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NCERT Solutions Curiosity Chapter 5 Chapter-end exercise, Questions 1–10 — Let Us Enhance Our Learning

Book page 69 Updated on2026-09-05

Q1.
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

(c) (i) and (iii)

StatementTrue of a physical change?Reason
(i) The state of the substance may or may not change.YesMelting ice changes state; folding paper does not. Both are physical.
(ii) A substance with different properties is formed.NoThat is a chemical change — the new substance has new properties.
(iii) No new substance is formed.YesThis is the defining test of a physical change.
(iv) The substance undergoes a chemical reaction.NoA chemical reaction is what produces a chemical change.
Why it happens: Statements (ii) and (iv) both describe a chemical change, so any option containing them must be wrong. Only (i) and (iii) survive, which is option (c).
Q2.
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
ChangeCan it be reversed?Reason
(i) Stitching cloth to a shirtYesThe stitches can be opened and the pieces of cloth come back — only tiny needle holes remain. A physical change.
(ii) Twisting of straight stringYesUntwist it and the string is straight again. Only the shape changed.
(iii) Making idlis from a batterNoFermentation and steaming form new substances. The cooked idli can never become batter again.
(iv) Dissolving sugar in waterYesEvaporate the water and the sugar is recovered. The sugar is only spread out among the water particles.
(v) Drawing water from a wellYesThe water can simply be poured back into the well. Only its position changed.
(vi) Ripening of fruitsNoNew substances give the ripe fruit its colour, softness, sweetness and smell. A ripe mango cannot become raw again.
(vii) Boiling water in an open panNot sure — see belowAs a change of state it is reversible; but in an open pan the steam escapes into the room and cannot be collected, so in practice the water is lost.
(viii) Rolling up a matYesUnroll it and the mat is flat again.
(ix) Grinding wheat grains to flourNoStill wheat, so it is a physical change — but the flour can never be pressed back into grains.
(x) Forming of soil from rocksNoWeathering involves chemical changes as well, and it takes thousands of years. Soil cannot be turned back into the parent rock.
Why I am not sure about (vii): The answer depends on the word open. Cover the pan with a lid and the steam condenses on the lid and drips back — the change is clearly reversed. Leave it open and the vapour mixes with the air of the room; the water still exists, but we have no way of collecting it back. So the change itself is reversible, while the experiment as described is not.
Tip: Item (ix) is a good reminder that irreversible does not mean chemical. Grinding wheat is a physical change that cannot be undone.
Q3.
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
StatementTrue / FalseCorrect statement and reason
(i) Melting of wax is necessary for burning a candle.TrueSolid wax cannot burn. It must first melt, rise up the wick and evaporate; it is the vapour of wax that burns.
(ii) Collecting water vapour by condensing involves a chemical change.FalseCorrect statement: Collecting water vapour by condensing involves a physical change. Only the state changes, from gas to liquid; the substance is water before and after.
(iii) The process of converting leaves into compost is a chemical change.TrueBacteria and fungi break the leaves down into completely new substances. Compost can never be turned back into leaves.
(iv) Mixing baking soda with lemon juice is a chemical change.TrueA new gas, carbon dioxide, is formed — it fizzes and turns lime water milky.
Q4.
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
StatementAnswer
(i) The brown deposits are due to ______, and this is a ______ change.rusting (the brown substance is rust, that is iron oxide); chemical
(ii) Folding a handkerchief is a ______ change and can be ______.physical; reversed
(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called ______, and this is a ______ change.combustion; chemical
(iv) Magnesium, when burnt in air, produces a substance called ______. The substance formed is ______ in nature. Burning of magnesium is a ______ change.magnesium oxide; basic; chemical
Why it happens: In (i), the iron of the cycle handle joins with the oxygen and moisture of the air to give a new brown substance — that is rusting, and a new substance means a chemical change. In (iv), the white powder left after the ribbon burns is magnesium oxide; when it is dissolved in water it turns red litmus blue, which shows that it is basic in nature.
Q5.
Are the changes of water to ice and water to steam, physical or chemical? Explain.
Answer

Both are physical changes.

Water (liquid) freezes → Ice (solid)   — and ice melts back into water
Water (liquid) boils → Steam (gas)   — and steam condenses back into water
Substance before = Substance after = water

Three reasons:

  1. No new substance is formed. Ice, water and steam are all the same substance in three different states.
  2. Only the state changes, which is a physical property.
  3. Both changes can be reversed — ice melts back to water, steam condenses back to water.
Why it happens: In a solid, the particles are packed closely and only vibrate. On heating they gain energy, move apart and slide over one another — that is a liquid. On further heating they fly far apart — that is a gas. Throughout, the particles themselves are unchanged, which is exactly why the change is physical.
Q6.
Is curdling of milk a physical or chemical change? Justify your statement.
Answer

Curdling of milk is a chemical change.

Justification:

  1. A new substance, curd, is formed. Its taste (sour), smell and texture (thick, set) are completely different from those of milk.
  2. The change cannot be reversed — curd can never be turned back into milk.
  3. The change is brought about by bacteria, which convert the sugar of milk into acid; the acid makes the milk protein set. This is a chemical reaction.
Why it happens: When a spoon of curd (the starter, jaman) is added to warm milk, the bacteria in it multiply and turn lactose, the sugar of milk, into lactic acid. The acid makes the protein of the milk clump together and set into a soft solid. New substances with new properties have appeared, so the change is chemical.
Did you know? Curdling is an undesirable change when milk goes sour on its own in summer, but exactly the same change is desirable when we set curd on purpose. Whether a change is welcome depends on the situation, not on the change.
Q7.
Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Answer

It is both — the formation of soil from rocks, called weathering, involves physical as well as chemical changes.

Physical part of weatheringChemical part of weathering
Temperature changes make rocks expand and contract until they crackWater, and chemicals dissolved in water, react with the minerals of the rock
Water freezing inside cracks expands and splits the rockIron in black basalt turns into red iron oxide (Fig. 5.10b)
Growing roots of trees widen cracks and break pieces offThe composition of the rock itself is altered — new substances are formed
Only size and shape change — no new substanceNew substances are formed
Why it happens: Wind and flowing water mostly break and carry away the rock, which is physical. Rain water, however, is not just water — it carries dissolved gases and chemicals that attack the rock and change what it is made of, which is chemical. Together, over thousands of years, they turn hard rock into soil. The change cannot be reversed.
Q8.
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
Part of the storyCorrect optionReason
He chops vegetables, peels potatoes, and cuts fruitsphysical changesOnly size and shape change; a chopped potato is still a potato
He collects the seeds, fruits, and vegetable peels into a clay potphysical changeOnly the position of the material changes; nothing new is formed
The materials decompose due to bacteria and fungi, forming compostchemical changeBacteria and fungi break the waste into completely new substances; compost cannot become peels again
The seeds germinate, plants grow and bloom into colourful flowerschemical changeGermination and growth are life processes in which new substances are built up inside the plant

A suitable title: “From Kitchen Waste to Colourful Flowers”.

Other equally good titles you may choose: “Prithvi's Compost Garden”, “Nothing is Waste”, “A Kitchen that Grows Flowers”, or “Waste to Wealth”.

Try This: Start a compost pot at home exactly as Prithvi did — vegetable peels and dry leaves in a clay pot, kept moist and turned over every few days. In about six to eight weeks you will have dark, crumbly compost, and you will have watched a chemical change happen with your own eyes.
Q9.
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 only B — Chemical only Tearing of paper Melting of ice Folding of clothes Rusting Curdling of milk Ripening of fruits Burning of magnesium Mixing baking soda with vinegar C — Both Process of burning a candle
The nine changes sorted into areas A, B and C.
AreaChangesReason
A — Physical changesTearing of paper; Melting of ice; Folding of clothesOnly shape, size or state changes — no new substance
B — Chemical changesRusting; Curdling of milk; Ripening of fruits; Burning of magnesium; Mixing baking soda with vinegarA new substance is formed in each case — iron oxide, curd, the substances of ripening, magnesium oxide, carbon dioxide
C — Both physical and chemicalProcess of burning a candleMelting, flowing, solidifying and evaporating of wax are physical; the burning of wax vapour is chemical
Q10.
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?
Answer

The lime water turns milky in (a) and (d) only.

Set-upMixture in the first test tubeGas produced?Lime water
(a)Vinegar and baking sodaYes — carbon dioxideTurns milky
(b)Lemon juice and vinegarNoStays clear
(c)Vinegar and common saltNoStays clear
(d)Lemon juice and baking sodaYes — carbon dioxideTurns milky
Why it happens: Carbon dioxide is released only when baking soda meets an acid. Vinegar and lemon juice are both acids, so (a) and (d) fizz and give off carbon dioxide, which turns the lime water milky. In (b) both substances are acids — two acids do not react with each other, so no gas is formed. In (c) common salt is neither an acid nor a hydrogen carbonate, so vinegar has nothing to react with. With no carbon dioxide reaching it, the lime water in (b) and (c) stays clear.
Baking soda + Acid → Carbon dioxide + Other substances
Carbon dioxide + Lime water → Calcium carbonate (white, insoluble) + Water → milky
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