Extra 20 important long questions and answers from the Chapter:6 Changes Around Us for Class 6 CBSE Science:

1. What is a physical change? Give examples.

Ans: A physical change is a change in which the physical properties of a substance, such as shape, size, or state, change without forming a new substance.

No new substance is formed.

It is usually reversible.

The chemical composition remains the same.

Examples include melting of ice, cutting of paper, and boiling of water.

It does not produce heat, light, or sound.

It can often be undone by simple methods like cooling or heating.

 

2. What is a chemical change? Give examples.

Ans: A chemical change is a change in which a new substance is formed with different properties.

A new substance is formed.

It is usually irreversible.

The chemical composition of the original substance changes.

Examples include burning of wood, rusting of iron, and digestion of food.

It may involve heat, light, or gas release.

Chemical changes are used in cooking, combustion, and various industrial processes.

 

3. How is melting of ice different from burning of wood?

Ans:

Melting of Ice	Burning of Wood
It is a physical change.	It is a chemical change.
No new substance is formed.	A new substance (ash, smoke) is formed.
It is reversible.	It is irreversible.
It occurs due to change in temperature.	It occurs due to combustion.
It does not produce heat or light.	It produces heat and light.
Example: Ice turning into water.	Example: Wood turning into ash.

 

4. Explain the differences between reversible and irreversible changes.

Ans:

Reversible Changes	Irreversible Changes
The original substance can be obtained back.	The original substance cannot be obtained back.
No new substance is formed.	A new substance is formed.
It is usually a physical change.	It is usually a chemical change.
Example: Freezing of water.	Example: Cooking of food.
Can be undone by heating, cooling, or other methods.	Cannot be undone by simple methods.
Example: Dissolving sugar in water.	Example: Rusting of iron.

 

5. What happens when iron rusts? How can rusting be prevented?

Ans: Rusting is a chemical change where iron reacts with oxygen and water to form iron oxide (rust).

Rusting occurs when iron is exposed to moisture and air.

A reddish-brown substance (iron oxide) is formed.

Rust weakens iron and damages structures.

It is an irreversible chemical change.

Rusting can be prevented by:

Painting

iron objects.

Oiling or greasing

to prevent air contact.

Galvanization

(coating iron with zinc).

Using stainless steel

, which does not rust.

 

6. What are slow and fast changes? Give examples.

Ans:

Slow changes

occur over a long period.

Example: Rusting of iron, growth of a plant.

Fast changes

occur quickly.

Example: Bursting of a balloon, burning of a matchstick.

Slow changes are usually natural and cannot be controlled.

Fast changes can be controlled or stopped quickly.

Both slow and fast changes can be reversible or irreversible.

Some changes, like weathering of rocks, take years to occur.

 

7. Why is cooking food an irreversible change?

Ans:

Cooking involves chemical reactions that change raw ingredients into new substances.

It changes the taste,

color

, and texture of food.

New substances like cooked vegetables, bread, or rice are formed.

We cannot get the raw ingredients back.

Heat is involved in the process, making it a chemical change.

Examples include boiling an egg or baking a cake.

 

8. What is expansion and contraction? Give examples.

Ans:

Expansion

is the increase in size of an object when heated.

Example: Railway tracks expand in hot weather.

Contraction

is the decrease in size of an object when cooled.

Example: A metal ring shrinks when placed in cold water.

Expansion occurs due to increased movement of particles.

Contraction occurs due to decreased movement of particles.

Expansion is used in thermometers, where mercury expands with heat.

Contraction is used in metal fitting techniques.

 

9. How is salt obtained from seawater?

Ans:

Seawater contains dissolved salt.

It is collected in shallow ponds.

The water is left to evaporate under the sun.

As the water evaporates, salt crystals are left behind.

The salt is collected and purified.

This method is widely used in coastal areas.

 

10. Why is breaking of glass a physical change?

Ans:

Breaking changes the shape and size of the glass.

No new substance is formed.

The chemical composition remains the same.

It is an irreversible change but still physical.

Broken glass pieces can be melted and reshaped.

It does not involve heat, light, or gas formation.

 

11. What happens when sugar dissolves in water?

Ans:

Sugar mixes with water to form a solution.

It is a physical change.

No new substance is formed.

It is reversible – sugar can be recovered by evaporation.

The taste of water changes but its properties remain the same.

This process is used in making sugar syrups.

 

12. Explain how heating can bring about changes in substances.

Ans:

Heating can cause

expansion

(metals expand).

It can cause

melting

(ice to water).

It can lead to

chemical changes

(baking, burning).

It can evaporate liquids (water to steam).

Some substances decompose when heated (wood burns to ash).

Heat plays an important role in cooking, industry, and daily life.

 

13. How is the stretching of a rubber band a physical change?

Ans:

It changes the shape of the rubber band.

No new substance is formed.

It is reversible – when released, it returns to its original shape.

The elasticity of rubber allows this.

It does not involve heat, light, or any chemical reaction.

This property is used in making elastic materials.

 

14. What happens when a candle burns?

Ans:

Wax melts

– a physical change.

Wax burns to form gas

– a chemical change.

The flame produces light and heat.

Carbon dioxide and water vapor are released.

It is irreversible as burnt wax cannot be recovered.

The burning process is a combination of physical and chemical changes.

 

15. Why is the formation of clouds a physical change?

Ans:

The formation of clouds occurs due to the condensation of water vapor.

Water changes from gaseous form to liquid droplets in the sky.

No new substance is formed, only a change in the state of matter.

The process is

reversible

– water can fall back as rain and evaporate again.

It does not involve any chemical reaction.

This is an important part of the

water cycle

.

 

16. Why is burning of paper an irreversible change?

Ans:

When paper burns, it turns into ash and smoke.

A new substance (ash) is formed with different properties.

The process cannot be reversed – ash cannot turn back into paper.

It is a

chemical change

as it involves a chemical reaction.

Heat and light are produced during burning.

Burning is an example of

combustion

, which is an irreversible change.

 

17. Explain how day and night is a periodic change.

Ans:

The Earth rotates on its axis, causing the cycle of day and night.

This change occurs

regularly

every 24 hours.

It is

reversible

– night changes into day and vice versa.

No new substance is formed.

The process continues indefinitely, making it

periodic

.

It affects life on Earth, influencing sleep cycles, weather, and biological processes.

 

18. What happens when a clay pot is baked? Why is it an irreversible change?

Ans:

Wet clay is shaped into a pot.

When baked at high temperature, water is removed, and the clay hardens.

A new, stronger material is formed.

The process

cannot be reversed

– the baked pot cannot return to soft clay.

This is a

chemical change

, as the properties of clay change permanently.

Baking makes the pot strong and water-resistant.

 

19. How do plants and animals show irreversible changes?

Ans:

A seed grows into a plant, which cannot change back into a seed.

A caterpillar transforms into a butterfly, which is irreversible.

A baby grows into an adult, which cannot be reversed.

Digestion of food leads to energy production, which is irreversible.

Death of living beings is also an irreversible process.

These changes involve biological and chemical processes.

 

20. How can expansion and contraction be useful in daily life?

Ans:

Metal bridges have expansion gaps

to allow expansion in summer.

Railway tracks expand

in heat and contract in cold, so gaps are left between them.

Thermometers work on expansion

– mercury expands when heated.

Electric wires sag in summer

due to expansion and become tight in winter due to contraction.

Rivets are fixed using expansion and contraction

– they are heated, inserted, and then contract to hold objects tightly.

Bimetallic strips in thermostats

use expansion and contraction to control temperature in electrical devices.