Class 7 Science Chapter: Changes Around Us – Physical and Chemical

Introduction

We observe many changes around us every day. Some changes happen naturally, while others are caused by humans.

Examples:

• Ice melts into water.
• Milk turns into curd.
• Fruits ripen.
• Wood burns.
• Iron rusts.
• Water freezes into ice.
• A flower blooms from a bud.

During these changes, the shape, size, colour, smell, state, or temperature of a substance may change.

Changes are mainly classified into two types:

1. Physical Changes
2. Chemical Changes

Physical Properties

The characteristics of a substance that can be observed without changing its composition are called. These include:

• Shape
• Size
• Colour
• State (solid, liquid, gas)
• Texture
• Mass
• Volume
• Smell

We observe these properties using our five sense organs.

Examples

• Colour of a flower
• Shape of a football
• Smell of perfume
• Hardness of stone

Physical Change

A physical change is a change in which only the physical properties of a substance change and no new substance is formed.

The substance remains the same even after the change.

Examples

• Melting of ice
• Freezing of water
• Boiling of water
• Stretching a rubber band
• Folding paper
• Breaking glass
• Dissolving sugar in water

Characteristics of Physical Changes

• No new substance is formed.
• Chemical composition remains unchanged.
• Usually reversible.
• Only shape, size or state changes.
• Temporary in nature.
• Very little energy change occurs.

Examples of Physical Changes

1. Melting of Ice

Ice absorbs heat and changes into water.

Ice → Water

The substance remains water (H2O), so it is a physical change.

2. Freezing of Water

Water loses heat and changes into ice.

Water → Ice

No new substance is formed.

3. Stretching a Rubber Band

Only the shape changes.

When released, it returns to its original shape.

4. Breaking Glass

The size changes but the material is still glass.

Chemical Change

A chemical change is a change in which one or more new substances are formed with different properties.

The original substance cannot be obtained easily.

Examples

• Rusting of iron
• Burning of wood
• Cooking food
• Digestion
• Milk changing into curd
• Burning of magnesium ribbon

 Chemical Reaction

A chemical reaction is the process in which one or more substances react to form new substances.

During a chemical reaction,

• New substances are formed.
• Energy may be released or absorbed.
• Colour may change.
• Gas may be produced.
• Heat or light may be produced.

Chemical Equation

A chemical equation is the short form of representing a chemical reaction using symbols and formulas.

Example 1

Carbon dioxide + Lime water → Calcium carbonate + Water

CO2 + Ca(OH)2 → CaCO3 + H2O

Observation

Lime water turns milky because calcium carbonate is formed.

Example 2

Baking Soda + Vinegar → Sodium acetate + Water + Carbon dioxide

NaHCO3 + CH3COOH → CH3COONa + H2O + CO2

Observation

Bubbles of carbon dioxide gas are produced.

Characteristics of Chemical Changes

• New substances are formed.
• Chemical composition changes.
• Usually irreversible.
• Heat, light or sound may be produced.
• Colour may change.
• Gas may be released.
• Precipitate may be formed.
• Energy change always occurs.

Difference Between Physical and Chemical Change

Physical Change	Chemical Change
No new substance formed	New substance formed
Usually reversible	Usually irreversible
Physical properties change	Chemical properties change
Composition remains same	Composition changes
Temporary	Permanent
Example: Melting ice	Example: Rusting iron

Rusting

When iron is exposed to air and moisture, a brown coloured substance called rust is formed.

This process is known as rusting.

Rust is hydrated iron oxide.

Conditions Necessary for Rusting

Iron needs

• Oxygen
• Water or moisture

Without these two, rusting cannot occur.

Equation

Iron + Oxygen + Water → Rust

Examples

• Rusted bicycle
• Rusted gate
• Rusted nail
• Rusted bridge

Prevention of Rusting

• Painting
• Oiling
• Greasing
• Galvanisation
• Electroplating

Combustion

Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and light.

Examples

• Burning wood
• Burning LPG
• Burning petrol
• Burning candle
• Burning magnesium ribbon

Burning of Magnesium Ribbon

Magnesium burns with a dazzling white flame.

Magnesium + Oxygen → Magnesium oxide

Observation

• Bright white flame
• White ash (magnesium oxide) is formed

Combustible Substances

Substances that burn easily in air are called combustible substances.

Examples

• Wood
• Paper
• Petrol
• Diesel
• LPG
• Kerosene
• Cotton

Non-Combustible Substances

Substances that do not burn are called non-combustible substances.

Examples

• Stone
• Glass
• Sand
• Water

Bioluminescence

Some living organisms produce light through chemical reactions.

This phenomenon is called bioluminescence.

Examples

• Fireflies
• Some jellyfish
• Certain fungi
• Deep-sea fishes

Unlike electric bulbs, this light is produced without much heat.

Ignition Temperature

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

Examples

• Paper catches fire at a lower temperature.
• Wood requires a higher temperature.

Below the ignition temperature, burning cannot start.

Conditions Necessary for Combustion

Three things are needed for combustion.

1. Fuel (Combustible substance)
2. Oxygen
3. Heat (Ignition temperature)

This is called the Fire Triangle.

If any one is removed, fire stops.

Burning of a Candle

A candle shows both physical and chemical changes.

Physical Change

Wax melts near the flame.

Liquid wax solidifies when cooled.

Chemical Change

Wax vapour burns.

New substances like carbon dioxide and water vapour are produced.

Heat and light are released.

Reversible Changes

Changes that can be reversed are called reversible changes.

Examples

• Melting ice
• Freezing water
• Inflating balloon
• Stretching rubber band
• Folding paper

Characteristics

• Original substance can be obtained.
• Mostly physical changes.
• Temporary.

Irreversible Changes

Changes that cannot be reversed are called irreversible changes.

Examples

• Cooking food
• Burning paper
• Rusting iron
• Cutting vegetables
• Ripening fruits
• Baking cake

Characteristics

• Original substance cannot be obtained.
• Mostly chemical changes.
• Permanent.

Desirable Changes

Useful changes are called desirable changes.

Examples

• Cooking food
• Making curd
• Ripening fruits
• Drying clothes
• Making butter
• Freezing ice cream

Undesirable Changes

Changes that are harmful are called undesirable changes.

Examples

• Rusting
• Food spoilage
• Decay of fruits
• Pollution
• Milk becoming sour

Slow Natural Changes

Weathering of Rocks

Weathering is the slow breakdown of rocks into smaller pieces due to

• Wind
• Rain
• Temperature changes
• Plant roots

Weathering helps in the formation of soil.

It may involve both physical and chemical changes.

Erosion

Erosion is the movement of soil, rocks and sediments from one place to another by natural forces.

Agents of erosion

• Wind
• Flowing water
• Glaciers
• Sea waves

Erosion is mainly a physical change.

Sediments

Small particles of sand, soil and stones that collect at the bottom of mountains, rivers or lakes are called sediments.

Sediments are formed due to weathering and erosion.

Summary

• A change is any alteration in the properties of a substance.
• Physical changes do not produce new substances.
• Chemical changes always produce new substances.
• Rusting is a chemical change requiring oxygen and water.
• Combustion produces heat and light.
• Fire requires fuel, oxygen and heat.
• Burning of a candle involves both physical and chemical changes.
• Reversible changes can be undone.
• Irreversible changes cannot be undone.
• Desirable changes are useful, while undesirable changes are harmful.
• Weathering and erosion are important natural processes that help shape the Earth's surface.