NCERT Solutions for Class 7 Science (Curiosity) Chapter 5: Changes Around Us – Physical and Chemical (NCERT 2026–27)

These Class 7 Science Curiosity Chapter 5 solutions cover Changes Around Us: Physical and Chemical from the new NCF-2023 textbook (session 2026–27). Every question of the end-of-chapter exercise “Let Us Enhance Our Learning” is reproduced exactly as printed and solved step by step, along with key concepts, extra practice, MCQs, assertion–reason questions and FAQs to help you explain the changes clearly.

Class: 7 Subject: Science Book: Curiosity Chapter: 5 Topic: Physical & Chemical Changes Session: 2026–27
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Class 7 Science Curiosity Chapter 5 Solutions – Overview

Chapter 5 of Curiosity, Changes Around Us: Physical and Chemical, explores the many changes we observe every day — ice melting, a bud blooming, a banana ripening, water boiling. It groups all such changes into two big types. In a physical change, only physical properties like shape, size or state change and no new substance is formed (folding paper, melting ice, crushing chalk). In a chemical change, one or more new substances are formed through a chemical reaction (lime water turning milky, rusting, burning of magnesium, curdling of milk). The chapter also studies combustion and the fire triangle (fuel + oxygen + ignition temperature), explains how some changes can be reversed while others cannot, looks at desirable vs undesirable changes, and ends with slow natural changes — the weathering and erosion of rocks that finally form soil.

Key Concepts & Definitions

Physical change: a change in which a substance undergoes a change in its physical properties (shape, size, state) and no new substance is formed. Examples: melting ice, folding paper, crushing chalk, boiling water.

Chemical change: a change in which one or more new substances are formed through a chemical reaction; it can be shown by a chemical equation. Examples: lime water turning milky, rusting, burning magnesium, curdling of milk.

Test for carbon dioxide: the gas turns colourless lime water milky (cloudy) due to the formation of insoluble calcium carbonate.
Calcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate (insoluble) + Water.

Combustion: a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Substances that undergo combustion are combustible substances (wood, paper, cotton, kerosene).

Ignition temperature: the lowest (minimum) temperature at which a substance catches fire.

Fire triangle: combustion needs three things together — (i) a combustible substance (fuel), (ii) oxygen, and (iii) heat to reach the ignition temperature.

Reversible vs irreversible: some changes can be reversed to get the original substance back (melting ice); some cannot (making popcorn).

Weathering: physical and chemical changes that break large rocks into smaller pieces, eventually forming soil. Erosion: the breaking and carrying away of rock, soil and sediments by wind and flowing water (a physical change).

“Let Us Enhance Our Learning” — NCERT Solutions

Original explanations for Questions 1–10 on pages 69–71 of NCERT Curiosity, Reprint 2026–27. The completed classification diagram is provided below.

1. Which of the following statements are the characteristics of a physical change? (i) The state of the substance may or may not change. (ii) A substance with different properties is formed. (iii) No new substance is formed. (iv) The substance undergoes a chemical reaction. (a) (i) and (ii)    (b) (ii) and (iii)    (c) (i) and (iii)    (d) (iii) and (iv)

ANSWER Correct option: (c) (i) and (iii). In a physical change the state may or may not change, e.g. melting ice changes state while folding paper does not — so (i) is correct. The key feature of a physical change is that no new substance is formed — so (iii) is correct. Statements (ii) and (iv) describe a chemical change, not a physical one.

2. Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these? (i) Stitching cloth to a shirt (ii) Twisting of straight string (iii) Making idlis from a batter (iv) Dissolving sugar in water (v) Drawing water from a well (vi) Ripening of fruits (vii) Boiling water in an open pan (viii) Rolling up a mat (ix) Grinding wheat grains to flour (x) Forming of soil from rocks

ANSWER
ChangeReversed?Reason
(i) Stitching cloth to a shirtYesStitching alone can be undone by removing the thread. If making the shirt also involved cutting cloth, the original uncut piece cannot be restored merely by unstitching.
(ii) Twisting of straight stringYesThe string can be untwisted to its original form (physical).
(iii) Making idlis from a batterNoCooking forms new substances; idlis cannot become batter again (chemical).
(iv) Dissolving sugar in waterYesSugar can be recovered by slow evaporation or crystallisation without heating it until it decomposes (physical).
(v) Drawing water from a wellYesIn principle its position can be restored; only location changed. Do not pour used or contaminated water into a drinking-water source.
(vi) Ripening of fruitsNoNew substances form; a ripe fruit cannot become unripe (chemical).
(vii) Boiling water in an open panYesThe steam (water vapour) can be condensed back to water (physical).
(viii) Rolling up a matYesThe mat can be unrolled to its original shape (physical).
(ix) Grinding wheat grains to flourNoThe flour cannot be put back together into whole grains.
(x) Forming of soil from rocksNoSoil cannot be turned back into the original rock (weathering, very slow).
 Where one may not be sure: “Boiling water in an open pan” can confuse us, because in an open pan the steam escapes and seems lost; but if we collect and condense that vapour we get water back, so it is reversible. State your assumptions: vapour escaping into the surroundings cannot easily be collected again, and stitching may or may not include cutting the cloth. Reversibility is separate from whether a new substance forms.

3. State whether the following statements are True or False. In case a statement is False, write the correct statement. (i) Melting of wax is necessary for burning a candle. (True/False) (ii) Collecting water vapour by condensing involves a chemical change. (True/False) (iii) The process of converting leaves into compost is a chemical change. (True/False) (iv) Mixing baking soda with lemon juice is a chemical change. (True/False)

ANSWER (i) True. The solid wax must first melt and then evaporate; the wax vapour is what actually burns to give the flame. (ii) False. Correct statement: Collecting water vapour by condensing is a physical change, because water vapour simply changes back to liquid water and no new substance is formed. (iii) True. Bacteria and fungi decompose leaves and form a mixture containing new substances (compost), so it is a chemical change. (iv) True. Baking soda reacts with lemon juice (an acid) to produce a new gas, carbon dioxide, so a new substance is formed — a chemical change.
A burning candle: solid wax melts to liquid and vaporises, which are physical changes; wax vapour reacts with oxygen in the flame, a chemical change.
A candle combines changes of state with combustion. Observe only with adult supervision. Tap to view at full size.

4. Fill in the blanks in the following statements: (i) Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to ________, and this is a ________ change. (ii) Folding a handkerchief is a ________ change and can be ________. (iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called ________, and this is a ________ change. (iv) Magnesium, when burnt in air, produces a substance called ________. The substance formed is ________ in nature. Burning of magnesium is a ________ change.

ANSWER (i) The brown deposits are due to rusting (iron oxide), and this is a chemical change. (ii) Folding a handkerchief is a physical change and can be reversed. (iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called combustion, and this is a chemical change. (iv) Magnesium, when burnt in air, produces a substance called magnesium oxide. The substance formed is basic in nature. Burning of magnesium is a chemical change.

5. Are the changes of water to ice and water to steam, physical or chemical? Explain.

ANSWER Both are physical changes. When water freezes to ice and when water boils to steam, only the state of the substance changes (liquid ↔ solid, liquid ↔ gas) — the substance remains water (H₂O) throughout. No new substance is formed, and both changes can be reversed: ice can melt back into water, and steam can be condensed back into water. Since there is no new substance, these are physical changes.

6. Is curdling of milk a physical or chemical change? Justify your statement.

ANSWERIt is a chemical change in this textbook context. During curd formation, bacteria convert some milk sugar (lactose) into lactic acid. The increased acidity makes milk proteins coagulate, changing texture and taste.We cannot restore the original milk by a simple physical step. The decisive evidence is chemical reaction and new substances, not irreversibility alone: tearing paper is physical even though it cannot readily be undone.

7. Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?

ANSWER The formation of soil from rocks (weathering) involves both physical and chemical changes. Physical changes: temperature changes, growing roots of trees and freezing of water in cracks break large rocks into smaller pieces; wind and flowing water carry and break them further (erosion). Chemical changes: water and chemicals present in water react with the minerals in the rock and change its composition — for example, iron in black basalt rock changes to red iron oxide. Together these slow physical and chemical changes are called weathering and finally form soil. The change cannot be reversed.

8. Read the following story titled ‘Eco-friendly Prithvi’, and tick the most appropriate option(s) given in the brackets. Provide a suitable title of your choice for the story. Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes/chemical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change/chemical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (physical change/chemical change). He decides to plant seeds in the compost and water them regularly. After a few days, he notices that the seeds begin to germinate and small plants start to grow, eventually blooming into colourful flowers (physical change/chemical change). His efforts are appreciated by all his family members.

ANSWER Chopping vegetables, peeling potatoes and cutting fruits → physical changes (only shape/size changes). Collecting the seeds, fruits and vegetable peels into a clay pot → physical change (only the position of the materials changes). Decomposing to form compost → chemical change (a new substance, compost, is formed by bacteria and fungi). Seeds germinating and growing into plants and flowers → chemical change (growth forms new substances and is irreversible). Suitable title (any one): “From Waste to Wealth” / “Prithvi’s Compost Garden” / “Nothing Goes Waste”. (Your own meaningful title is accepted.)

9. Some changes are given here. Write physical changes in the area marked ‘A’ and chemical changes in the area marked ‘B’. Enter the changes which are both physical and chemical in the area marked ‘C’. Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium and Mixing baking soda with vinegar.

ANSWER
A — Physical changesB — Chemical changesC — Both physical & chemical
Tearing of paper; Melting of ice; Folding of clothes Rusting; Curdling of milk; Ripening of fruits; Burning of magnesium; Mixing baking soda with vinegar Process of burning a candle
Burning of a candle goes in ‘C’ because the melting, flowing and solidifying of wax are physical changes, while the burning of the wax vapour is a chemical change — both happen together.
Physical area A contains tearing paper, melting ice and folding clothes. Chemical area B contains rusting, curdling, ripening, burning magnesium and baking soda with vinegar. Shared area C contains the process of burning a candle.
Question 9: completed A–C–B classification. The overlap means one process contains both kinds of change. Tap to view at full size.

10. The experiments shown in Fig. 5.11a, b, c, and d were performed. Find out in which case(s) did lime water turn milky and why? (a) Vinegar and baking soda — Lime water (b) Lemon juice and vinegar — Lime water (c) Vinegar and common salt — Lime water (d) Lemon juice and baking soda — Lime water

ANSWER Lime water turns milky only in cases (a) and (d). Why: Lime water turns milky when carbon dioxide gas is passed through it. Carbon dioxide is produced only when an acid reacts with baking soda (sodium hydrogen carbonate). In (a) vinegar (an acid) + baking soda and in (d) lemon juice (an acid) + baking soda, this reaction occurs and CO₂ is released, turning the lime water milky. In (b) lemon juice + vinegar are both acids that do not react to give CO₂, and in (c) vinegar + common salt do not produce CO₂; so the lime water stays clear in (b) and (c).

Extra Practice Questions

These questions and MCQs are ClearStudy practice, separate from the NCERT exercise.

Short Answer Type Questions

Q1. Define a physical change with one example.

ANSWERA physical change is one in which only the physical properties (shape, size, state) of a substance change and no new substance is formed, e.g. melting of ice.

Q2. How can you test whether a gas is carbon dioxide?

ANSWERIn this classroom experiment, pass the gas into fresh lime water using the teacher’s apparatus. Carbon dioxide produces a milky calcium carbonate precipitate. With prolonged passage of excess carbon dioxide the milkiness can disappear as soluble hydrogen carbonate forms; observe the initial change.

Q3. What are the three requirements for combustion (the fire triangle)?

ANSWER(i) A combustible substance (fuel), (ii) oxygen, and (iii) heat that allows the fuel to reach its ignition temperature.

Q4. Why does a candle covered with a glass tumbler stop burning after some time?

ANSWERCovering the candle prevents a continuing supply of fresh air. Burning consumes oxygen, and the flame goes out when there is too little oxygen to sustain combustion; it need not consume every oxygen molecule. Hot glass and flames must be handled by an adult.

Q5. Why are some changes called undesirable? Give one example.

ANSWERUndesirable changes are harmful or unwanted, such as the rusting of iron or the decay of stored food. They damage or spoil useful materials.

Long Answer Type Questions

Q1. Distinguish between physical and chemical changes with two examples of each.

ANSWERA physical change alters only the physical properties of a substance — its shape, size or state — and no new substance is formed; these changes may be reversible or irreversible. Examples: melting of ice and folding of paper. A chemical change produces one or more new substances through a chemical reaction and is usually irreversible; it can be shown by a chemical equation. Examples: rusting of iron and burning of magnesium ribbon. The key difference is that no new substance forms in a physical change, whereas a new substance always forms in a chemical change.

Q2. Explain, with the activity of blowing air into lime water, how a chemical change is identified.

ANSWERWhen we blow exhaled air through a straw into a glass of tap water, only bubbles form and the water stays clear — a physical change. But when we blow the same air into lime water, the lime water turns milky and a white substance later settles at the bottom. This white substance is calcium carbonate, formed when carbon dioxide in our breath reacts with calcium hydroxide (lime water): Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water. The formation of this new, insoluble substance shows that a chemical reaction has taken place. Therefore, the milkiness of lime water is clear evidence of a chemical change.

Follow the teacher’s lime-water demonstration. Never suck liquid through a straw; lime water must not enter the mouth or eyes. Bubbling alone does not prove a chemical reaction—for example, boiling water forms bubbles without making a new substance.

Q3. Describe the slow natural changes of weathering and erosion and how soil is formed.

ANSWERLarge rocks slowly break into smaller pieces through weathering, which includes both physical and chemical changes. Physically, changing temperatures, growing tree roots and the freezing of water in cracks break the rocks apart. Chemically, water and substances dissolved in it react with the rock — for example, iron in black basalt changes into red iron oxide. Erosion then occurs when wind and flowing water break down rock pebbles, soil and sediments and carry them from one place to another; this is a physical change. River pebbles become smooth, and when the water or wind slows down, the carried material settles and hardens into new rock over thousands of years. Together, these slow weathering and erosion processes eventually form the soil we use.

MCQs & Answer Key

1. Which of the following is a chemical change?

(a) Melting of ice    (b) Folding of paper    (c) Rusting of iron    (d) Tearing of cloth

2. In a physical change:

(a) a new substance is formed    (b) no new substance is formed    (c) the substance always burns    (d) heat and light are always given out

3. Lime water turns milky when we pass through it:

(a) oxygen    (b) hydrogen    (c) carbon dioxide    (d) nitrogen

4. The white substance that makes lime water milky is:

(a) calcium carbonate    (b) calcium hydroxide    (c) magnesium oxide    (d) sodium chloride

5. Burning of magnesium ribbon produces:

(a) magnesium chloride    (b) magnesium oxide    (c) magnesium carbonate    (d) calcium oxide

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

(a) boiling point    (b) melting point    (c) ignition temperature    (d) freezing point

7. Which gas of air supports combustion?

(a) Nitrogen    (b) Oxygen    (c) Carbon dioxide    (d) Hydrogen

8. The burning of a candle involves:

(a) only a physical change    (b) only a chemical change    (c) both physical and chemical changes    (d) no change

9. Which of the following changes can be reversed?

(a) Curdling of milk    (b) Making popcorn    (c) Melting of ice    (d) Rusting of iron

10. The breaking and carrying away of rock and soil by wind and flowing water is called:

(a) combustion    (b) erosion    (c) condensation    (d) rusting

Answer key: 1-(c), 2-(b), 3-(c), 4-(a), 5-(b), 6-(c), 7-(b), 8-(c), 9-(c), 10-(b).

Assertion–Reason Questions

For each question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Rusting of iron is a chemical change.

Reason: During rusting a new substance, iron oxide, is formed.

A-R 2. Assertion: Melting of ice is a physical change.

Reason: A new substance is formed when ice melts.

A-R 3. Assertion: A candle covered with a glass tumbler stops burning after some time.

Reason: The supply of oxygen needed for combustion is cut off.

A-R 4. Assertion: Lime water turns milky when carbon dioxide is passed through it.

Reason: Carbon dioxide reacts with lime water to form insoluble calcium carbonate.

A-R 5. Assertion: Curdling of milk is a physical change.

Reason: Curd formation involves chemical reactions and cannot be undone by a simple physical process.

Answer key: 1-(A), 2-(C), 3-(A), 4-(A), 5-(D).

How to write a clear answer

Always justify whether a change is physical or chemical by checking the single test — was a new substance formed? If yes, it is chemical (rusting, burning, curdling, ripening); if not, it is physical (melting, folding, dissolving). Remember the exact word equations for the lime-water test and the burning of magnesium, and note that the burning of a candle is both physical and chemical. For combustion answers, name all three parts of the fire triangle and define ignition temperature precisely.

Common Mistakes to Avoid

Watch out for these

  • Thinking a change of state (water to steam) is chemical — it is physical, since no new substance forms.
  • Saying dissolving sugar in water is irreversible — sugar can be recovered by evaporation, so it is reversible and physical.
  • Forgetting why lime water turns milky in these experiments: carbon dioxide reacts with it to form insoluble calcium carbonate. This is a useful classroom test in the stated context, not an infallible identification of any unknown gas.
  • Writing that burning a candle is purely chemical — the melting and evaporation of wax are physical, so it is both.
  • Confusing weathering (breaking and chemically altering rock in place) with erosion (carrying the broken material away).
  • Assuming all chemical changes are undesirable — cooking food and forming compost are useful chemical changes.

Frequently Asked Questions

What is Class 7 Science Curiosity Chapter 5 about?

Chapter 5, Changes Around Us: Physical and Chemical, explains the two main types of changes — physical changes (no new substance, e.g. melting ice) and chemical changes (new substances form, e.g. rusting). It also covers the carbon dioxide and lime-water test, combustion and the fire triangle, reversible and irreversible changes, desirable and undesirable changes, and slow natural changes like weathering and erosion.

How do you tell a physical change from a chemical change?

Ask whether a new substance is formed. In a physical change only properties like shape, size or state change and no new substance is formed (and it is usually reversible). In a chemical change one or more new substances form through a chemical reaction, and it is usually irreversible.

Why does lime water turn milky?

Lime water (calcium hydroxide) turns milky when carbon dioxide is passed through it, because the gas reacts to form insoluble calcium carbonate, which makes the liquid look cloudy. This is the standard test for carbon dioxide.

Are these Class 7 Science Curiosity Chapter 5 solutions free?

Yes. All solutions are free and follow the official NCERT Curiosity textbook for session 2026–27.

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