers (IEEE) create standard symbols used worldwide, ensuring everyone understands the same diagrams.
Electrical Conductors and Insulators
Materials are classified based on whether they allow electric current to flow through them.
Conductors
Materials that allow electric current to flow easily are called conductors or good conductors of electricity.
Examples: Metals like copper, silver, gold, aluminum, and objects like keys, coins, and sewing needles.
Use in Circuits: Conductors, especially copper, are used for wires because they allow current to flow efficiently.
Dive Deeper – Best Conductors:
Silver, copper, and gold are the best conductors, but copper is used most often due to its lower cost and availability.
Insulators
Materials that do not allow electric current to flow are called insulators or poor conductors of electricity.
Examples: Plastic, rubber, glass, wood, cork, paper, wax, ceramics, and objects like plastic scales, erasers, and candles.
Use in Circuits: Insulators are used to cover wires, make plug tops, and switches to prevent electric shocks and ensure safety.
(a) Conduction tester (b) Using the conduction tester for testing a materialHere are the results of testing materials for electrical conductivity:
Importance of Both:
Conductors: Used in wires, switches, plugs, and sockets to carry current.
Insulators: Protect users from electric shocks by covering conductive parts.
Caution: The human body is a conductor, so electric current passing through it can cause severe injury or death. Never touch switches or plugs with wet hands, use devices in wet areas, or handle damaged electrical equipment.
Dive Deeper – Types of Electricity:
Direct Current (DC): Produced by batteries, used in small devices like torchlights and phones.
Alternating Current (AC): Supplied from power plants to wall sockets, used for larger appliances.
Points to Remember
An electric cell provides portable electrical energy and has a positive terminal (metal cap, +) and a negative terminal (metal disc, -).
A battery is formed by connecting two or more cells, with the positive terminal of one cell touching the negative terminal of the next, to supply more energy or last longer.
The term “battery” is sometimes used for a single cell, like in mobile phones.
An incandescent lamp has a filament that glows when heated by current, connected to two terminals (metal case and tip), and glows regardless of terminal connections.
A “fused” incandescent lamp doesn’t glow because its filament is broken, stopping current flow.
An LED has no filament, only glows when its positive terminal (longer wire) connects to the battery’s positive terminal and negative terminal (shorter wire) to the negative terminal, as current flows in one direction.
A switch completes (ON) or breaks (OFF) a circuit, controlling current flow, and can be placed anywhere in the circuit.
An electrical circuit is a closed path for current, flowing from the positive to the negative terminal of a cell, making devices like lamps glow.
Circuit diagrams use standard symbols (e.g., long/short lines for cell terminals, triangle for LED) set by organizations like IEC, ANSI, and IEEE, making circuits easy to understand globally.
Conductors (e.g., copper, silver) allow current to flow and are used for wires, while insulators (e.g., plastic, rubber) block current and are used for safety coverings.
Copper is widely used for wires due to its good conductivity, lower cost, and abundant supply compared to silver or gold.
The human body conducts electricity, so handling electrical devices unsafely (e.g., with wet hands) can cause injury or death.
Battery-powered devices use Direct Current (DC), while wall sockets supply Alternating Current (AC) for larger appliances.
Difficult Words and Their Meanings
Electricity: A form of energy that powers devices, like lights or fans, by flowing through wires or circuits.
Circuit: A complete path that allows electric current to flow from a power source (like a cell) through a device and back.
Electric Cell: A small, portable device that produces electrical energy, with positive and negative terminals, used in torchlights or remotes.
Battery: Two or more electric cells connected together to provide more energy or last longer.
Terminal: The ends of a cell or device (positive or negative) where current enters or leaves.
Incandescent Lamp: A light bulb with a filament that glows when heated by electric current, used in older torchlights.
Filament: A thin wire inside an incandescent lamp that glows to produce light when current passes through it.
LED (Light Emitting Diode): A modern lamp that glows when current flows in one direction, with no filament, used in many torchlights.
Switch: A device that controls current by opening (stopping) or closing (allowing) a circuit.
Conductor: A material, like metal, that allows electric current to flow easily, used for wires.
Insulator: A material, like plastic or rubber, that blocks electric current, used to cover wires for safety.
Circuit Diagram: A drawing using standard symbols to show how electrical components are connected in a circuit.
On 28 February, the school celebrated National Science Day with a science fair. At the entrance, Ashwin and Keerthi were given plain white sheets of paper. They were puzzled—why a blank sheet?
Soon, a volunteer sprayed a liquid on their sheets, and like magic, the words “Welcome to the Wonderful World of Science” appeared! This amazing trick made them very curious.
At the Colourful World of Substances stall, they saw many experiments where mixing things changed their colour. Excited to learn more, they decided to explore the science behind it.
Let’s join them on this fun journey!
Nature – Our Science Laboratory
We can determine if a substance is acidic, basic, or neutral by using special tools called indicators, which change color or smell when mixed with different substances.
1. Litmus as an Indicator
What is Litmus? Litmus is a natural material obtained from lichens, which are organisms formed by a fungus and an alga living together, often found on rocks and trees in rainy, clean areas.
Forms of Litmus: It is available as blue and red litmus paper strips, used to test substances.
How Litmus Works:
If a substance turns blue litmus paper red, it is acidic.
If a substance turns red litmus paper blue, it is basic.
If there’s no color change in either litmus paper, the substance is neutral.
Here are the test results showing the nature of samples using blue and red litmus papers.
Try yourself:
What does blue litmus paper turning red indicate?
A.The substance is acidic.
B.The substance is basic.
C.The substance is neutral.
D.The substance is colorful.
View Solution
Properties of Acids and Bases
Acids:
Taste sour, like lemon juice, tamarind, or vinegar.
Contain specific acids, e.g., citric acid in lemon, lactic acid in curd, tartaric acid in tamarind, acetic acid in vinegar.
Turn blue litmus red and red rose extract red.
Bases:
Feel slippery or soapy when rubbed, like baking soda solution or soap.
Often taste bitter, but not all bitter things are bases (e.g., bitter gourd is not basic).
Turn red litmus blue, red rose extract green, and turmeric paper red.
Neutral Substances:
Do not have a strong taste or slippery feel.
Do not change the color of indicators like litmus, red rose extract, or turmeric.
2. Red Rose as an Indicator
Preparation of Red Rose Extract Red rose extract is prepared by collecting fresh rose petals and washing them properly. The petals are then crushed and soaked in hot water. After some time, the mixture is filtered to get a red-colored liquid known as red rose extract.
How It Works:
In acidic substances, the extract changes to a shade of red.
In basic substances, it changes to a shade of green.The changes in colour of the red rose extract on adding lemon juice (A) and soap solution (B)
In neutral substances, the extract’s color remains unchanged.
Red rose extract is an acid-base indicator because it shows different colors for acidic and basic substances, similar to litmus.
Some examples:
Lemon juice (acidic) turns the extract red.
Soap solution (basic) turns it green.
Neutral substances like sugar solution don’t affect the color.
Try yourself:
What color does red rose extract turn in basic substances?
A.Red
B.Green
C.Yellow
D.Blue
View Solution
3. Turmeric as an Indicator
Preparation
Turmeric powder is mixed with water to make a paste, spread on filter paper, and dried to create yellow turmeric paper strips.
How It Works:
Basic substances turn turmeric paper red.
Acidic and neutral substances do not change the yellow color of turmeric paper.
A turmeric stain on a shirt changes color when soap (basic) is applied, showing its indicator property.
Turmeric paper can only identify basic substances, not distinguish between acidic and neutral ones.
Here are the test results showing the nature of samples using turmeric paper:
Fascinating Facts: Why is Turmeric Called the ‘Golden’ Spice?
Turmeric, also known as Haldi, is a bright yellow spice that belongs to the ginger family. Grown widely in India and other countries, it’s a common ingredient in everyday cooking. But turmeric is much more than just a flavouring agent!
In the Ayurvedic system of medicine, turmeric is believed to offer several health benefits. That’s why it plays a key role in many traditional home remedies. Its rich golden color and healing properties have earned it the name ‘Golden Spice’.
Researchers today are also studying turmeric for its potential benefits beyond taste and color—making this age-old spice even more special!
4. Olfactory Indicators
Olfactory indicators are special substances whose smell changes when they come in contact with acidic or basic substances.
For example, a cloth soaked in onion juice loses its smell when mixed with tamarind water (which is acidic) or baking soda solution (which is basic).
These indicators are useful because they help us identify whether a substance is acidic or basic just by observing changes in smell.
5. Other Natural Indicators
Substances like beetroot, purple cabbage, red hibiscus (gudhal), and Indian blackberry (jamun) can also act as acid-base indicators, changing colors in acidic or basic solutions.
Did You Know ?
Hydrangea plants in the Himalayas or North-eastern states produce blue flowers in acidic soil and pink or red flowers in basic soil, showing how soil nature affects plants.
Know a Scientist: Acharya Prafulla Chandra Ray
Acharya Prafulla Chandra Ray is known as the Father of Modern Indian Chemistry. He earned his chemistry doctorate in the UK and later returned to India. In 1901, he started India’s first pharmaceutical company.
He wrote about the history of Indian science to show the world the achievements of ancient Indian scientists. A true reformer, he also supported teaching in the mother tongue to make education easier and more meaningful.
Let’s Revise
Q: How does turmeric act as a natural indicator? View Answer
Q: What are olfactory indicators and how do they work? View Answer
What Happens When Acidic Substances Mix with Basic Substances?
1. Neutralization Process
When an acid (e.g., lemon juice) is mixed with a base (e.g., lime water) in the right amount, they react to form a solution that is neither acidic nor basic.
This reaction is called neutralization.
Neutralization shows how acids and bases balance each other, creating a neutral substance that doesn’t affect indicators.
This process is key to understanding how substances interact chemically.
Activity
Let’s see how acids and bases react using litmus solution:
First, add lemon juice (which is acidic) to a blue litmus solution. It turns red, showing the presence of an acid.
Now, slowly add lime water (a base) to the same solution. As more base is added, the red color starts changing back to blue, showing the solution is becoming neutral or basic.
This happens because the base cancels the effect of the acid, resulting in a neutral solution.
2. Products of Neutralization
The reaction produces salt, water, and releases heat.
Acid + Base → Salt + Water + Heat.
Neutralization in Daily Life
Situation 1: Ant Bite
When a red ant bites, it injects formic acid into the skin, causing redness and a stinging pain.
Applying moist baking soda, which is a base, neutralizes the formic acid, relieving the pain and reducing swelling.
Different regions may use other basic remedies, like lime water, for ant bites.
Situation 2: Soil Treatment
Farmers may notice poor plant growth if the soil becomes too acidic due to excessive use of chemical fertilizers.
Adding lime (a base, like calcium oxide) neutralizes the acidic soil, making it suitable for plant growth.
If the soil is too basic, organic matter like manure or composted leaves is added, which releases acids to neutralize the basic soil.
Neutral soil may still need nutrients if plants are unhealthy, showing that soil health involves more than just acidity or basicity.
Situation 3: Factory Waste
Acidic waste from factories can pollute lakes, harming fish and other aquatic life.
Before releasing waste into lakes, basic substances are added to neutralize the acidity, making the water safe for fish.
Q: What is a neutralization reaction and what are its products? View Answer
Q: How does baking soda help relieve pain from an ant bite? View Answer
Points to Remember
Substances are grouped into acidic, basic, or neutral based on how they interact with indicators.
Litmus, from lichens, comes as blue and red paper: acids turn blue litmus red, bases turn red litmus blue, and neutral substances cause no change.
Red rose extract, made from crushed petals, turns red in acidic solutions and green in basic solutions, acting as an acid-base indicator.
Turmeric paper, prepared from turmeric paste, turns red in basic solutions but remains yellow in acidic or neutral solutions, making it useful only for detecting bases.
Olfactory indicators, like onion-soaked cloth, change smell when mixed with acidic or basic substances, helping identify their nature.
Other natural indicators, such as beetroot, purple cabbage, red hibiscus, and Indian blackberry, also show color changes in acidic or basic solutions.
Acids taste sour (e.g., lemon juice contains citric acid, vinegar contains acetic acid) and turn blue litmus red.
Bases feel slippery, often taste bitter, and turn red litmus blue, red rose extract green, and turmeric paper red.
Neutral substances, like tap water or sugar solution, don’t change the color or smell of indicators.
Neutralization is the reaction between an acid and a base, forming salt, water, and releasing heat, resulting in a neutral solution.
Neutralization is used in daily life to:
Relieve ant bites by applying baking soda to neutralize formic acid.
Treat acidic soil with lime or basic soil with organic matter to help plants grow.
Neutralize acidic factory waste to protect aquatic life in lakes.
Hydrangea flowers change color based on soil: blue in acidic soil, pink or red in basic soil, showing nature’s use of acid-base properties.
Creative uses, like writing messages with basic solutions on turmeric paper, show how indicators can be applied in art or communication.
Difficult Words and Their Meanings
Acidic: A substance that tastes sour, turns blue litmus paper red, and red rose extract red, like lemon juice or vinegar.
Basic: A substance that feels slippery, turns red litmus paper blue, red rose extract green, and turmeric paper red, like soap or baking soda.
Neutral: A substance that doesn’t affect indicators’ color or smell and is neither acidic nor basic, like sugar solution or tap water.
Indicator: A tool that changes color or smell to show if a substance is acidic, basic, or neutral, such as litmus, red rose extract, or turmeric.
Litmus: A natural substance from lichens, used as blue or red paper strips to test if a substance is acidic (turns blue litmus red) or basic (turns red litmus blue).
Lichens: Organisms made of a fungus and an alga living together, found on rocks and trees, used to make litmus.
Neutralization: A chemical reaction where an acid and base mix to form salt, water, and heat, creating a neutral solution.
Olfactory: Related to the sense of smell, used for indicators like onion that change odor in acidic or basic substances.
Extract: A liquid obtained by crushing and soaking a substance (like red rose petals) in water and filtering it, used as an indicator.
Formic Acid: An acidic substance injected by ants during a bite, causing pain and redness, which can be neutralized by a base like baking soda.
Science is everywhere—from tiny cells inside a leaf to the movement of the sun and stars. You might test materials at home or learn how water flows underground. Each chapter will bring new adventures that challenge your thinking, expand your knowledge, and help you become a little explorer making your own discoveries.
Before starting, take a moment to notice something special—the page numbers follow the playful flight of a butterfly and a soaring paper plane! Just like a butterfly flutters freely and a paper plane flies high, learning takes flight when guided by curiosity.
Did you know that simple things like paper planes inspired real scientific studies of flight? From early inventors watching bird wings to modern engineers designing aircraft, flying began with simple observations and experiments.
Let’s explore! As you turn each page, let your imagination soar—explore new ideas, discover wonders, and reach for the skies!
Science is an Adventure
Exploring the World: Science helps us understand both small and big things. For example, we can study tiny cells inside a leaf or the way the sun and stars move in the sky.
Asking Questions: Science starts with curiosity. When you wonder “why” or “how” something happens, you’re thinking like a scientist.
Doing Experiments: Experiments let you see how things work. For example, testing materials at home can teach you about their properties.
Learning Takes Flight: The textbook compares learning to a butterfly fluttering or a paper plane flying. Just like a paper plane inspired scientists to study flight (like how bird wings led to airplanes), your curiosity can lead to new discoveries.
Imagination is Key: As you read this book, let your imagination soar. Each page is a chance to explore new ideas and find wonders in the world.
Exploring Science Beyond Facts
To answer these, it’s important to step beyond the classroom and experience the world through activities and experiments. These hands-on experiences help build a deeper understanding of our environment and our place on Earth.
Science is not just about discovery; it’s also about responsibility. As young explorers, you will see how human activities affect the natural world and our society. You will learn how science can help solve environmental challenges and contribute to a sustainable future.
Try yourself:
What does science help us understand?
A.Only scientific theories
B.Only small things
C.Only big things
D.Small and big things
View SolutionScience as a Way of Thinking
Science is a process—a way of thinking that encourages curiosity, asks questions, and stays open to the unknown. Exploration is not just about discovering new facts or learning about nature.
In Grade 7, the focus will be on asking deeper questions like:
How do things work?
Why do events happen the way they do?
What can we learn from patterns in nature?
Step Outside the Book
To understand science, you need to explore the world. Doing experiments and observing nature helps you learn better than just reading.
A Never-Ending Journey
Science is always growing. Every discovery leads to new questions, making it an ongoing adventure.
Responsibility to Nature
Science shows how human actions affect the environment. For example, pollution can harm nature, but science can help us find ways to protect the planet and make it more sustainable (better for the future).
Connecting with Society
What we do affects the world, and science helps us understand our role in keeping nature and society balanced.
Let’s Revise : Why is it important to “step outside the book” when learning science? View Answer
Exploring Substances
We often interact with various materials in our daily lives—fruits, clothes, spices, utensils—without stopping to think why they behave the way they do. Science encourages us to observe, question, and understand these common occurrences by studying the properties of materials. Let’s look at a couple of everyday examples and the science behind them:
Why are some fruits sour?
What happens when we wash a haldi (turmeric) stain from our uniform?
These simple questions are not just curiosities—they are gateways into deeper scientific ideas. By investigating the familiar, we begin to understand important concepts in chemistry and develop a scientific way of thinking.
Exploring Properties of Materials
After studying basic properties of everyday materials, the book moves on to experiments with electric batteries, wires, and lamps.
Objective: To discover what kinds of materials allow current to pass and make a lamp glow.
This exploration helps us to:
Classify materials based on their properties (like conductivity).
Enter the study of metals and non-metals.
We also observe that devices like torch batteries eventually stop working.
This leads to the study of changes in materials.
What are Changes?: The world is always changing. Some changes we can see, like ice melting, and some we can’t, like water moving underground.
These changes differ in nature:
Some are physical (like melting ice)
Some are chemical (like ripening of fruits or battery discharge)
Some happen quickly (melting), others are slow (weathering of rocks)
Some changes are reversible (e.g., melting ice).
Others are irreversible (e.g., cooking food or a used battery
Let’s Revise
Q: What is the importance of studying the conductivity of materials? View Answer
Q: Why are some changes in materials considered irreversible? View Answer
Understanding Changes and the Role of Heat
1. We observe many changes around us in daily life, such as:
Ice melting into water
Fruits ripening
Rocks breaking into pebbles
2. Role of Heat in Causing Changes
Heat often causes or speeds up changes in materials.
Examples:
Ice cube melting on a warm day
Massive glaciers slowly melting over years
These examples show how heat affects the state and structure of materials.
3. Introduction to Heat Transfer
To understand how these changes occur, we explore the concept of heat transfer:
Heat always flows from a hotter object to a colder one
This flow of heat leads to changes in temperature and state of substances
4. Heat and the Water Cycle
The Water Cycle is a perfect example of how heat drives natural processes:
Evaporation: Heat from the Sun causes water in oceans, lakes, and rivers to evaporate
Condensation: Water vapor cools in the atmosphere and forms clouds
Precipitation: Water falls as rain
Infiltration: Rainwater seeps into the ground and continues the cycle
Let’s Revise : How does heat play a role in the water cycle? View Answer
Changes in Living Things
Not all changes are in materials—our bodies also undergo changes, especially during middle-school years (puberty).
1. Life Processes: Staying Alive
Living things carry out certain vital activities called life processes, which help them survive. In animals (including humans)
Eating (nutrition)
Breathing (respiration)
Blood circulation
Growth and reproduction
2. Plants Also Undergo Life Processes
Life processes are not limited to animals:
Plants also need food to grow (they make their own through photosynthesis).
They undergo respiration, though differently from animals.
They also grow, reproduce, and respond to changes in their surroundings.
4. The Bigger Picture
Over millions of years, life on Earth has evolved into complex, interdependent systems.
These systems are balanced, ensuring survival for a wide variety of organisms.
Understanding life processes helps us: 1. Know how our bodies work 2. Stay healthy and aware of changes 3. Appreciate how all living things—plants, animals, and humans—are connected through nature’s systems
Let’s Revise: What are life processes, and why are they important for living things? View Answer
Measuring Time
What is Time?
Time helps us organize our day, like knowing when to go to school or sleep.
How We Measure Time:
Today, we use clocks and watches to tell time.
Long ago, people used the sun’s shadows to measure time.
For example, they looked at how shadows moved as the sun changed position in the sky.
Time in Daily Life
Time affects when we wake up, eat, or sleep, and it’s connected to nature, like day and night.
Why It Matters?
Understanding time helps us plan our lives and learn how nature works, like how day and night happen.
Light and Shadows
Importance of Light:
Light helps us see the world around us.
We use light to do things like read at night or play with shadows (like making shadow puppets).
Shadows in Nature:
Shadows happen when something blocks light.
The Earth and Moon can cast shadows, causing eclipses (when the sun or moon is blocked).
Examples:
Long ago, people used shadows to tell time by watching how they moved.
Today, we use light in many ways, like in bulbs or lasers.
Try yourself:
What does science encourage us to do with common occurrences?
A.Avoid and dismiss
B.Observe and question
C.Ignore and forget
D.Accept and memorize
View Solution
Earth’s Movements
How the Earth Moves?
Rotation: The Earth spins on its axis (an imaginary line through its center) once every 24 hours, causing day and night.
Revolution: The Earth moves around the Sun once every year, causing seasons.
Moon’s Movement
The Moon goes around the Earth, which affects things like tides and how we see the Moon’s phases.
Effects on Life:
Day and night happen because the Earth rotates, giving us time to work and rest.
Seasons (like summer or winter) happen because of the Earth’s revolution around the Sun.
Eclipses happen when the Earth or Moon blocks sunlight, creating shadows in space.
Let’s Revise: How do Earth’s movements affect our daily life? View Answer
Points to Remember
Science is an adventure where you ask questions, do experiments, and explore the world.
Science is a process of thinking, observing patterns, and being curious about how things work.
All sciences (physics, chemistry, biology, earth sciences) are connected, and ideas from one help us understand others.
Materials have different properties, like conducting electricity, and we group them as metals or non-metals.
Changes can be reversible (like ice melting) or irreversible (like a fruit ripening).
Heat causes or speeds up changes, like melting ice or evaporating water in the water cycle.
Living things (humans, animals, plants) go through changes and need processes like eating and breathing to survive.
Time is measured with clocks today, but long ago, people used shadows from the sun.
Light helps us see and understand the universe, and shadows cause eclipses.
The Earth’s rotation causes day and night, its revolution causes seasons, and the Moon’s movement affects tides and eclipses.
Difficult Words and Their Meanings
Curiosity: Wanting to know or learn more about something, like wondering why the sky is blue.
Properties: The qualities of something, like whether it’s hard, soft, or conducts electricity.
Classifying: Sorting things into groups based on what they’re like, such as grouping materials as metals or non-metals.
Reversible: A change that can go back to its original form, like water freezing into ice and melting back into water.
Irreversible: A change that cannot go back, like a fruit becoming ripe.
Evaporates: When a liquid, like water, turns into a gas and rises into the air, like water disappearing from a puddle.
Nutrients: Things in food that help living things grow and stay healthy, like vitamins or proteins.
Eclipses: When one object in space blocks light from another, like the Moon blocking the Sun during a solar eclipse.
Rotation: When something spins around a center point, like the Earth spinning to cause day and night.
Revolution: When something moves in a circle around another object, like the Earth moving around the Sun.
Sustainable: Doing things in a way that keeps the environment healthy for the future, like using less plastic.
Lime Water Turns Milky
Candle (a) burning (b) covered with a glass tumbler
Paper catching fire
View Answer
Metals such as copper, aluminium, and iron have a characteristic shiny appearance known as metallic lustre. This means they reflect light well, giving them a bright, polished look.
In contrast, non-metals like coal, sulfur, and wood do not have this shiny appearance. Instead, they look dull or matte because they do not reflect light like metals.
Electrical Fittings:
Jewellery and Ornaments:
Musical Instruments:
The ringing sound of ghungroos (the small bells worn by dancers) is because of the sonorous nature of metals.
The school bell produces its loud ringing sound due to the sonority of the metal it is made from.
People can also use the difference in sound between hitting wood or metal to help navigate or identify objects, like using a stick to find their way by the sound it makes when it hits different materials.
On the other hand, materials like sulfur, coal, wood, stone, rubber, and nylon do not allow electricity to flow freely and are called poor conductors of electricity or insulators.
Metal Characteristics
Battery made up of (a) two cells (b) four cells 
An LED lamp for torch
LEDs of different colours
Electric Cell: This provides the electrical energy required to make the lamp glow. The energy comes from the chemical reaction inside the cell.
Incandescent Lamp: The lamp is a device that uses electricity to produce light. It has a filament that glows when electricity passes through it.
Cell Holder: A holder is used to securely place the electric cell in the circuit. It ensures the proper connection of the battery’s terminals to the wires.
Prediction of Lamp Glow: Depending on how the circuit is set up, the lamp may or may not glow. If the circuit is properly connected (with all components in place), the lamp will glow. Otherwise, it will not light up.
The changes in colour of the red rose extract on adding lemon juice (A) and soap solution (B)
Objective: To discover what kinds of materials allow current to pass and make a lamp glow.
Enter the study of metals and non-metals.
Rocks breaking into pebbles
Infiltration: Rainwater seeps into the ground and continues the cycle
Growth and reproduction
They also grow, reproduce, and respond to changes in their surroundings.
