Chapter 5: Changes Around Us: Physical and Chemical Class 8th Science (Curiosity) NCERT Solution

July 13, 2026 • Admin ETeam • Class 7, Class 7 NCERT Solution

Class 7 Science Chapter 5 – Changes Around Us: Physical and Chemical | Solutions

Curiosity • Textbook of Science • Grade 7

Chapter 5 — Changes Around Us: Physical and Chemical

All activities, tables and in-text questions, plus the complete “Let Us Enhance Our Learning” exercise — solved with the textbook’s own figures.

In-Text Questions, Activities & Tables

The students are describing some changes. What kinds of changes are they talking about?

SOLUTION

Students describing changes: ice melting, bud to flower, cold water warming, banana browning — The four changes described by the students (from the chapter)

What the student says	What is changing	Type of change
“A cube of ice has now become water.”	The state changes (solid → liquid). Water is still water.	Physical change (reversible — it can be refrozen)
“A bud has become a flower.”	A natural growth change — new substances form inside the plant.	Chemical change (cannot be reversed)
“The bottle of cold water is not cold anymore.”	Only the temperature changed.	Physical change (reversible — cool it again)
“The banana has more brown spots and a strong smell.”	New substances are formed — colour and smell change (over-ripening/decay).	Chemical change (cannot be reversed)

So the students are talking about two kinds of changes — some in which no new substance is formed (physical), and some in which a new substance is formed (chemical). That is exactly what this chapter is about.

Activity 5.1 — Let us think and reflect (Table 5.1)

A5.1

Reflect on each change listed in Table 5.1 and record your observations.

SOLUTION — TABLE 5.1 (COMPLETED)

Table 5.1 from the textbook — Table 5.1 — as printed in the book (from the chapter)

S.No.	Change	Observation(s) — what changes?	Type
1	Melting ice cubes	State changes: solid ice → liquid water. Substance stays water.	Physical
2	Chopping vegetables	Size and shape change; the vegetable itself is unchanged.	Physical
3	Boiling water	State changes: liquid water → water vapour (steam).	Physical
4	Making popcorn from corn	Size, shape, colour, smell and taste all change — a new substance is formed.	Chemical
5	Cutting a piece of paper	Size and shape change; it is still paper.	Physical
6	Adding beetroot extract to water	The colour of water changes (it becomes pink); no new substance — the extract simply mixes/dissolves.	Physical
7	Burning wood	New substances — ash, smoke, gases — are formed, with heat and light.	Chemical
8	Drying wet clothes	Water evaporates — a change of state only.	Physical
9	Making small balls of dough	Only the shape and size change.	Physical
10	Rolling balls of dough into chapatis	Only the shape changes (the dough stays dough).	Physical
11	Any other — e.g. curdling of milk / rusting of a nail / burning a candle	New substances with new smell, taste and colour are formed.	Chemical

These changes may be in the size, shape, smell, colour, state or other property of a substance. We notice them using our five senses — sight, smell, touch, hearing and taste.
Can we arrange these changes into categories? Yes — into physical and chemical changes (and also into reversible / irreversible, and desirable / undesirable).

5.1 Physical changes — Activity 5.2

A5.2

(A) Do you get the same paper back when you unfold the objects? (B) Do you get the uninflated balloon back after the air escapes — and after it is pricked with a pin? (C) Can you get the chalk piece back from the powder? What is the similarity in A, B and C?

SOLUTION

Objects made by folding paper — Fig. 5.1 — Objects made from paper (from the chapter)

Activity	Observation	New substance formed?	Can it be reversed?
A. Folding paper into objects	Only the shape changes.	No — it is still the same paper	Yes — unfold it and you get the paper back
B1. Inflating a balloon and letting the air escape	Only the size/shape changes.	No — the rubber is unchanged	Yes — the uninflated balloon comes back
B2. Pricking the inflated balloon with a pin	The balloon bursts into pieces.	No — the pieces are still rubber	No — the torn balloon cannot be joined back
C. Crushing chalk into powder	Only the size changes.	No — the powder is still chalk	No — the powder cannot be pressed back into the same piece

The similarity: in all of A, B and C the material (paper, rubber, chalk) remains the same — only its appearance (shape or size) has changed and no new substance is formed.
Such changes, in which only physical properties like shape, size and state change, are called PHYSICAL CHANGES.
Note: a physical change need not always be reversible (the burst balloon and the chalk powder prove it!).

5.2 Chemical changes — Activities 5.3 & 5.4

A5.3

Blow air through a straw into tumbler A (tap water) and tumbler B (lime water). Do you notice any changes?

SOLUTION

Boy blowing air through a straw into tap water and into lime water — Fig. 5.2 — Blowing air into (a) tap water (b) lime water (from the chapter)

Tumbler	Observation	Conclusion
A — tap water	Only bubbles form; no change in the appearance of the water.	No new substance → physical change only
B — lime water	Bubbles form and the lime water turns MILKY (cloudy). On standing, a white substance settles at the bottom.	A new substance is formed → CHEMICAL CHANGE

Why does the lime water turn milky?

The carbon dioxide in the air we exhale reacts with lime water and forms a new white substance, calcium carbonate, which is insoluble in water — so the liquid looks milky. A little water is also formed.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(lime water) (insoluble — makes it milky)

A change in which one or more new substances are formed is a CHEMICAL CHANGE. New substances are formed through a chemical reaction, which is written in short as a chemical equation.
Turning lime water milky is the standard TEST for carbon dioxide.

A5.4

Add a pinch of baking soda to vinegar/lemon juice. What do you observe? Pass the gas through lime water — what do you infer? Then repeat with baking soda and water: do you get bubbles? Is it physical or chemical?

SOLUTION

Reaction of vinegar and baking soda, gas passed into lime water — Fig. 5.3 — Reaction of vinegar and baking soda (a) with test tubes (b) with bottles and a straw (from the chapter)

Step 1 — Observation. A fizzing, bubbling sound is heard and gas bubbles are seen.

Step 2 — Passing the gas into lime water. The lime water turns milky.

Step 3 — Inference. Since only carbon dioxide turns lime water milky, the gas formed is CARBON DIOXIDE.

Step 4 — Type of change. A new substance (carbon dioxide) is formed → it is a CHEMICAL CHANGE.

Vinegar + Baking soda → Carbon dioxide + Other substances

Repeating with baking soda and water

No bubble formation is seen — the baking soda simply dissolves in the water.
No new substance is formed, and the baking soda can be got back by evaporating the water.
So this is a PHYSICAL CHANGE, not a chemical one.

Big idea: bubbling with an acid (vinegar/lemon juice) means a reaction; simple dissolving in water does not.

5.3 Rusting and Combustion — Activity 5.5

Is rusting a chemical change? Is the burning of magnesium ribbon physical or chemical? What is combustion?

SOLUTION

Fig. 5.4 — Rusted iron nails (from the chapter)

Rusting is a CHEMICAL CHANGE — a new brown-coloured substance, rust (iron oxide), is formed on the iron.

Fig. 5.5 — Burning magnesium ribbon (from the chapter)

Burning of magnesium ribbon is a CHEMICAL CHANGE — a new substance, magnesium oxide (a white powder), is formed, along with heat and light.

Magnesium + Oxygen → Magnesium oxide + Heat + Light
(ribbon) (air) (white powder)

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

A5.5

Two lighted candles: one open, one covered with a glass tumbler. What happens to the flames? Why? How can you test that carbon dioxide is present?

SOLUTION

Candle burning in the open and candle covered with a glass tumbler — Fig. 5.6 — Candle (a) burning (b) covered with a glass tumbler (from the chapter)

Candle	Observation	Reason
(a) Not covered	Keeps burning	It has a continuous supply of air, hence of oxygen.
(b) Covered with a glass tumbler	The flame is extinguished after some time	The air (oxygen) inside the tumbler is used up and there is no fresh supply.

Testing for carbon dioxide

Step 1. Pour a small amount of lime water into the petri dish under the inverted tumbler.

Step 2. The lime water turns milky → carbon dioxide is present.

Step 3. This CO₂ was formed from the carbon of the wax and the oxygen of the air.

Conclusion: Oxygen is required for combustion. The component of air that supports burning is oxygen.
Science and Society: If a person’s clothes catch fire, wrap a blanket or cloth around them — this cuts off the supply of air and the fire is put out. ⚠ Never use a synthetic blanket — it can melt and stick to the skin.

Fireflies glowing at night — **Fascinating fact:** Fireflies produce light by a **chemical change**. Light production without heat in living organisms is called **bioluminescence** (from the chapter)

A5.6

Paper catches fire at once with a matchstick, but it can also be burnt by focusing sunrays with a magnifying glass. What do you observe, and how do you explain it? What are the three requirements for combustion?

SOLUTION

Focusing sunrays on paper with a magnifying glass and the paper catching fire — Fig. 5.7 — (a) Focusing the sunrays using a magnifying glass (b) Paper catching fire (from the chapter)

Observation. The paper first starts to emit smoke and then catches fire — even though there was no fire touching it.

Explanation. Focusing the sunrays heats the paper. Its temperature keeps rising, and after some time it becomes so hot that it starts burning.

Ignition temperature. The minimum temperature at which a substance catches fire is called its ignition temperature.

Why the matchstick works instantly. The flame of the lighted matchstick is already hotter than the ignition temperature of paper, so the paper catches fire almost immediately. We do not need a fire to start burning — we need enough heat.

Fire triangle showing oxygen, heat and fuel — Fig. 5.8 — The fire triangle (from the chapter)

The three requirements for combustion:
(i) a combustible substance (fuel) • (ii) oxygen • (iii) heat to raise the fuel to its ignition temperature.
Remove any one side of this fire triangle and the fire goes out — that is why a blanket (cutting off oxygen) or water (cooling below the ignition temperature) puts out a fire.

5.4 Can both changes occur together? — Activity 5.7

A5.7

Analyse what the students are discussing about the burning candle. What do you think?

SOLUTION

Students discussing what changes occur when a candle burns — Fig. 5.9 — What changes occur when a candle burns? (from the chapter)

All the students are partly right — the burning of a candle involves BOTH physical and chemical changes.

Step in the process	What happens	Type
The wax near the flame melts	Solid wax → liquid wax (state change)	Physical
The liquid wax is drawn up the wick and later solidifies in different shapes	Liquid → solid; wax remains wax	Physical
The wax evaporates due to the heat of the flame	Liquid wax → wax vapour	Physical
The wax vapour burns and produces the flame	New substances (carbon dioxide, water vapour, soot) are formed, with heat and light	Chemical (combustion)

Conclusion: melting, solidification and evaporation of wax are physical changes; the burning of the wax vapour is a chemical change. So both types of change occur in the same process.

**Know a Scientist:** **Michael Faraday** delivered a famous series of lectures, *Chemical History of a Candle*, using the candle to explain melting, vapourisation and combustion (from the chapter)

5.5 Are changes permanent? — Activity 5.8 (Table 5.2)

A5.8

In which of the changes can we get the object or substance back in the form we started with? Complete Table 5.2.

SOLUTION — TABLE 5.2 (COMPLETED)

Table 5.2 from the textbook — Table 5.2 — as printed in the book (from the chapter)

S.No.	Change	Original state can be brought back?	Reason
1	Melting ice cubes	Yes	Water can be refrozen into ice.
2	Chopping vegetables	No	Pieces cannot return to their original size and shape.
3	Boiling water	Yes	The vapour can be condensed back into water (if it is collected).
4	Making popcorn from corn	No	Popcorn cannot go back to being corn — a new substance formed.
5	Cutting a piece of paper	No	The cut pieces cannot be joined into the original sheet.
6	Adding beetroot extract to water	Yes	The water can be evaporated to get the extract back.
7	Burning wood	No	Ash and gases cannot be turned back into wood.
8	Drying wet clothes	Yes	The clothes can be wetted again; the vapour can condense.
9	Making small balls of dough	Yes	The balls can be joined back into a lump of dough.
10	Rolling dough into chapatis	Yes	The rolled dough can be made into a ball again (before cooking).
11	Rusting of iron	No	A new substance (rust) has formed.
12	Curdling of milk	No	Curd cannot be turned back into milk.

Changes can therefore be grouped into those that can be reversed and those that cannot be reversed.

5.6 & 5.7 Desirable changes and slow natural changes

Are all changes desirable? Give examples. What are weathering and erosion?

SOLUTION

Desirable changes	Undesirable changes
Milk → curd, ripening of fruits, cutting of fruits, cooking of food, germination of seeds, making compost, baking bread	Rusting of iron, decay/spoiling of stored food, souring of milk, landslides, increase of carbon dioxide from burning fuels, pollution from drying paint

Important idea: the same change can be undesirable in one situation and desirable in another — the decomposition of food is bad in the kitchen but very useful when it turns food waste into compost.

Weathering of rocks (5.7.1)

Sediments at the base of a cliff and a red sediment layer — Fig. 5.10 — (a) Sediments at the base of a cliff (b) red sediment layer (from the chapter)

The heaps of sand, soil and stones at the base of mountains are called sediments.
Physical part: large rocks break into smaller pieces because of temperature changes, the growing roots of trees, and the freezing of water inside cracks.
Chemical part: water and the chemicals in it react with rocks. For example, black basalt (which contains iron) turns into a red layer because iron oxide is formed.
These physical and chemical changes together are called WEATHERING, and they eventually lead to the formation of soil.

Erosion (5.7.2)

The breaking down of rock pebbles, soil and sediments and their movement from one place to another by natural forces like wind and flowing water is called EROSION.
Erosion during a landslide is an example of a physical change. River pebbles become smooth because of constant erosion.
Where the water or wind slows down (a lake or an ocean), the material settles at the bottom; over time these sediments harden into new rocks. Such changes take thousands of years and cannot be reversed.

Let Us Enhance Our Learning (Exercise)

Which statements are characteristics of a physical change?
(i) The state 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

Answer: (c) (i) and (iii)

Statement	Correct for a physical change?	Why
(i) The state may or may not change	✔ Yes	Melting ice → state changes; folding paper → state does not change. Both are physical.
(ii) A substance with different properties is formed	✘ No	That is a chemical change.
(iii) No new substance is formed	✔ Yes	This is the defining feature of a physical change.
(iv) The substance undergoes a chemical reaction	✘ No	Again a chemical change.

Predict which changes can be reversed and which cannot. If you are not sure, say so — and say why you are not sure.

SOLUTION

Change	Reversible?	Reason
(i) Stitching cloth to a shirt	Can be reversed	The stitches can be opened and the pieces of cloth got back (the cloth itself is unchanged).
(ii) Twisting of a straight string	Can be reversed	Untwist it — the straight string comes back.
(iii) Making idlis from a batter	Cannot be reversed	Cooking forms new substances — the idli can never become batter again (chemical change).
(iv) Dissolving sugar in water	Can be reversed	Evaporate the water and the sugar is recovered.
(v) Drawing water from a well	Can be reversed	The water can simply be poured back — only its position changed.
(vi) Ripening of fruits	Cannot be reversed	New substances form (taste, smell, colour change) — a ripe fruit never becomes raw again.
(vii) Boiling water in an open pan	Not sure / cannot be reversed in practice	Boiling is a physical change and steam can be condensed back to water — but the pan is open, so the vapour escapes into the air and cannot be collected. Why not sure: it depends on whether we cover the pan and collect the steam or not.
(viii) Rolling up a mat	Can be reversed	Unroll it — only the shape changed.
(ix) Grinding wheat grains to flour	Cannot be reversed	Flour cannot be turned back into grains (though it is still wheat — a physical change that is irreversible).
(x) Forming of soil from rocks	Cannot be reversed	Weathering takes thousands of years and involves chemical changes too.

Remember: “physical” does not automatically mean “reversible” — grinding wheat and crushing chalk are physical changes that cannot be undone.

True or False? Correct the false statements.

SOLUTION

Statement	T/F	Reason / Corrected statement
(i) Melting of wax is necessary for burning a candle.	True	The wax must first melt, rise up the wick and evaporate; it is the wax vapour that actually burns.
(ii) Collecting water vapour by condensing involves a chemical change.	False	Correct statement: Collecting water vapour by condensing involves a PHYSICAL change — only the state changes (gas → liquid) and no new substance is formed.
(iii) Converting leaves into compost is a chemical change.	True	Bacteria and fungi decompose the leaves into new substances (compost); it cannot be reversed.
(iv) Mixing baking soda with lemon juice is a chemical change.	True	A new substance, carbon dioxide, is formed (fizzing) — it turns lime water milky.

Fill in the blanks.

SOLUTION

Statement	Answer
(i) Nalini observed brown deposits on her cycle handle. The brown deposits are due to ______, and this is a ______ change.	rusting (formation of rust / 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

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

SOLUTION

Both are PHYSICAL changes.

Reason 1 — No new substance. Ice, water and steam are all the same substance (water), only in three different states — solid, liquid and gas.

Reason 2 — Reversible. Ice can be melted back into water; steam can be condensed back into water.

Reason 3 — Only physical properties change — the shape, volume and state, not the chemical nature.

Ice ⇌ Water ⇌ Steam (all physical changes)

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

SOLUTION

Curdling of milk is a CHEMICAL change.

Justification

A new substance — curd — is formed from milk (bacteria act on the milk).
The curd has completely different properties from milk: a sour taste, a different smell, and a thick, semi-solid texture.
The change cannot be reversed — curd can never be turned back into milk.

It is also a desirable change, since curd is nutritious and used in our daily food.

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

SOLUTION

It involves BOTH physical and chemical changes. The whole process is called weathering.

Type	What happens
Physical change	Big rocks break into smaller pieces because of temperature changes, the freezing of water inside cracks, the growing roots of trees, and the rubbing action of wind and flowing water (erosion). The rock material is only broken — it is still the same substance.
Chemical change	Water and the chemicals dissolved in it react with the minerals of the rock. For example, black basalt (which contains iron) is changed into a red layer of iron oxide. The composition of the rock itself changes.

Why: because a new substance (iron oxide) is formed in part of the process, it cannot be called purely physical. Weathering is very slow and cannot be reversed, and it finally produces soil.

Read the story ‘Eco-friendly Prithvi’ and tick the correct option in each bracket. Also suggest a suitable title.

SOLUTION

Part of the story	Correct option	Why
He chops vegetables, peels potatoes and cuts fruits	Physical changes ✔	Only the size and shape change — no new substance is formed.
He collects the seeds, peels and fruits into a clay pot	Physical change ✔	Only the position of the material changes.
The peels decompose by bacteria and fungi, forming compost	Chemical change ✔	New substances (compost) with new properties are formed; it cannot be reversed.
The seeds germinate, plants grow and flowers bloom	Chemical change ✔	Growth involves the formation of new substances inside the plant; it cannot be reversed.

Suitable titles for the story

“From Kitchen Waste to Blooming Flowers”
“Prithvi’s Compost Garden”
“Nothing is Waste — Changes that Grow a Garden”

Place the given changes in the Venn diagram: A = physical, B = chemical, C = both.
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

Venn diagram with regions A, C and B — The Venn diagram as given in the book (from the chapter)

The completed Venn diagram

Region	Changes	Why
A — Physical only	Tearing of paper • Melting of ice • Folding of clothes	Only the shape/size/state changes; no new substance is formed.
B — Chemical only	Rusting • Curdling of milk • Ripening of fruits • Burning of magnesium • Mixing baking soda with vinegar	A new substance is formed in each case (rust, curd, CO₂, magnesium oxide …).
C — Both	Process of burning a candle	Melting, solidifying and evaporating of wax are physical; the burning of the wax vapour is chemical.

Q10

In the experiments of Fig. 5.11 (a, b, c, d), in which case(s) did the lime water turn milky, and why?

SOLUTION

Four experiments passing gas into lime water — Fig. 5.11 — the four set-ups (from the chapter)

Answer: the lime water turns milky only in cases (a) and (d).

Case	Substances mixed	Gas produced?	Lime water	Reason
(a)	Vinegar + Baking soda	Yes — carbon dioxide	Turns milky ✔	An acid (vinegar) reacts with baking soda (a carbonate) and releases CO₂, which turns lime water milky.
(b)	Lemon juice + Vinegar	No	No change ✘	Both are acids — they do not react with each other, so no gas is produced.
(c)	Vinegar + Common salt	No	No change ✘	Common salt is not a carbonate — no reaction, no CO₂.
(d)	Lemon juice + Baking soda	Yes — carbon dioxide	Turns milky ✔	Lemon juice is an acid and reacts with baking soda, releasing CO₂.

Rule: lime water turns milky only when carbon dioxide is passed through it. CO₂ is produced when an acid (vinegar or lemon juice) is mixed with baking soda.

Exploratory Projects — guidance

GUIDANCE

1. Invisible ink with lemon juice

Step	What happens	Type of change	Reversible?
Writing with lemon juice and letting it dry	The water in the juice evaporates, leaving an invisible film.	Physical	Yes (the juice could be re-wetted)
Warming the paper (iron / candle flame)	The dried juice chars — the letters turn dark brown. A new substance is formed.	Chemical	No — the brown letters can never become invisible again

⚠ Do this only with an adult, and keep the paper from catching fire.

2. Steps to reduce landslides and rock erosion

Plant trees and grass on hill slopes — roots hold the soil together.
Build retaining walls and terraces on steep slopes.
Make proper drainage channels so rainwater does not soak the slope.
Avoid deforestation, unplanned construction, mining and blasting in hilly areas.
Follow early-warning systems and do not build houses on unstable slopes.

3. Changes in the kitchen

Can be reversed (physical)	Cannot be reversed (mostly chemical)
Melting butter/ghee, freezing water into ice, dissolving sugar or salt, boiling water (if the steam is condensed), making dough balls	Cooking rice, boiling an egg, making chapati, curdling milk, burning gas on the stove, frying, roasting, burning food

4. How does yeast work? (the balloon experiment)

Observation 1. After about an hour, the balloon inflates on its own — a gas has been produced inside the bottle.

Observation 2. When the gas from the balloon is shaken with lime water, the lime water turns milky.

Conclusion. The gas is carbon dioxide. The yeast feeds on the sugar and produces carbon dioxide (this process is called fermentation). It is the CO₂ bubbles that make bread fluffy and soft.

Change in the experiment	Type
Sugar dissolving in water	Physical
The balloon inflating (stretching)	Physical
Yeast acting on sugar to produce carbon dioxide (fermentation)	Chemical
Lime water turning milky (calcium carbonate formed)	Chemical

5. Do chameleons change colour reversibly?

A chameleon on a branch — Fig. 5.12 — A chameleon (from the chapter)

Yes — this change can be reversed. A chameleon can go back to its earlier colour once it is calm or its surroundings change again.
The colour changes because special pigment cells in the skin spread out or shrink, changing how light is reflected. No new substance is permanently formed, so it behaves like a reversible (physical) change.
It helps them blend with the surroundings (camouflage) and also signals when they are angry or sense danger.

Chapter 5: Changes Around Us: Physical and Chemical Class 8th Science (Curiosity) NCERT Solution

Chapter 5 — Changes Around Us: Physical and Chemical

Why does the lime water turn milky?

Repeating with baking soda and water

Testing for carbon dioxide

Weathering of rocks (5.7.1)

Erosion (5.7.2)

Justification

Suitable titles for the story

1. Invisible ink with lemon juice

2. Steps to reduce landslides and rock erosion

3. Changes in the kitchen

4. How does yeast work? (the balloon experiment)

5. Do chameleons change colour reversibly?

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