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.
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).
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
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.
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.
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.
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).
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.
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
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.
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.
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.
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
Thunder: Lightning heats the air rapidly, causing it to expand and produce a loud sound called thunder.
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?
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.
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.
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.
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.
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
Pressure: Pressure is defined as the force exerted per unit area.
SI Unit of Pressure: The SI unit of pressure is the newton per square metre (N/m²), also called the pascal (Pa).
Pressure by Liquids and Gases: Liquids and gases exert pressure on the walls of their container.
Atmospheric Pressure: The pressure exerted by the air around us is known as atmospheric pressure.
Cause of Winds: Differences in air pressure cause winds to blow.
Low and High Pressure: Warm air rises to create a low-pressure area; cooler air from high-pressure areas moves in to replace it.
Formation of Thunderstorms: Moisture and strong winds are important for the formation of thunderstorms.
Development of Electric Charges: Upward and downward strong winds rub ice particles with water droplets in clouds, creating electric charges.
Lightning Formation: Lightning is caused by the collision of electric charges within a cloud, between different clouds, or between a cloud and the ground.
Effects of Lightning: Lightning strikes can cause destruction to life and property.
Lightning Conductors: Lightning conductors are devices that protect buildings from the damaging effects of lightning.
IMD (India Meteorological Department): The IMD constantly monitors cyclones and thunderstorms in India.
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.
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 ofelectric 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):
Wrap insulated wire around an iron nail to form a coil.
Connect both ends of the wire to a cell.
When current flows, the iron nail can attract iron objects.
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.
Have you ever wondered what it really means to be healthy—is it just not being sick, or is there more to it? This chapter will help you discover how good health includes not only your body, but also your mind, your habits, and your relationships with others. Let’s explore how to stay healthy and keep diseases away in simple, practical ways!
Health: Is It More Than Not Falling Sick?
Health is not just about not being sick. It means feeling good in your body, mind, and social life. According to the World Health Organization (WHO), health is a state of complete physical, mental, and social well-being, not just the absence of disease.
A healthy person can:
Do tasks efficiently.
Handle tough situations well.
Get along with friends and others in society.
Key Aspects of Health
Physical Health: Taking care of your body through proper food, exercise, and sleep.
Mental Health: Staying positive and managing stress.
Social Health: Having good relationships with friends and others.
A Student’s Story
A Class 8 boy moved to a new city and school. He felt lonely with no friends and busy parents. He spent more time on his phone and social media, which made him feel worse and caused:
Headaches.
Weight loss.
Trouble sleeping.
Cause and Solution A doctor suggested less screen time and seeing a counsellor. The counsellor helped him make friends, and his health improved.
Loneliness and too much screen time caused both physical and mental health problems.
Making new friends and support from adults helped him feel better.
Ayurveda and Health
Ayurveda teaches that health is a balance of body, mind, and surroundings.
Follow a daily routine (dinacharya) and seasonal routine (ritucharya).
Eat fresh, wholesome food suited to your prakriti (body type).
Practice regular exercise, cleanliness, restful sleep, and a calm mind through yoga, meditation, and mindfulness.
How Can We Stay Healthy?
Staying healthy involves:
Eating nutritious food.
Keeping clean (personal hygiene and surroundings).
Exercising regularly.
Getting enough sleep.
Spending time with family and friends.
Having a positive attitude.
Good and Bad Habits
Good Habits:
Keep yourself clean and maintain personal hygiene.
Eat a healthy and balanced diet.
Exercise regularly.
Make time to relax or meditate every day.
Bad Habits (harmful to health):
Spending too much time on mobile phones or screens.
Eating fast food or junk food every day.
Sleeping very late or not getting enough sleep.
Skipping meals, especially breakfast.
Maintain a Healthy Lifestyle
Our health depends on many factors. These factors include our lifestyle (how we live) and our environment (our surroundings).
We should Eat a balanced diet with: Plenty of fruits, vegetables, and whole grains. Balanced Diet
Avoid processed, fatty, or sugary foods and drinks.
Stay physically active andPlay outdoors, walk, run, cycle, or exercise.
Limit screen time and spend more time in nature.
Get enough sleep to help our body and mind rest.
Practice yoga or pranayama (simple breathing exercises) regularly.
Say ‘NO’ to harmful substances like tobacco, alcohol, and addictive drugs.
Keep the Environment Clean
A clean, well-maintained playground is better for playing because it’s safe and healthy.
A dirty, polluted playground has flies, mosquitoes, and can make you sick.
Why Clean Surroundings Matter:
Clean air and water are important for health.
Pollution from vehicles or factories can cause coughing or asthma.
The Air Quality Index (AQI) shows how clean or polluted the air is.
A clean environment helps you stay healthy and feel good.
Keeping Surroundings Clean:
Avoid littering and keep your area tidy.
A clean environment reduces the risk of diseases.
Importance of Feelings and Relationships
Health is not just about the body; feelings and relationships matter too.
Even with good food and a clean place, you may feel bad if you’re lonely or upset.
Spend time with friends and family, talk, laugh, and have fun to keep your mind healthy.
Try yourself:
What is one way to maintain a healthy lifestyle?
A.Skip breakfast often
B.Stay indoors all day
C.Exercise regularly
D.Eat fast food every day
View SolutionHow Do We Know That We Are Unwell?
Our body usually works in a specific way to keep us healthy.
When we feel unwell, something inside our body may not be working properly.
We experience symptoms and signs that show we are not healthy.
Symptoms of Dehydration
Symptoms and Signs
They act as clues to show that our body is not working as it should.
Doctors use these clues to understand the cause of our illness and suggest treatment.
Symptoms:
What we feel when we are unwell.
Examples: Pain, tiredness, dizziness.
Signs:
Things that can be seen or measured by others, like a doctor.
Examples: Fever (high body temperature), rash, high blood pressure, swelling.
Diseases – What Are the Causes and Types?
A disease is a condition that stops the body or mind from working normally.
It happens when one or more organs or organ systems do not function properly.
Causes of Diseases
Pathogens: Germs like bacteria, viruses, fungi, worms, or protozoa (single-celled organisms) that cause diseases.
Poor Nutrition: Not eating enough healthy food can lead to diseases.
Unhealthy Lifestyle: Bad habits like lack of exercise or poor diet can cause diseases.
Types of Diseases
Diseases can be grouped into two major types based on their causes and how they spread:
1. Communicable Diseases: Can spread from one person to another. Caused by pathogens. Examples: Typhoid, dengue, flu, chickenpox, COVID-19. Spread through: Communicable Diseases
Air: Coughing or sneezing (e.g., flu, tuberculosis).
Direct contact: Touching an infected person (e.g., shaking hands).
Indirect contact: Sharing items like towels or handkerchiefs.
Contaminated food or water: Eating or drinking unsafe food/water (e.g., cholera, typhoid).
Vectors: Insects like mosquitoes or houseflies (e.g., malaria, dengue).
2. Non-Communicable Diseases (NCDs): Not caused by pathogens and do not spread from person to person.Linked to lifestyle, diet, or environment.Examples: Cancer, diabetes, asthma, heart disease. Non- Communicable Diseases
Deficiency diseases: Caused by lack of nutrients (e.g., scurvy, anaemia, goitre).
Often chronic (last more than 3 months).
NCDs are Common in India due to: Eating more processed food. Less physical activity. Longer lifespans.
How Are Communicable Diseases Caused and Spread?
All communicable diseases are caused by pathogens. These pathogens can enter our body through the air we breathe or by consuming contaminated food or water and more.
Diseases Spread Through Air: Diseases that are transmitted when pathogens in tiny droplets are released into the air by coughing, sneezing, or talking, and then breathed in by others.
Diseases Spread Through Contaminated Water/Food: Diseases that occur when people consume water or food contaminated with pathogens like bacteria, viruses, or parasites.
Diseases Spread by Insects (Vectors): Diseases that are passed on when insects such as mosquitoes or flies carry pathogens from one host to another, causing infection.
Prevention of Communicable Diseases
Keep yourself and surroundings clean.
Practice good hygiene (e.g., wash hands with soap).
Cover mouth and nose when coughing or sneezing.
Wear a mask in crowded places.
Avoid sharing personal items (e.g., towels, handkerchiefs).
Keep food and water clean.
Stay home and rest when unwell to recover and avoid spreading disease.
Use mosquito nets/repellents and control insect breeding.
How Are Non-Communicable Diseases Caused?
Non-communicable diseases (NCDs) like cancer, diabetes, and asthma are not caused by infections. Instead, they are linked to factors such as lifestyle, diet, environment, and body functions:
Lifestyle Factors: Unhealthy habits like eating too much junk food, not exercising, smoking, or excessive stress can lead to diseases such as diabetes, heart disease, or some cancers.
Diet: Not getting enough essential nutrients can cause deficiency diseases (e.g., scurvy from lack of vitamin C, anaemia from lack of iron, goitre from lack of iodine). These are also non-communicable.
Environmental Factors:Pollution, exposure to harmful chemicals, or living in unhealthy surroundings can contribute to diseases like asthma or cancer.
Hormonal Imbalances: Sometimes, diseases like diabetes happen due to imbalance of hormones in the body.
Chronic Nature: NCDs usually last a long time (more than 3 months) and need ongoing care.
How to Prevent and Control Diseases?
You may have heard the saying: “Prevention is better than cure.” This means it’s easier and wiser to stop a disease from happening than to try to cure it after you get sick.
Good sanitation and cleanliness are some of the simplest and most effective ways to reduce the spread of diseases, especially those that spread from person to person or through contaminated water and food. Example:
Odisha — community-led sanitation campaign
In Bhadrak district, Odisha, a community campaign encouraged people to build and use toilets.
As more families stopped open defecation, child health improved, and diseases like diarrhoea and infections greatly reduced.
Simple steps like building and using toilets, keeping surroundings clean, and good waste disposalprevent germs from spreading.
Community action can have a big impact on public health—when people work together, the whole area becomes healthier.
Ability of the body to fight diseases
Why do some people get sick more often than others, even in the same environment?
The ability of the body to fight diseases is called immunity.
Our immune system is a group of special cells, tissues, and organs that recognise and fight off harmful germs (pathogens) that can make us sick.
Role of Vaccines
Vaccines are special injections or drops that help your immune system become stronger against specific diseases.
They do this by “teaching” your body how to recognise and destroy certain germs before they can make you sick. This is called acquired immunity.
Some vaccines use tiny, harmless parts of the germ (or a dead/weakened germ) to “train” your immune system.
If you’re exposed to the real germ in future, your body is ready and can fight off the disease quickly.
Tetanus shot: Contains a safe (inactive) form of a bacterial toxin, which helps your immune system learn how to fight tetanus without making you actually sick.
Other common vaccines: Polio, measles, hepatitis, and more—many are given in childhood.
Edward Jenner and the smallpox vaccine
Edward Jenner and smallpox vaccine: In the late 1700s, Jenner realized that people who got cowpox didn’t get smallpox. He created the first vaccine by using material from cowpox sores. This discovery eventually helped eradicate smallpox worldwide.
In olden days, India also had a method called variolation, where a bit of material from a mild smallpox infection was used to protect healthy people.
Why are vaccines important?
Vaccines prevent diseases: They are used before you get sick, to stop serious illnesses from starting at all.
Community protection: When more people get vaccinated, diseases have less chance to spread—so even those who can’t get vaccinated are protected.
Vaccines are safe and tested: Scientists and doctors check them carefully before use.
India’s Role in Vaccines
India is a major global vaccine producer, supplying vaccines to many countries.
Indian vaccine companies played a key role during the COVID-19 pandemic and continue to support global health efforts
Try yourself:
What is the main focus of the text?
A.Improving technology
B.Studying animal behavior
C.Developing new medicines
D.Preventing and controlling diseases
View SolutionTreatment of Diseases
If our immune system cannot fight off infection, we fall ill and need help from a doctor. The doctor gives us medicines, and sometimes these are called antibiotics.
What are antibiotics?
Antibiotics are special medicines that kill bacteria in our body.
They help cure diseases caused by bacteria, such as tuberculosis and typhoid.
Important: Antibiotics do NOT work against diseases caused by viruses (like colds, flu, or COVID-19) or protozoa.
Discovery of Antibiotics
In 1928, Alexander Fleming, a scientist in London, discovered the first antibiotic called penicillin.
He found that a mould on a petri dish killed harmful bacteria growing there. This mould made a substance (penicillin) that stopped bacteria from growing.
Since then, antibiotics have saved millions of lives.
What is Antibiotic Resistance?
Antibiotic resistance happens when bacteria change and stop being killed by antibiotics that used to work.
This makes infections much harder to treat and can cause longer illnesses and more complications.
How does antibiotic resistance develop and spread?
Taking antibiotics when we don’t need them (like for viral infections).
Not finishing all the doses as prescribed by the doctor.
Overuse of antibiotics in animals and plants.
Spread can happen through people, animals, food, water, and soil.
If you take antibiotics for a sore throat caused by a virus, bacteria in your body may become resistant to antibiotics.
When farmers give antibiotics unnecessarily to animals (like cows), resistant bacteria can spread through meat, milk, or even through the soil.
How to Prevent Antibiotic Resistance?
Take antibiotics only when prescribed by a doctor.
Always finish the full course of medicine, even if you feel better.
Never use leftover antibiotics or someone else’s prescription.
Avoid buying antibiotics without a doctor’s prescription.
Farmers should avoid giving antibiotics unnecessarily to animals.
Remember: Using antibiotics wisely keeps them working for everyone!
Traditional Medicine Systems
Systems like Ayurveda, Siddha, and Unani use natural remedies (herbs, oils, minerals) and focus on good food, exercise, and overall healthy living.
They can be helpful in treating simple health problems and for everyday wellness.
However, for some serious or advanced diseases, modern medicine may work better.
Managing Non-Communicable Diseases Diseases like diabetes, cancer, and heart disease are managed by:
Taking medicines (as advised by doctors)
Making healthy lifestyle changes (diet, exercise)
Rehabilitation (special help to get better)
Early diagnosis and regular check-ups are very important.
Key Points to Remember
Health means complete physical, mental, and social well-being—not just being free from diseases.
Happiness and health go together: Being happy can help you stay active and healthy, and good health can improve your mood.
A disease is any condition that stops the body or mind from working normally.
Symptoms are feelings we notice ourselves (like pain or tiredness); signs are visible or measurable changes (like fever, rash, or swelling).
Non-communicable diseases (like diabetes, cancer, and heart disease) are not caused by germs, but by lifestyle factors and environment. They can be prevented by eating healthy, staying active, and avoiding harmful habits.
Communicable (Infectious) diseases are caused by pathogens—germs such as bacteria, viruses, fungi, or worms—and can spread from person to person.
Good hygiene and clean surroundings are essential to prevent many diseases.
Our immune system is the body’s natural defense that fights harmful germs and keeps us safe.
Vaccines help our immune system “learn” how to fight certain diseases and provide protection before we actually get sick.
Early diagnosis and proper treatment help manage, control, or cure many diseases.
Healthy habits—like regular exercise, balanced diet, enough sleep, and positive relationships—keep both our body and mind strong.
Antibiotics treat bacterial infections only, not viral ones. Overuse can cause antibiotic resistance—so take them only as prescribed.
Prevention is better than cure: Practising good habits, getting vaccinated, and staying informed are the best ways to maintain health.
Have you ever wondered about the incredibly tiny living things that fill our world, but remain hidden from our sight?
This chapter takes you on a journey into the “invisible living world”—the microscopic universe full of life, and reveals how scientific tools such as lenses and microscopes have helped us discover, understand, and utilize these unseen organisms.
You’ll also learn about the basic unit of life—the cell, and how microorganisms are deeply interconnected with our lives.
The Limits of Human Vision and Discovery of Lenses
Our eyes can only see objects above a certain size. Many tiny things around us are invisible to the naked eye.
Long ago, people did not know about the many small organisms living around (and even on) us!
People learned that a curved piece of glass can make small things look bigger.
This piece of glass is called a lens (because it’s thick in the middle like a lentil seed).
Over time, better lenses led to the creation of magnifying glasses and, later, microscopes.
Magnifying glasses and microscopes allow us to observe things that are too small for the naked eye.
How a Magnifying Glass Works
Round Bottom Flask
If you fill a round-bottom flask with water and seal it, you can use it as a simple magnifying glass.
When you look through this flask at a book, the letters appear bigger.
This is because the flask acts like a lens, bending light and making objects look larger.
Using a real magnifying glass, you can see the details of tiny organisms (like ants) more clearly.
Such tools help us explore the tiny living world that is otherwise hidden.
The Invention of the Microscope People were always curious about small things but couldn’t see them until the invention of the microscope. A microscope makes tiny things appear much bigger and reveals details not seen by the naked eye.
Robert Hooke (1665): Rober Hooke and his Microscope
Published the book Micrographia, with detailed drawings of tiny living things as seen under a microscope.
Used a microscope that made objects appear 200–300 times bigger.
Observed thin slices of cork and saw small, empty compartments.
Called these compartments cells (the first use of the word in science).
Realized that all living things are made up of such basic units.
Antonie van Leeuwenhoek (1660s): Antonie van Leeuwenhoek
Improved the lenses and built better microscopes.
First to clearly observe and describe living cells like bacteria and blood cells.
Known as the Father of Microbiology for his discoveries of the invisible living world.
What Is a Cell?
Cells are the basic building blocks of all living beings—plants, animals, and even humans. Cells cannot always be seen with the naked eye. We observe them using a microscope.
Basic Building Blocks: Cells
Observing Plant and Animal Cells
1. Studying Onion Peel Cells (Plant Cell)
An onion bulb has a thin, transparent layer called the onion peel. Staining (with red safranin) helps make cells visible. Under the microscope, onion peel cells appear rectangular and are packed closely together.
These cells have:
A cell wall (extra outer layer). Cell wall gives strength and rigidity to plant cells.
Cell membrane (inside the cell wall, outer lining of cell)
Cytoplasm (jelly-like substance filling the cell)
Nucleus (round structure in the center)
Onion Peel Cell
2. Studying Cheek Cells (Animal Cell)
Scraping the inside of your cheek gives cheek cells. These are stained with methylene blue to make them easier to see. Cheek cells are polygon-shaped (irregular), flat, and have:
Cheek Cell
Cell membrane (outer boundary)
Cytoplasm
Nucleus
Animal cells do NOT have a cell wall.
Main Parts and Structure of a Cell
All cells have three basic parts:
1. Cell Membrane:
Thin, flexible layer forming the boundary of the cell.
Separates one cell from another.
Porous, allowing certain materials to enter or exit; keeps harmful substances out and retains useful molecules.
2. Cytoplasm:
Jelly-like substance filling most of the cell inside the membrane.
Contains dissolved nutrients like carbohydrates, proteins, fats, and mineral salts.
Most cellular life processes occur here.
3. Nucleus:
Spherical or oval structure, usually located centrally.
Controls all cell activities, including growth and division.
Surrounded by its own thin membrane.
Plant cells have extra parts:
1. Cell Wall (in plants only):
A tough, outer layer surrounding the cell membrane.
Provides rigidity, strength, and protection to plant cells.
Makes cells appear firm and compactly arranged.
2. Plastids (including chloroplasts, in plants only):
Tiny rod-shaped structures within plant cells.
Chloroplasts contain chlorophyll and enable photosynthesis.
Other plastids help store food and substances.
3. Vacuole:
Large, clear cavity in plant cells; smaller or absent in animal cells.
Stores nutrients, waste, and helps maintain cell shape and turgidity.
Differences Between Plant and Animal Cells
(a) An animal cell and (b) A plant cellFeaturePlant CellAnimal CellCell wallPresentAbsentNucleusPresent, usually at one sidePresent, often centralVacuoleLarge, centralSmall or absentPlastidsPresent (e.g., chloroplasts)AbsentShapeRectangular, rigidIrregular, flexibleVariation in Shape and Structure of Cells
Cells within living organisms can have different shapes, sizes, and structures, depending on their role and location.
Muscle cells: Spindle-shaped (tapered ends), designed for contracting and relaxing to enable movement.
Nerve cells (neurons): Long and branched, suitable for carrying messages quickly across the body.
Cheek cells: Thin and flat, forming a protective lining.
Plant cells: May be rectangular, elongated, oval, or tube-shaped; some form long tubes for transporting water.
Why Do Cells Differ in Shape and Structure?
The unique shape and size of each cell type help it perform specific functions for the organism.
Nerve cells must reach distant parts, so they are long and extended.
Muscle cells contract and relax, so their spindle shape helps this motion.
Plant tube cells transport water, so they are elongated and tube-like.
Role of Different Cells in Body Functions
In humans, muscle cells in the digestive tract move food by contracting in waves.
The stomach contains muscle cells for churning food and specialized cells for producing digestive juices and acids.
In plants, tube-like cells help move water and nutrients up the stem and into leaves.
What Are the Levels of Organisation in the Body of a Living Organism?
The structure of every living organism—from tiny plants to complex animals—is organized in a systematic and hierarchical way. This organisation allows the body to function efficiently, as each level is built from the previous, simpler level.
1. Cell – The Basic Unit of Life
The cell is the smallest, most fundamental unit of all living beings.
Just as a brick is the basic building block of a wall, a cell is the building block of life.
Each cell performs all the basic processes necessary for life, such as taking in nutrients, producing energy, and reproducing.
2. Tissue – Group of Similar Cells
A tissue is a group of similar cells that work together to perform a specific function.
Muscle tissue (made of muscle cells for movement), nerve tissue (made of nerve cells for message transmission), etc.
Tissues provide structure and support to organs and help in carrying out specialized tasks.
3. Organ – Structure Formed by Different Tissues
An organ is formed when different types of tissues combine and work together to perform a particular function.
Heart (pumps blood), stomach (helps in digestion), leaf (photosynthesis in plants).
Each organ has a specific structure and role in the body.
4. Organ System – Group of Organs for Major Functions
An organ system is a group of organs that work together to perform a major life function.
Digestive system (mouth, stomach, intestines, etc.), respiratory system (lungs, windpipe), circulatory system (heart, blood vessels).
Each organ system takes care of major tasks, ensuring the survival and wellbeing of the organism.
5. Organism – Complete Living Being
All the organ systems together make up a complete, multicellular organism—like a plant, animal, or human.
The organism is able to perform all the functions of life: growth, development, response to the environment, and reproduction.
Summary Table: Levels of OrganisationLevelDescription / DefinitionExampleCellBasic unit of lifeMuscle cell, nerve cellTissueGroup of similar cellsMuscle tissue, nervous tissueOrganStructure formed by different tissuesHeart, stomach, leafOrgan systemGroup of organs working for a major functionDigestive system, circulatory systemOrganismAll organ systems combined as a living beingPlant, human, animal (dog, bird, etc.)What Are Microorganisms?
Some living organisms are so incredibly small that they are invisible to the naked eye. Unlike the plants, animals, and cells we’ve seen before, these tiny forms of life require special tools just to be observed.
Microorganisms (or microbes):
Living beings made up of just one cell (unicellular) or only a few cells.
They are so tiny that we cannot see them without the aid of a microscope.
The word “microorganism” comes from “micro” (very small) and “organism” (living being).
Key Points About Microorganisms
Size: Too small to see without a microscope; invisible to unaided eyes.
Cellularity: Some (like bacteria and Amoeba) are unicellular (single-celled). Others (like some fungi and algae) can be multicellular (made of many cells) but still remain extremely small.
Everywhere: in water, soil, air, on and inside our bodies, and in extreme places too.
How Do We See Microorganisms?
Microscopes are needed to observe the cells of microorganisms. Microscopes enlarge (magnify) the image of microbes so they become visible.
Types of Microscopes:
Laboratory Microscopes: High-powered, show clear details but are expensive.
Foldscope: A low-cost, foldable paper microscope that allows more people to study microbes, though it may not show as much detail as advanced microscopes.
Are Microorganisms the Same as Plant or Animal Cells?
Like plant and animal cells, microbial cells may have a cell membrane, cytoplasm, and sometimes a nucleus.
However, there are many differences in size, structure, and the way they perform life processes.
Not all microbes have the same features as plant or animal cells; some can be quite unique.
Where Do We Find Microorganisms?
In Water: Lakes, rivers, oceans, ponds, even in a drop of water!
In Soil: The earth is full of bacteria, fungi, and protozoa.
In Air: Some float in the atmosphere.
Inside Living Beings: Many live in our intestines or on our skin—helping or sometimes causing illness.
Why Are Microorganisms Important? Microbes play crucial roles in nature—including recycling nutrients, decomposing waste, supporting plant growth, and even digesting food in our bodies!How Are We Connected to Microbes?
Microorganisms (microbes) are found all around us—not just in laboratories or textbooks, but in every corner of our daily lives.
You may have noticed when fruits like lemons, tomatoes, or oranges are left out, they sometimes develop a powdery or cotton-like growth.
This is microbial growth (usually fungi); the food has been infected by microbes.
These microbes reach food from the air, water, soil, or even by landing on the food surface.
Where Can Microorganisms Be Found?
On food: Responsible for spoilage and rotting (mouldy bread, spoiled fruits).
On plant surfaces: Leaves, stems, and roots all have microbial residents.
In water, soil, and air: Microbes thrive in ponds, rivers, soil, and even float in the air.
Inside living beings: The human body (especially the intestine) is home to many bacteria that help with vital functions like digestion.
Extreme environments: Some microbes live in very hot springs, icy cold zones, or salty lakes—showing their incredible adaptability and diversity.
Microbial Diversity
Microbes come in many shapes: spherical, rod-shaped, spiral, and irregular.
Like animals and plants, they also vary in size, structure, and function.
This diversity allows them to survive everywhere and play different roles.
Key Players in Cleaning the Environment
Microbes, especially bacteria and fungi, break down dead plant and animal matter, turning it into simpler substances (decomposition).
This process is what turns fallen leaves and fruit peels into manure (compost), enriching the soil for healthy plant growth.
Ancient texts (the Vedas) recognized both visible and invisible “tiny entities” (Krimi), mentioning their helpful and harmful effects—showing the long-standing appreciation of microbes.
Manure Formation
When organic waste (like fruit and vegetable peels) is left in moist soil for a few weeks, microbes decompose it, forming dark-coloured, nutrient-rich manure.
This process needs optimal temperature and moisture.
The nutrients released go back into the soil, supporting new plant growth.
Microbes also decompose animal waste (like dung), cleaning up the environment naturally.
They even break down dead animal bodies, ensuring nature recycles its resources.
Without microbes, waste and dead matter would accumulate and the recycling of nutrients would stop.
Why Don’t Microbes Spoil Pickles and Murabbas?
Preservation with Salt and Sugar: Pickles and murabbas are made with high concentrations of salt or sugar.
These act as preservatives and prevent the growth of microbes, so the food does not spoil easily.
Microbes as a Source of Biogas
Some bacteria and fungi can grow in places where there is no oxygen (anaerobic conditions).
These microbes decompose plant and animal waste, releasing gases—mainly methane and carbon dioxide.
Methane (biogas) is an important renewable fuel used for cooking, heating, electricity, and even running vehicles.
Real-Life Example: Dr. Ananda Mohan Chakrabarty developed a special bacterium that can break down oil spills (patented in 1980), showing how microbes can help solve real-world pollution problems.
Microorganisms in Food Preparation
Fermentation and Rising of Dough
Yeast (a fungus) is used to make dough rise for breads, cakes, and some Indian foods:
When mixed with flour and warm water, yeast ferments sugars, releasing carbon dioxide (which forms bubbles, making the dough soft and fluffy) and a little alcohol (which adds to the smell).
Bowl experiment: Dough with yeast becomes fluffy and airy; without yeast, it stays dense.
Formation of Curd and Other Fermented Foods
Lactobacillus (a type of bacteria) is used in curd formation:
Added to warm milk, it multiplies and converts sugars (lactose) into lactic acid, making the milk sour and thick (curd).
This needs a warm environment to work well.
Warm milk forms curd quickly; cold milk does not.
Bacteria like Lactobacillus and yeast help in fermentation for foods like idli, dosa, and bhatura.
All these organisms make our food tastier, more nutritious, and sometimes easier to digest.
Nitrogen Fixation
Rhizobium bacteria live in the root nodules of legumes (peas, beans).
These bacteria convert nitrogen from the air into forms plants can use, improving soil fertility naturally (without chemical fertilizers).
This is why farmers grow legumes in crop rotation.
Amazing Microalgae: Tiny Helpers in Water
Microalgae are microscopic, plant-like organisms found in water, soil, air, and even on tree bark.
They perform photosynthesis (making food from sunlight) and release oxygen—more than half of the Earth’s oxygen comes from microbes like these.
They are a major food source for aquatic animals.
Some, like Spirulina, Chlorella, and Diatoms, are used as dietary supplements and medicines for humans.
Other Benefits
Microalgae help in cleaning water and are being developed as a source of biofuel (clean energy).
However, they are threatened by pollution, climate change, and loss of habitat.
Conserving microalgae is important for maintaining oxygen supply, food security, and the health of aquatic ecosystems.
Example: Spirulina
Known as a “superfood” because it is rich in protein (over 60%) and vitamin B12, while being low in fat and sugar.
Spirulina can be easily farmed in tanks with pond water, moderate temperature, and sunlight.
Spirulina farming is a growing livelihood option for communities.
Why Is the Cell Considered the Basic Unit of Life?
The cellis called the basic unit of life because it is the smallest structure that can carry out all the functions necessary for survival. All living organisms—plants, animals, and microorganisms—are made up of cells.
Multicellular organisms (plants and animals):
The bodies of all plants and animals are made up of many cells so, they are called Multicellular organisms.
These cells are specialized to perform different functions (e.g., skin cells, muscle cells, nerve cells).
Cells cooperate and communicate with each other to keep the whole organism alive.
Each type of cell plays a unique role, but all are essential for survival.
Examples of multicellular organisms are: Plants, animals, humans.
Unicellular organisms (bacteria, protozoa, some fungi and algae):
They are made up of only one single cell.
This single cell performs all life processes—nutrition, movement, reproduction, growth, and response to the environment.
Examples of unicellular organisms are: Bacteria, amoeba, yeast.
Examples of Microbial Organization
Bacteria and protozoa are usually unicellular (single-celled).
Some algae and fungi can be unicellular or multicellular:
Yeast: A unicellular fungus (lives as single cells).
Moulds: Multicellular fungi (made of many cells).
Components and Structure of Cells
All cells are typically surrounded by a cell membrane (keeps contents in and controls movement of substances).
Fungal cells have an extra cell wall (for protection and shape), but unlike plant cells, they don’t have chloroplasts and cannot make their own food.
Bacterial cells are different: They do not have a well-defined nucleus or a nuclear membrane. Instead, their genetic material is found in a region called the nucleoid.
In plant and animal cells, the nucleus is clearly defined and surrounded by a membrane.
Special Features in Cells
There are many parts inside cells (called organelles), each with a special function.
To see even smaller details, you need stronger microscopes:
Electron microscopes can magnify cells up to 10,00,000 times (revealing structures not seen with ordinary microscopes).
Diversity in Cells
Cells vary in size, shape, and structure based on their function and the organism they belong to.
Even plant and animal cells are different in how they look and what structures they contain.
Welcome to Class 8 Science, where curiosity fuels exploration! This chapter invites you to become young scientists, asking “Why?” and “How?” about the world around you.
From wondering why one side of a puri is thinner than the other to pondering if there are more stars in the galaxy than grains of sand on Earth, science begins with questions that spark investigation.
Each chapter of Class 8 Science is designed to ignite your curiosity, with roots symbolizing grounded observation and kites representing unique ideas.
Curiosity: The Heart of Scientific Exploration
Science is like an adventure—whether we’re looking at tiny microbes in a drop of water or trying to understand the motions of big objects like the Moon and cyclones. It all begins with being curious: asking questions, exploring new ideas, and noticing patterns around us.
Exploring the World: As young investigators, you’ll discover how science helps us make sense of both the things we can see (like the changing phases of the Moon) and the things we can’t (like the invisible life in a water drop). From wondering why a puri puffs up more on one side, to exploring why nature has so many different kinds of plants and animals—turn curiosity into real discoveries.
Phases of Moon
Asking Questions: Every discovery in science starts with a question. “Why?” and “How?” are powerful words that kick off your scientific journey. When you wonder about simple things, you open up the door to deeper exploration and investigation.
Systematic Investigation: Science is more than just wondering—it’s about investigating step-by-step. This means: – Asking focused questions, – Controlling variables in experiments (like adjusting the thickness of the dough for a puri) – Observing carefully what happens, – Recording your results, and – Using what you observe to improve your understanding.
Roots and Kites: Just like the symbols in your book: – The root at the bottom of the left page reminds us to stay connected—to build our ideas on careful observations and solid facts. – The kite at the top of the right page encourages our curiosity to take flight, to be creative and daring in our explorations.
Hidden Patterns: Some lines and patterns at the bottom of the pages even carry hidden scientific ideas—making sure science stays both fun and meaningful!
Try yourself:
What do you need to start a scientific journey?
A.Reading books
B.Following rules
C.Asking questions
D.Writing notes
View SolutionJourney Through Science
This year, we’ll explore a range of topics, connecting small-scale observations to global challenges, using curiosity and investigation as our guides.
1. Exploring the Investigative World of Science
Science is a journey of curiosity: ask “Why?” and “How?” about everything.
Good science uses both careful observation (roots) and imaginative thinking (kites).
Questions, experiments, and patterns lead to discoveries about our world.
2. The Invisible Living World: Beyond Our Naked Eye
In a drop of water, countless microbes live—some helpful, some harmful.
Microbes help in digestion and medicine-making; others cause disease.
Discovering the invisible world explains food spoilage, fermentation, and infections.
Try yourself:
What do microbes in a drop of water do?
A.They help in digestion.
B.They cause disease.
C.They absorb sunlight.
D.They create food spoilage.
View Solution
3. Health: The Ultimate Treasure Aspects of Health
Health means complete physical, mental, and social well-being.
Good food, hygiene, exercise, and healthy habits keep us well.
The immune system defends us; vaccines and medicines help fight disease.
Prevention is better than cure—cleanliness and vaccination are key.
4. Electricity: Magnetic and Heating Effects
Electricity has both heating and magnetic effects.
We use the heating effect for warmth (like in heaters), and the magnetic effect to run motors and machines.
Understanding electric circuits is the basis of much modern technology.
5. Exploring Forces
Forces can make objects start, stop, speed up, slow down, or change direction.
Gravity pulls objects towards Earth, friction slows motion, and applied forces move objects.
Everyday activities (walking, throwing, cycling) all depend on forces.
6. Pressure, Winds, Storms, and Cyclones
Pressure is force spread over an area; it moves air and water.
Air pressure differences create winds, breezes, and, when extreme, storms and cyclones.
Weather events affect people, farming, and safety; understanding them helps us prepare better.
7. Particulate Nature of Matter
All matter is made of tiny particles.
Particles are tightly packed in solids, loosely arranged in liquids, and move freely in gases.
Changes in state (solid, liquid, gas) depend on how these particles move or are arranged.
8. Nature of Matter: Elements, Compounds, and Mixtures
Elements are pure substances (like oxygen, iron).
Compounds are two or more elements chemically joined (like water).
Mixtures are physical blends (like air or salt water) that can be separated.
9. The Amazing World of Solutes, Solvents, and Solutions
A solution is a mix where one substance dissolves into another (sugar in tea).
The substance that dissolves is the solute; the one doing the dissolving is the solvent.
Understanding solutions helps explain things in daily life, like sweetened drinks or salty water.
10. Light: Mirrors and Lenses
Light travels in straight lines and can reflect (bounce off mirrors) or refract (bend through lenses).
Images in shiny surfaces, and the way lenses correct our vision, are all about light behavior.
Reflected and refracted light helps us see objects, mirrors, and use cameras or eyeglasses.
11. Keeping Time with the Skies
The apparent movement of the Sun and Moon helps measure days, months, and years.
Phases of the Moon create lunar calendars.
Eclipses occur when shadows are cast by the Moon or Earth blocking sunlight.
12. How Nature Works in Harmony
All living things are connected, forming ecosystems.
Plants, animals, water, air, and sunlight interact to support life.
Simple changes in one part of an ecosystem can affect the whole system.
13. Our Home: Earth, a Unique Life Sustaining Planet
Earth’s distance from the Sun and its atmosphere make it suitable for life.
Human activities can change Earth’s climate and harm its balance.
Science helps us recognize planet-wide challenges and work towards their solutions.
Investigating Everyday Phenomena
Science starts with being curious and asking questions like: Why does a puri puff up? Why is one side thinner? You don’t need a lab—your kitchen is a great place to observe and experiment.
Step 1: Ask a scientific question E.g., What changes how a puri puffs up when fried?
Step 2: Identify things you can change (variables):
Thickness of the dough
Size of the puri
Type of flour (atta, maida, etc.)
Temperature of the oil
How you put the puri into the oil (dropped straight or slid in)
Step 3: Decide what to observe or measure:
Does the puri puff up (yes or no)?
How long does it take to puff?
Does the thickness affect puffing?
Step 4: Change only one thing at a time so you know what causes the difference. (For example, test different oil temperatures but keep dough thickness the same.)
Step 5: Write down what you see (Does the oil splatter? Does it smoke? What smells do you notice?)
Step 6: Ask new questions based on what you observe. Example: Does fresh dough puff better than stored dough? What if there’s a hole in the puri?
This step-by-step way of experimenting is called systematic investigation.
Remember, science is about being curious and carefully observing even simple things around you, like a puffing puri!
Key Points to Remember
Science begins with curiosity, asking simple questions like “Why?” and “How?” about the world around us.
Science is an ongoing journey: every answer leads to new questions, making our understanding evolve and grow deeper.
In Grade 8, you learn not just facts, but how to investigate—by asking focused questions, doing simple experiments, observing carefully, and explaining results.
You can become a young scientist by observing everyday phenomena—from why a puri puffs up in hot oil, to studying changes in Earth’s climate.
Scientific investigation balances careful observation (like roots keeping us grounded) and creative thinking (like a kite soaring to new heights).
The hidden world of microbes affects our health—some microbes help us, others cause disease.
Staying healthy depends on nutritious food, exercise, medicines, and vaccines.
Electricity’s heating and magnetic effects help in many daily tasks.
Forces cause objects to change speed, direction, or stop.
Pressure affects air movement, leading to breezes, winds, and cyclones.
Everything is made of tiny particles — solids have fixed particles, gases have freely moving particles.
Materials are classified as elements, compounds, and mixtures, helping us understand the nature around us.
Light behaves in interesting ways—it reflects and bends, helping us see objects and understand vision.
The phases of the Moon and movements of celestial bodies help humans keep time and create calendars.
Life on Earth is interconnected; ecosystems depend on balanced relationships among living things and their environment.
Earth has unique conditions making life possible, but human activities are causing climate changes that threaten this balance.
Scientific methods—observing, measuring, experimenting—are vital tools to understand and address these challenges.
Always remember: science isn’t just for classrooms or labs; your ordinary surroundings and everyday questions are starting points for discovery!
Saturated Solution: A solution where no more solute can dissolve at a particular temperature, and excess solute settles at the bottom.
Concentrated Solution: A solution with a relatively large amount of solute in the solvent (e.g., more spoons of salt make it more concentrated).
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.
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.
Sea Breeze
Land Breeze
Blowing harder (faster air) makes them move even closer, more quickly.
Heavier, negatively charged water droplets gather at the bottom.
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.
Balanced Diet
Diseases Spread by Insects (Vectors): Diseases that are passed on when insects such as mosquitoes or flies carry pathogens from one host to another, causing infection.
