CLASS 7 · SCIENCE · CHAPTER 5

Changes Around Us: Physical and Chemical

Simple answers to all 10 end questions, in-text activities, and five exploratory projects.

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Textbook pages 69–71

Question 1

Which statements describe a physical change? (i) The state may or may not change. (ii) A substance with different properties forms. (iii) No new substance forms. (iv) A chemical reaction takes place.

Steps only

A physical change changes appearance or state but forms no new substance.

Answer

Question 2

Say whether each change can be reversed: (i) stitching cloth to a shirt, (ii) twisting a straight string, (iii) making idlis from batter, (iv) dissolving sugar in water, (v) drawing water from a well, (vi) ripening fruit, (vii) boiling water in an open pan, (viii) rolling a mat, (ix) grinding wheat into flour, (x) forming soil from rocks. Explain any uncertainty.

Steps only

Usually reversible: (i), (ii), (iv), (v), (vii, with condensation), (viii). Not practically reversible: (iii), (vi), (ix), (x).

Answer

Reversibility depends on whether the original can be recovered and what extra steps are allowed.

Question 3

Mark True or False and correct the false statements: (i) Melting wax is necessary for a candle to burn. (ii) Condensing water vapour is a chemical change. (iii) Turning leaves into compost is a chemical change. (iv) Mixing baking soda with lemon juice is a chemical change.

Steps only

(i) T (ii) F (iii) T (iv) T

Answer

True, False, True, True.

Question 4

Fill in the blanks: (i) Brown deposits on a cycle handle are due to _ and are a _ change. (ii) Folding a handkerchief is a _ change and can be _. (iii) Reaction with oxygen releasing heat is called _ and is a _ change. (iv) Burning magnesium produces _, which is _ in nature; burning magnesium is a ___ change.

Steps only

(i) rusting, chemical (ii) physical, reversible (iii) combustion, chemical (iv) magnesium oxide, basic, chemical

Answer

Rusting; physical/reversible; combustion/chemical; magnesium oxide/basic/chemical.

Question 5

Are changing water to ice and changing water to steam physical or chemical changes? Explain.

Steps only

Water ⇄ ice and water ⇄ steam are changes of state; no new substance forms.

Answer

Both changes are physical and reversible.

Question 6

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

Steps only

New substances form and the change cannot easily be reversed.

Answer

Curdling milk is a chemical change.

Question 7

Is the formation of soil from rocks by wind and rain a physical or chemical change? Why?

Steps only

Breaking rocks is physical; reactions that make new minerals are chemical.

Answer

Soil forms through both physical and chemical weathering.

Question 8

In the story, classify chopping vegetables, collecting peels in a pot, decomposition into compost, and seeds growing into plants as physical or chemical changes. Give a suitable title.

Steps only

Chopping/collecting—physical; decomposition—chemical; growth involves chemical changes.

Answer

A suitable title is “From Kitchen Scraps to a Garden.”

Question 9

Sort these into physical changes (A), chemical changes (B), and changes that involve both (C): burning a candle, tearing paper, rusting, curdling milk, ripening fruit, melting ice, folding clothes, burning magnesium, mixing baking soda with vinegar.

Steps only

A: tearing, melting ice, folding. B: rusting, curdling, ripening, burning magnesium, baking soda + vinegar. C: burning candle.

Answer

A: tearing paper, melting ice, folding clothes. B: rusting, curdling, ripening, burning magnesium, baking soda + vinegar. C: burning candle.

Question 10

In which experiments will lime water turn milky: (a) vinegar and baking soda, (b) vinegar and common salt, (c) lemon juice and vinegar, or (d) lemon juice and baking soda? Why?

Steps only

(a) and (d): acid + baking soda releases CO₂, which turns lime water milky.

Answer

Cases (a) and (d).

Chapter activities and learning prompts

Textbook pages 58–68, 71–72

Question Activities 5.1–5.2

Classify common changes such as melting ice, chopping vegetables, boiling water, making popcorn, cutting paper, adding beetroot extract to water, burning wood, drying clothes, making dough balls, and rolling chapatis.

Steps only

Physical: melting, chopping, boiling, drying, mixing colour, shaping dough. Chemical: popping corn, burning wood.

Answer

Decide whether a new substance forms.

Question Sections 5.1–5.2

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

Steps only

Physical: same substance, different form. Chemical: new substance forms.

Answer

Check whether a new substance is made.

Question Activities 5.5–5.6

Why does a covered candle go out, and what three things are needed for combustion?

Steps only

Fire needs fuel + oxygen + heat to ignition temperature.

Answer

A covered candle goes out when its oxygen supply becomes too low.

Question Activities 5.7–5.8

Which changes happen when a candle burns, and which can be reversed?

Steps only

Wax melts (physical) and vapour burns (chemical); only the melting can be reversed by cooling.

Answer

Candle burning includes reversible physical and irreversible chemical changes.

Question Sections 5.7.1–5.7.2

What are weathering and erosion, and how can they help form soil?

Steps only

Weathering breaks rock down; erosion carries the pieces away.

Answer

Weathering and erosion help make and move soil and sediments.

Question Opening discussion

Classify the changes in the opening examples: ice becomes water, cold water warms, a bud becomes a flower, and a banana ripens.

Steps only

Melting and warming water are physical; flowering and fruit ripening involve biological and chemical changes.

Answer

The first two are physical changes; the last two involve chemical changes.

Question Activity 5.2

What happens when paper is folded and unfolded, a balloon is inflated or pricked, and chalk is crushed? Are these physical changes, and can they be reversed?

Steps only

The material remains the same, so all are physical changes; folding and deflating are reversible, but pricking and crushing are hard to reverse.

Answer

Physical changes can be reversible or difficult to reverse.

Question Activities 5.3–5.4

What is seen when air is blown into tap water and lime water? What happens when baking soda is mixed with water, and when it is mixed with lemon juice?

Steps only

Carbon dioxide turns lime water milky. Baking soda fizzes with lemon juice, but not with plain water.

Answer

The milky lime water is evidence of carbon dioxide.

Question Section 5.6

Give examples of desirable and undesirable changes. Can the same change be useful in one situation and harmful in another?

Steps only

Cooking and composting can be useful; food spoilage and rusting can be harmful.

Answer

A change may be desirable or undesirable depending on the situation.

Question Exploratory project

Why does a dried lemon-juice message turn brown when warmed? Can this change be reversed?

Steps only

Heating makes the lemon-juice writing brown through a chemical change; it is not easily reversed.

Answer

The message appears because heating causes browning.

Question Exploratory project

Suggest steps that can reduce landslides and soil or rock erosion in hilly regions.

Steps only

Protect vegetation, manage water safely, avoid risky slope changes, and follow warnings.

Answer

Reduce erosion and avoid unsafe activity on unstable slopes.

Question Exploratory project

What happens in the yeast, sugar, and water experiment, and how can the gas be tested?

Steps only

Yeast + sugar releases CO₂; it inflates the balloon and turns lime water milky.

Answer

The yeast experiment shows a chemical change that produces carbon dioxide.

Question Exploratory project

Can a chameleon’s colour change be reversed? Is it physical or chemical?

Steps only

The colour can change back; it is a reversible change in how the skin displays colour.

Answer

The colour change is generally reversible and does not form a new substance.

Question Exploratory project: Kitchen observation

Observe activities in the kitchen and note changes that can be reversed. Are these physical or chemical changes?

Steps only

Melting and condensing water are reversible physical changes. Cutting vegetables is physical; cooking food is chemical and cannot be undone.

Answer

Kitchen changes can be physical or chemical. Melting and condensing water are reversible physical changes; cooking food is a chemical change.