Chapter 5: Changes Around Us: Physical and Chemical (Chemistry)
Answers to all "Let Us Enhance Our Learning" questions of Chapter 5, Changes Around Us: Physical and Chemical (NCERT Class 7 Science, Curiosity, 2026-27): reversible and irreversible changes, physical and chemical changes, rusting, combustion, and the lime-water test for carbon dioxide. All 10 questions are answered, with the key answer highlighted.
In a physical change no new substance is formed (only the shape, size or state changes, and it can often be reversed). In a chemical change one or more new substances with new properties are formed; it is usually not reversible. Carbon dioxide turns lime water milky.
Which of the following statements are 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)
Solution
(c) (i) and (iii). In a physical change no new substance is formed, and the state may or may not change (melting changes the state; tearing paper does not).
Predict which of the following changes can be reversed and which cannot. If you are not sure, write that down and say why. (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
Solution
Change
Can it be reversed?
(i) Stitching cloth to a shirt
Yes (unstitch it), though the cut pieces cannot become the original whole cloth again
(ii) Twisting a straight string
Yes
(iii) Making idlis from batter
No (new substance on cooking)
(iv) Dissolving sugar in water
Yes (evaporate the water)
(v) Drawing water from a well
Yes (pour it back)
(vi) Ripening of fruits
No
(vii) Boiling water in an open pan
Yes in principle (the steam can be condensed back), but in an open pan the steam escapes into the air
(viii) Rolling up a mat
Yes
(ix) Grinding wheat into flour
No (the flour cannot be made into grains again), though no new substance is formed
(x) Forming of soil from rocks
No (takes very long and cannot be undone)
Why we may be unsure: some changes (like stitching, grinding wheat or boiling water in an open pan) are physical, since no new substance forms, but are practically difficult to reverse completely. Reversibility and "physical or chemical" are not always the same thing.
Reversible: twisting string, dissolving sugar, drawing water, rolling a mat (and unstitching). Not reversible: making idlis, ripening fruits, grinding wheat, soil formation. Stitching and boiling in an open pan are partly reversible, which is why one may be unsure.
State whether True or False; correct the false statements. (i) Melting of wax is necessary for burning a candle. (ii) Collecting water vapour by condensing involves a chemical change. (iii) The process of converting leaves into compost is a chemical change. (iv) Mixing baking soda with lemon juice is a chemical change.
Solution
(i) True. The wax first melts and rises up the wick, where the wax vapour burns.
(ii) False. Condensing water vapour into water is a physical change (only the state changes).
(iii) True. Leaves decompose into a new substance, compost.
(iv) True. A new gas (carbon dioxide) is formed with fizzing.
(i) T (ii) F, condensing is a physical change (iii) T (iv) T
Fill in the blanks: (i) Nalini observed brown deposits on the handle of her cycle. 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.
Solution
(i) rusting; chemical
(ii) physical; reversed
(iii) combustion; chemical
(iv) magnesium oxide; basic; chemical
(i) rusting, chemical (ii) physical, reversed (iii) combustion, chemical (iv) magnesium oxide, basic, chemical
Are the changes of water to ice and water to steam physical or chemical? Explain.
Solution
Both are physical changes. Only the state of water changes (liquid to solid, liquid to gas); no new substance is formed: ice and steam are still water. Both changes can be reversed: ice melts back into water and steam condenses back into water.
Physical: only the state changes, no new substance forms, and both can be reversed.
Is curdling of milk a physical or chemical change? Justify your statement.
Solution
Chemical change. Curd is a new substance with a different taste, smell and texture from milk. It is formed by the action of bacteria on milk, and curd cannot be turned back into milk.
Chemical: curd is a new substance with different properties, formed by bacteria, and cannot be changed back into milk.
Natural factors such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Solution
It is both. Rocks are broken into smaller pieces by wind, water, heating and cooling: a physical change. At the same time, rainwater and air react with minerals in the rocks and change them into new substances (for example, minerals in rocks slowly react with water and oxygen), and decaying plants add humus: chemical changes. So soil formation involves both physical and chemical changes. It is also very slow and not reversible.
Both: rocks are broken into small particles (physical), and their minerals react with water and air while humus is added (chemical).
Read the story 'Eco-friendly Prithvi', tick the most appropriate option(s) and give a suitable title.
Solution
Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (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 (chemical change). His efforts are appreciated by all his family members.
Title: "From Peels to Petals" (or "Waste to Wonder").
Chopping and cutting: physical; collecting in a pot: physical; forming compost: chemical; germination and growth into flowers: chemical. Title: e.g. "From Peels to Petals".
Write physical changes in 'A', chemical changes in 'B', and changes that are both physical and chemical in '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; mixing baking soda with vinegar.
Solution
A (physical): tearing of paper, melting of ice, folding of clothes.
B (chemical): rusting, curdling of milk, ripening of fruits, burning of magnesium, mixing baking soda with vinegar.
C (both): process of burning a candle: the wax melts and vaporises (physical) and the wax vapour burns (chemical).
A: tearing paper, melting ice, folding clothes. B: rusting, curdling milk, ripening fruits, burning magnesium, baking soda with vinegar. C: burning a candle.
The experiments in Fig. 5.11 were performed: (a) vinegar and baking soda (b) lemon juice and vinegar (c) vinegar and common salt (d) lemon juice and baking soda, each with the gas passed through lime water. In which case(s) did lime water turn milky and why?
Solution
Lime water turns milky in (a) and (d). In both, an acid (vinegar or lemon juice) reacts with baking soda, producing carbon dioxide gas, which turns lime water milky.
In (b) two acids are mixed, and in (c) an acid is mixed with common salt; no carbon dioxide is produced, so the lime water stays clear.
(a) and (d): acid + baking soda produces carbon dioxide, which turns lime water milky; (b) and (c) produce no carbon dioxide.
