CBSE Class 7 Science Revision Notes Chapter 5: Changes Around Us – Physical and Chemical

Changes can affect the size, shape, state, smell, colour or composition of a substance. A physical change forms no new substance, while a chemical change produces one or more new substances.

Changes happen around us all the time. Ice melts, water boils, fruits ripen, wood burns, iron rusts and wet clothes dry. Some changes affect only the appearance or state of a substance, while others form completely new substances.

These CBSE Class 7 Science Revision Notes Chapter 5 follow the current 2026–27 chapter. Revise physical and chemical changes, combustion, ignition temperature, reversible and irreversible changes, desirable and undesirable changes, weathering and erosion. The notes also explain how lime water helps identify carbon dioxide and why a burning candle shows both physical and chemical changes.

Key Takeaways

• Physical change: Only physical properties change, and no new substance is formed.
• Chemical change: One or more new substances are produced.
• Combustion: A substance reacts with oxygen and releases heat or light.
• Fire triangle: Fuel, oxygen and sufficient heat are required for burning.

Access Class 7 Science Chapter 5 Changes Around Us Notes in 30 Minutes

Spend the first 10 minutes revising physical and chemical changes with examples. Use the next 10 minutes for carbon dioxide testing, rusting, combustion, oxygen and ignition temperature.

Use the final 10 minutes for candle burning, reversible and irreversible changes, desirable and undesirable changes, weathering and erosion.

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Changes Around Us in Class 7 Science Chapter 5 Notes

A change is any alteration observed in a substance, material or object. It may involve a change in shape, size, state, colour, smell, temperature or another property.

We observe changes using our senses of sight, smell, touch, hearing and taste. However, unknown substances should never be tasted during an experiment.

Examples of Changes

Common changes include:

• Melting ice
• Boiling water
• Chopping vegetables
• Making popcorn
• Burning wood
• Drying wet clothes
• Rolling dough into chapatis
• Ripening fruits
• Forming flowers from buds
• Rusting of iron

These changes do not all happen in the same way. They can be grouped according to whether a new substance forms, whether the original form can be recovered, and whether the change is useful.

Physical Changes Revision Notes

A physical change is one in which only the physical properties of a substance change. No new substance is formed during the process.

Physical properties include:

• Shape
• Size
• State
• Appearance
• Volume
• Texture

Folding Paper

When paper is folded to make an object, only its shape changes. Unfolding it gives back the same paper, so no new substance is formed.

Inflating a Balloon

When a balloon is inflated, its shape and size change. When the air is allowed to escape, the balloon returns close to its original form.

If the balloon is pricked, its material remains rubber even though its form cannot be restored. The change is physical because no new substance is produced.

Crushing Chalk

Crushing a piece of chalk changes its size and shape. Chalk powder is still made of the same substance as the original chalk piece.

The original piece cannot easily be recovered, but this does not make the change chemical. The main test is whether a new substance has formed.

Changes in the State of Water

Ice melting into water and water changing into steam are physical changes. Water remains the same substance even though its state changes.

These changes can also be reversed:

• Water can freeze into ice.
• Water vapour can condense into liquid water.

Main Features of a Physical Change

• No new substance is formed.
• Shape, size or state may change.
• The substance remains chemically the same.
• The change may be reversible or irreversible.
• Energy may be absorbed or released.

Examples of Physical Changes

• Melting ice
• Boiling water
• Folding clothes
• Cutting paper
• Chopping vegetables
• Crushing chalk
• Rolling dough
• Drying wet clothes
• Twisting a string
• Breaking rocks into smaller pieces

Chemical Changes in Chapter 5 Science Revision Notes

A chemical change is one in which one or more new substances are formed. The new substance has properties different from the original material.

The process through which new substances form is called a chemical reaction. A chemical equation is a short way of representing the substances involved in a chemical reaction.

Common Signs of a Chemical Change

A chemical change may involve:

• Formation of a new substance
• Change in colour
• Formation of gas
• Change in smell
• Production of heat
• Production of light
• Formation of a solid deposit
• Sound or bubbling

Not every colour or temperature change proves a chemical reaction. The strongest evidence is the formation of a new substance.

Carbon Dioxide and Lime Water Test

Carbon dioxide can react with lime water and turn it milky. This change is used as a test for the presence of carbon dioxide.

Blowing Air into Lime Water

When exhaled air is passed through tap water, only bubbles are observed. The water does not show a major visible change.

When exhaled air is passed through lime water, the liquid becomes milky. Carbon dioxide in the exhaled air reacts with calcium hydroxide in lime water and forms calcium carbonate, which is insoluble and white.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Since calcium carbonate is a new substance, the reaction is a chemical change.

Why Does Lime Water Turn Milky?

The newly formed calcium carbonate remains suspended in the liquid and makes it appear cloudy or milky. If the mixture is left undisturbed, some of the white material may settle at the bottom.

Vinegar and Baking Soda Reaction

When vinegar or lemon juice is mixed with baking soda, fizzing and bubbling occur. The gas produced can be passed into lime water.

The lime water turns milky, showing that the gas is carbon dioxide.

Vinegar + Baking soda → Carbon dioxide + Other substances

This is a chemical change because a new gas is formed.

When baking soda is mixed only with water, the same bubbling is not observed. This shows that the acid in vinegar or lemon juice is involved in the reaction.

Rusting as a Chemical Change

Rusting is a chemical change because iron reacts with oxygen and water to form a new brown substance called rust. Rust has properties different from iron.

The product is commonly described as iron oxide. Since a new substance forms and the original iron cannot easily be recovered, rusting is an irreversible chemical change.

Rusting can damage:

• Gates
• Tools
• Machines
• Vehicles
• Bridges
• Pipes
• Bicycle parts

Methods such as painting, oiling, greasing and galvanisation help reduce rusting by limiting contact with air and water.

Combustion in Class 7 Science Chapter 5 Revision Notes

Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Burning wood, paper, cotton, kerosene, wax vapour and magnesium are examples.

Substances that undergo combustion are called combustible substances. A combustible substance may also be called a fuel when it is used to produce useful heat or energy.

Burning Magnesium Ribbon

Magnesium ribbon burns in oxygen with a dazzling white flame. It forms a new white substance called magnesium oxide.

Magnesium + Oxygen → Magnesium oxide + Heat + Light

This is a chemical change because magnesium oxide is different from magnesium.

Combustible Substances

Examples include:

• Wood
• Paper
• Cotton
• Kerosene
• Candle wax
• Cooking gas
• Petrol
• Coal

Not every material catches fire under the same conditions. Different substances require different temperatures to begin burning.

Oxygen and Combustion

Oxygen is essential for combustion. Without a continuous supply of air, a burning flame goes out.

Candle and Glass Tumbler Activity

Two similar candles are lit. One remains uncovered, while the other is covered with a glass tumbler.

The uncovered candle continues burning because it receives air. The covered candle stops burning after some time because the oxygen trapped inside the tumbler gets used up.

Testing the Gas Formed

When a candle burns, carbon in the wax reacts with oxygen and forms carbon dioxide. Lime water placed in the covered setup turns milky and confirms the presence of carbon dioxide.

Why Does Covering a Fire Help?

Covering a fire blocks the supply of oxygen. Since oxygen is required for combustion, the flame becomes weaker and goes out.

If a person’s clothes catch fire, a thick blanket or cloth may be wrapped around the person to cut off the air supply. Synthetic cloth should not be used because it can melt and stick to the skin.

Ignition Temperature and Fire Triangle

A combustible substance does not catch fire simply because oxygen is present. It must first reach a minimum temperature.

The lowest temperature at which a substance catches fire is called its ignition temperature.

Paper and Matchstick

A lighted matchstick is already hot enough to raise paper above its ignition temperature. Therefore, paper catches fire quickly when touched by the flame.

Paper and Magnifying Glass

A magnifying glass can focus sunlight on a small spot. The focused energy heats the paper until it reaches its ignition temperature and begins to burn.

This shows that a flame is not always required to start combustion. Sufficient heat is enough.

Fire Triangle

Three conditions are necessary for combustion:

1. A combustible substance or fuel
2. Oxygen
3. Enough heat to reach the ignition temperature

These three conditions form the fire triangle.

Removing any one of them can stop combustion:

• Remove fuel
• Cut off oxygen
• Reduce temperature below the ignition point

Bioluminescence in Fireflies

Fireflies produce light through a chemical change inside their bodies. This production of light by living organisms is called bioluminescence.

Unlike an ordinary flame, this light is produced with very little heat. It is another example of a chemical process producing light.

Physical and Chemical Changes in a Burning Candle

Burning a candle involves both physical and chemical changes. Several processes occur at the same time.

Physical Changes

The heat of the flame melts solid wax near the wick. The liquid wax moves up the wick and changes into vapour.

Melting, solidification and evaporation of wax are physical changes because wax remains the same substance.

Chemical Change

Wax vapour reacts with oxygen and burns. New substances such as carbon dioxide and water vapour form.

The burning of wax vapour is therefore a chemical change.

Summary

Michael Faraday used the candle to explain scientific ideas involving melting, evaporation and combustion. His lectures were called The Chemical History of a Candle.

Reversible and Irreversible Changes

Changes can also be classified according to whether the original substance or object can be obtained again.

A change that can be brought back to its original form is called reversible. A change that cannot be brought back easily is called irreversible.

Reversible Changes

Examples include:

• Melting and freezing water
• Evaporation and condensation
• Inflating and deflating a balloon
• Folding and unfolding cloth
• Rolling and unrolling a mat
• Twisting and untwisting a string

Irreversible Changes

Examples include:

• Chopping vegetables
• Making popcorn
• Burning wood
• Ripening fruit
• Rusting iron
• Curdling milk
• Making compost
• Grinding wheat into flour

Important Difference

A physical change is not always reversible. Cutting paper and crushing chalk are physical but cannot be easily reversed.

A chemical change is usually irreversible, but reversibility and chemical nature are different ways of classifying a change.

Desirable and Undesirable Changes

A desirable change is useful or wanted in a particular situation. An undesirable change causes damage, loss or inconvenience.

Desirable Changes

Examples include:

• Cooking food
• Ripening fruits
• Setting curd
• Cutting vegetables
• Germination of seeds
• Formation of compost
• Drying wet clothes

Undesirable Changes

Examples include:

• Rusting of iron
• Spoiling of food
• Accidental fire
• Decay of stored materials
• Pollution from fuel burning
• Damage caused by erosion

Same Change, Different Situation

A change may be undesirable in one situation and useful in another. Food decomposition is undesirable during storage, but decomposition of food waste is useful for making compost.

Environmental Impact of Changes

Some changes caused by human activity may have long-term effects on the environment. Burning more fuel in vehicles and machines increases the amount of carbon dioxide released into the atmosphere.

Paints may also release substances into the air while drying. Such processes can contribute to atmospheric pollution.

Scientific understanding helps us decide which changes should be encouraged and which ones need to be controlled.

Weathering of Rocks

Weathering is the breakdown or alteration of rocks due to physical and chemical changes. Over long periods, weathering helps form soil.

Physical Weathering

Large rocks may break into smaller pieces because of:

• Temperature changes
• Growing plant roots
• Freezing water in rock cracks
• Wind
• Flowing water

No new substance may form during physical breaking.

Chemical Weathering

Water and substances dissolved in it may react with minerals in rocks. This changes the composition and colour of the rock.

For example, iron-containing basalt may develop a reddish layer because iron oxide forms after long exposure to air and moisture.

Formation of Soil

Physical breaking produces smaller rock particles. Chemical changes alter the minerals in these particles.

Together with organic matter, these materials gradually contribute to soil formation.

Erosion

Erosion is the breaking down and movement of soil, rock particles and sediments from one place to another by wind, water or other natural forces.

Flowing rivers carry pebbles, soil and sand. Continuous movement may make stones smoother over time.

Sediment Formation

When the speed of water or wind decreases, the transported material settles. These deposited materials are called sediments.

Over long periods, sediments may harden and form new rocks.

Erosion as a Physical Change

Erosion mainly changes the size, shape and location of materials. It is therefore generally considered a physical change.

A landslide moving rocks and soil is another example of physical change caused by natural forces.

Physical and Chemical Changes Comparison

Reversible and Irreversible Changes Comparison

Class 7 Science Chapter 5 Summary Notes

Important Terms from Changes Around Us

Physical change: A change in which no new substance is formed.

Chemical change: A change in which one or more new substances are formed.

Chemical reaction: A process through which substances change into new substances.

Chemical equation: A short representation of a chemical reaction.

Combustion: A reaction in which a substance reacts with oxygen and releases heat or light.

Combustible substance: A substance that can undergo combustion.

Fuel: A combustible material used to produce energy.

Ignition temperature: The minimum temperature at which a substance catches fire.

Fire triangle: The three requirements of fuel, oxygen and heat.

Reversible change: A change in which the original form can be obtained again.

Irreversible change: A change in which the original form cannot be obtained easily.

Desirable change: A change that is useful in a particular situation.

Undesirable change: A change that causes harm or loss.

Weathering: Physical and chemical breakdown of rocks.

Erosion: Movement of rock particles, soil and sediments by wind or water.

Sediments: Rock, soil and sand particles deposited in a place.

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