NCERT Solutions for Class 6 Science Chapter 6 — Changes Around Us

Key Concepts

Intext Questions (Page-by-Page)

Intext Question 1 (Page 50)

Q: Can all changes always be reversed? Give examples.

No, all changes cannot always be reversed. Changes can be classified into two types:

Reversible changes (can be reversed):

• Melting of ice into water — water can be frozen back into ice
• Folding of paper — the fold can be unfolded
• Dissolving salt in water — salt can be recovered by evaporation
• Stretching a rubber band — it comes back to its original shape
• Opening and closing a door — it can be done repeatedly

Irreversible changes (cannot be reversed):

• Burning of paper — ash cannot be turned back into paper
• Cooking of food — cooked food cannot be changed back to raw food
• Ripening of fruit — ripe fruit cannot become raw again
• Breaking of a glass tumbler — pieces cannot be joined back
• Cutting a piece of wood — cut wood cannot be joined back to its original form

Intext Question 2 (Page 52)

Q: A thick coating of Plaster of Paris (POP) is applied on a wooden board. After it dries, you scratch the design of a tree on it. Is this a reversible change?

No, this is an irreversible change.

• Once the Plaster of Paris dries and hardens, the design scratched on it cannot be erased or undone.
• The POP changes from a soft paste to a hard solid — this hardening itself is irreversible.
• The scratched design removes material from the surface, which cannot be restored to its original state.
• Therefore, both the drying of POP and the scratching of the design are irreversible changes.

Intext Question 3 (Page 53)

Q: A piece of chalk is taken and broken into two pieces. Is this a reversible or irreversible change?

Breaking a piece of chalk into two pieces is an irreversible change.

• Once the chalk is broken, the two pieces cannot be joined back to form the original chalk in its exact form.
• Even if we stick the pieces together with adhesive, the chalk will not be exactly the same as before.
• The broken chalk has new edges and surfaces that were not there originally.
• Since we cannot get back the original piece of chalk, this is an irreversible change.

Intext Question 4 (Page 55)

Q: You have a ball of dough. You press it to make a flat disc. Is this a reversible or irreversible change?

Pressing a ball of dough into a flat disc is a reversible change.

• The dough ball changes shape when pressed, but it is still dough.
• No new substance is formed — only the shape has changed.
• The flat disc of dough can be rolled back into a ball shape.
• Since the change can be reversed by reshaping, it is a reversible change.

Note: This is also a physical change because the composition of the dough remains the same; only its form has changed.

Intext Question 5 (Page 56)

Q: What happens when you burn a piece of paper? Is this a reversible or irreversible change?

When a piece of paper is burnt, it turns into ash, smoke, and gases (carbon dioxide and water vapour).

This is an irreversible change because:

1. The ash formed cannot be converted back into paper. 2. New substances (ash, smoke, gases) are formed which have completely different properties from paper. 3. The change is permanent and cannot be undone. 4. This is also a chemical change because new substances are formed.

Intext Question 6 (Page 57)

Q: Classify the following as reversible or irreversible changes:

(a) Breaking of a glass tumbler — Irreversible (pieces cannot be joined back perfectly)

(b) Melting of ice — Reversible (water can be frozen back to ice)

(c) Burning of wood — Irreversible (ash and smoke are formed; cannot get wood back)

(d) Stretching of a rubber band — Reversible (it returns to original shape when released)

(e) Cooking of food — Irreversible (raw food cannot be obtained from cooked food)

(f) Dissolving salt in water — Reversible (salt can be recovered by evaporation)

(g) Making a biogas plant — Irreversible (the process produces new substances)

(h) Growth of a plant — Irreversible (a grown plant cannot become a seed again)

(i) Rusting of iron — Irreversible (rust cannot be converted back to iron easily)

(j) Boiling of water — Reversible (steam can be condensed back to water)

Intext Question 7 (Page 58)

Q: Is burning of paper a physical change or a chemical change? Explain.

Burning of paper is a chemical change.

• When paper burns, it reacts with oxygen in the air.
• New substances are formed: ash, carbon dioxide, water vapour, and smoke.
• These new substances have completely different properties from the original paper.
• The change is permanent and cannot be reversed.
• Heat and light are also produced during burning.
• Since new substances are formed with different properties, this is a chemical change.

Intext Question 8 (Page 59)

Q: Is freezing of water a physical change or a chemical change? Why?

Freezing of water is a physical change.

• When water freezes, it changes from liquid state to solid state (ice).
• No new substance is formed — ice is still H₂O, just in a different state.
• The change can be reversed — ice melts back to water when heated.
• The chemical composition of water remains the same before and after freezing.
• Since only the physical state changes and no new substance is formed, it is a physical change.

Intext Question 9 (Page 60)

Q: Why is burning of LPG considered a chemical change?

Burning of LPG (Liquefied Petroleum Gas) is a chemical change because:

1. New substances are formed: When LPG burns, it reacts with oxygen to form carbon dioxide and water vapour. These are entirely new substances with different properties from LPG. 2. The change is irreversible: The carbon dioxide and water vapour produced cannot be converted back into LPG. 3. Heat and light are produced: Burning releases energy in the form of heat and light, which is a characteristic of chemical changes. 4. Chemical composition changes: The molecular composition of the products is different from that of LPG.

Intext Question 10 (Page 61)

Q: Explain how a metal rim is fixed on a wooden wheel. Can you think of any other examples where expansion is used?

How a metal rim is fixed on a wooden wheel: 1. The metal rim is made slightly smaller than the wooden wheel. 2. The metal rim is heated. On heating, the metal expands and its circumference increases. 3. The hot expanded rim is then placed around the wooden wheel. 4. On cooling, the metal rim contracts and fits tightly around the wheel.

This process uses the principle of expansion on heating and contraction on cooling.

Other examples where expansion is used:

• A glass bottle with a tight metal lid can be opened by running hot water over the lid. The metal lid expands on heating and becomes loose.
• Railway tracks have small gaps between sections to allow for expansion during hot weather. Without gaps, the tracks would bend.
• An electric fuse wire melts and breaks when excessive current flows, protecting the circuit.

Exercise Questions and Answers

Q1. Classify the following as reversible or irreversible changes:

(a) Drying of a wet shirt — Reversible The shirt can become wet again. Only the state (wet/dry) has changed.

(b) Painting of an iron gate — Reversible The paint can be removed, though it is difficult. The gate itself does not change chemically.

(c) Ploughing of a field — Reversible The field can be levelled back. The change is in the physical arrangement of soil.

(d) Melting of ice candy — Reversible The melted ice candy (water) can be frozen back into ice candy.

(e) Baking of a chapati — Irreversible Once the chapati is baked, the dough has been cooked and cannot be turned back into raw dough.

(f) Folding of a paper — Reversible The folded paper can be unfolded back to its original form.

(g) Cutting of a tree — Irreversible A cut tree cannot be joined back to its original form.

(h) Making a boat from a sheet of paper — Reversible The paper boat can be unfolded back into a flat sheet of paper.

Q2. Can you think of changes that are both reversible and irreversible? Can you give one example each?

Change that is reversible: Melting of ice

• When ice melts, it becomes water. When water is cooled, it freezes back into ice.
• The original substance (ice) can be obtained again.

Change that is irreversible: Burning of paper

• When paper burns, it turns into ash, smoke, and gases.
• The original substance (paper) cannot be obtained again.

Q3. Give two examples each of reversible and irreversible changes.

Reversible changes: 1. Melting of ice — ice turns to water; water can be frozen back to ice. 2. Stretching a rubber band — it can return to its original shape when released.

Irreversible changes: 1. Burning of wood — ash and smoke are formed; wood cannot be obtained again. 2. Cooking of rice — raw rice changes to cooked rice; cannot be reversed.

Q4. A drawing sheet changes when you draw a picture on it. Can you reverse this change? Is this a reversible or irreversible change?**

It depends on what is used to draw:

If drawn with a pencil: This is a reversible change — the drawing can be erased with an eraser and the sheet can be restored close to its original state.

If drawn with a pen or permanent marker: This is an irreversible change — the ink marks cannot be completely erased. The sheet cannot be restored to its original blank state.

If painted with watercolours: This is generally an irreversible change — the paper absorbs the paint and cannot be fully restored to its original state.

Q5. Is cutting of a piece of paper a reversible or irreversible change?

Cutting of a piece of paper is an irreversible change.

Reason: Once a piece of paper is cut, the two pieces cannot be joined back to form the original piece of paper in exactly the same way. Even if we use tape or glue, the paper will not be the same as before. The cut creates new edges that were not there originally. Since we cannot get back the original piece of paper, this is an irreversible change.

Q6. State whether the following are physical or chemical changes:

(a) Melting of ice — Physical change Only the state changes from solid to liquid. No new substance is formed. The composition remains H₂O.

(b) Burning of a candle — Chemical change The wax burns to produce carbon dioxide, water vapour, and soot. New substances are formed.

(c) Rusting of iron — Chemical change Iron reacts with oxygen and moisture to form iron oxide (rust). A new substance with different properties is formed.

(d) Cooking of food — Chemical change Raw food changes to cooked food. New substances are formed, and the taste, texture, and composition change.

(e) Boiling of water — Physical change Only the state changes from liquid to gas. No new substance is formed. Steam is still H₂O.

(f) Digestion of food — Chemical change Food is broken down into simpler substances by digestive enzymes. New substances are formed.

(g) Breaking of a glass tumbler — Physical change The glass breaks into pieces but no new substance is formed. Only the shape changes.

(h) Formation of curd from milk — Chemical change Milk changes into curd due to the action of bacteria. A new substance (curd) with different properties is formed.

Q7. Which of the following are physical changes and which are chemical changes?

(a) Baking of bread — Chemical change New substances are formed. The dough changes into bread with different properties.

(b) Freezing of water — Physical change Only the state changes from liquid to solid. No new substance is formed.

(c) Burning of LPG — Chemical change LPG reacts with oxygen to form carbon dioxide and water vapour. New substances are formed.

(d) Melting of wax — Physical change Only the state changes from solid to liquid. The wax is still wax.

(e) Breaking of a chalk piece — Physical change The chalk breaks but no new substance is formed. Only the shape and size change.

(f) Souring of milk — Chemical change Milk turns sour due to bacterial action. New substances (lactic acid) are formed.

(g) Stretching of a spring — Physical change The spring returns to its original shape. No new substance is formed.

(h) Ripening of fruit — Chemical change The fruit's composition changes. New substances are formed, changing taste and texture.

Q8. Explain why a burning candle shows both physical and chemical changes.

A burning candle shows both physical and chemical changes:

Physical changes observed: 1. The solid wax near the flame melts into liquid wax. This is a change of state from solid to liquid, which is a physical change. 2. The liquid wax rises up the wick by capillary action. 3. Some of the melted wax solidifies again when it moves away from the flame.

Chemical changes observed: 1. The wax vapour near the flame burns (combustion) and produces carbon dioxide, water vapour, and sometimes soot. These are new substances. 2. Heat and light are produced during burning, which are characteristics of chemical changes. 3. The wax that burns cannot be recovered — it is permanently changed.

Conclusion: The melting of wax is a physical change, but the burning of wax is a chemical change. Since both types of changes occur simultaneously, a burning candle shows both physical and chemical changes.

Q9. When a candle burns, both physical and chemical changes take place. Explain.

When a candle burns, the following changes take place:

Physical changes:

• The solid wax near the wick melts due to the heat of the flame and forms liquid wax.
• The liquid wax rises through the wick by capillary action.
• The melted wax changes from solid state to liquid state — only the physical state changes.
• This change can be reversed: if the flame is extinguished, the liquid wax solidifies again.

Chemical changes:

• The liquid wax evaporates and the wax vapour burns in the flame.
• The burning wax reacts with oxygen in the air to produce carbon dioxide gas and water vapour.
• New substances (carbon dioxide, water vapour, soot) are formed that have different properties from wax.
• Heat and light are released during the combustion.
• This change is irreversible — the burnt wax cannot be recovered.

Thus, a burning candle demonstrates both physical change (melting of wax) and chemical change (burning of wax vapour).

Multiple Choice Questions (MCQs)

Common Mistakes to Avoid

Exam Checklist

• [ ] Can you define reversible and irreversible changes with two examples each?
• [ ] Can you define physical and chemical changes with two examples each?
• [ ] Can you explain why a burning candle shows both physical and chemical changes?
• [ ] Can you classify any given change as reversible/irreversible and physical/chemical?
• [ ] Can you explain how expansion and contraction are used to fix a metal rim on a wheel?
• [ ] Do you know that some physical changes can be irreversible (e.g., breaking glass)?
• [ ] Can you explain rusting as a chemical change?
• [ ] Can you distinguish between evaporation (physical) and burning (chemical)?
• [ ] Can you explain why cooking of food is a chemical change?
• [ ] Can you give examples of changes caused by heating and cooling?

Quick Reference Table: Types of Changes