NCERT Solutions Class 7 Science Chapter 5 Changes Around Us Physical and Chemical

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 7 · SCIENCE

NCERT Solutions

Chapter 5: Changes Around Us:
Physical and Chemical

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

57 – 72 17 44 English

Solutions, notes, sample papers & more at 48 pages

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

CLASS 7 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 5: Changes Around Us:
Physical and Chemical
Complete NCERT Solutions for Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical from
the NCERT textbook Curiosity. Every question of the chapter is answered — all eight Activities (5.1 to 5.8), the
two data tables, the in-text and Science and Society questions from pages 57 to 68, all 10 questions of Let Us
Enhance Our Learning, and the Exploratory Projects — with the reasoning behind each answer spelt out.

TEXTBOOK BOOK PAGES

Curiosity (Class 7) 57 – 72

SECTIONS QUESTIONS

17 44

MEDIUM

English

In-text Questions — Page 57
Chapter opener — students describing changes

Q1 These students are describing some changes. What kinds of changes are they
talking about?

They are talking about everyday changes that we notice with our senses. Two of them are
physical changes (nothing new is formed) and two are chemical changes (a new substance is
formed).

Page 1 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

WHAT THE STUDENT SAYS WHAT IS CHANGING KIND OF
CHANGE

“I placed a cube of ice here half an hour Solid ice → liquid water (only the state Physical, and it can be
ago. It has now become water!” changes) reversed by freezing

“A bud that I saw yesterday on this rose The bud grows and opens; new colour, Chemical (a life
plant has become a flower today.” new scent, new substances are made process); it cannot be
inside the plant reversed

“The bottle containing cold water is not Only the temperature of the water has Physical, and it can be
cold anymore.” risen reversed by cooling

“Yesterday, I saw some brown spots on a Ripening and decay — colour and Chemical; it cannot be
banana, but today it has more brown smell change because new substances reversed
spots and a strong smell.” form

Why it happens: We spot a change with our senses — sight (ice becoming water,
brown spots), touch (the bottle no longer cold), smell (the strong banana smell). But
the sense only tells us that something changed. To decide the kind of change we
must ask one question: has a new substance been formed? If yes, the change is
chemical; if only shape, size, state or temperature changed, it is physical.

Tip: A strong new smell is one of the easiest clues to a chemical change. Milk turning
sour, a banana over-ripening and food going bad all announce themselves by their
smell.

Activity 5.1: Let us think and reflect — Page 58

Page 2 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Table 5.1: Some changes observed around us

ACTIVITY

Q1 You might have observed various changes happening around you. Some of them
are listed in Table 5.1, you may notice that something is changing in each case. Take
a moment to reflect on the changes in each case. Record your observations in Table
5.1.

S.NO. CHANGE OBSERVATION(S)

1. Melting ice cubes

2. Chopping vegetables

3. Boiling water

4. Making popcorn from corn

5. Cutting a piece of paper

6. Adding beetroot extract to water

7. Burning wood

8. Drying wet clothes

9. Making small balls of dough

10. Rolling small balls of dough into chapatis

11. Any other

Table 5.1, page 58: Some changes observed around us — the Observation column is left
blank for you to fill in.

Here is Table 5.1 filled in. Look carefully at what exactly changes in each case — that is what
the observation column is asking for.

Page 3 of 48

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

co m
m.
S.NO. CHANGE OBSERVATION(S)

m as e
1.
.co
Melting ice cubes
a g l
Hard, solid ice slowly turns into liquid water. Only the state changes;

a s em the substance is still water.

a2.g l Chopping vegetables One big piece becomes many small pieces. Only size and shape
change; it is still the same vegetable.

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3. Boiling water
e m .
Bubbles rise, steam comes out, the level of water falls. Liquid water ag
g l as
becomes water vapour — a change of state.

4. Making popcorn from
a
The kernel bursts with a pop, becomes white, soft and fluffy, and
corn smells different. New substances are formed.
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m l as
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5. Cutting a piece of paper Size and shape change. Every piece is still paper.

a g
se m Adding beetroot extract
l a
6. The colourless water turns pink or deep red. The colour spreads

a g to water through the water; no new substance is made — it is a mixture.

7. Burning wood
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Flame, heat, light, smoke, and grey ash are left behind. New
a s
.co agl
substances are formed and the wood is gone for ever.

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a lighter and dry; the cloth itself does not change.
8. Drying wet clothes The water in the cloth evaporates into the air. The cloth becomes

co m
.
9. Making small balls of Only the shape of the dough changes; it is still dough.
dough
se m
omRolling small balls of l a
g only the shape changes —
10. .c aAgain
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The ball becomes a flat, round disc.

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dough into chapatis the raw chapati can be rolled back into a ball.

11. A rough, brown, flaky layer (rust) appears on the shining nail. A new
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Any other — an iron nail
left in the rain
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substance has formed.
g l a
m . a
gl ase
a
Check it yourself: Read down your observation column. Wherever you have written
words like new smell, new colour, ash, cannot be undone, you are almost certainly
looking at a chemical change.
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a s emQ2
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Can you think of some other changes that happen in your surroundings? Make a list
a c
.
of those changes too.

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Yes. Here is a list you can add to Table 5.1, sorted so that you can see the pattern.
a

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Changes in the kitchen: milk setting into curd, grinding wheat into atta, kneading atta into
dough, baking a chapati on the tawa, roasting groundnuts, boiling milk, dissolving sugar in
tea, cutting an apple and watching it turn brown.
Changes at home and school: sharpening a pencil, tearing an old notebook, folding a paper
boat, breaking a piece of chalk, a wet blackboard drying, an iron gate rusting, a candle
burning, an LPG stove burning.
Changes in nature: a seed germinating, a leaf turning yellow and falling, fruits ripening,
dew forming on grass in winter, clouds forming and rain falling, a river making pebbles
smooth, dead leaves rotting into compost.
Changes in ourselves: our height increasing, hair and nails growing, a wound healing,
digestion of the food we eat.

Why it happens: Every one of these can be sorted with the same single test — is a
new substance formed? Sharpening a pencil, folding paper and dew forming are
physical. Curd setting, rusting, burning, ripening, rotting and digestion are chemical.

Q3 Can we arrange these changes into categories?

Yes. Changes can be grouped in more than one useful way, and the same change can appear in
several groupings.

BASIS OF GROUPING CATEGORIES EXAMPLES

Is a new substance Physical change / Chemical Melting ice (physical) · Burning wood
formed? change (chemical)

Can the original be Reversible / Irreversible Boiling water (reversible) · Making popcorn
brought back? (irreversible)

Do we want it to happen? Desirable / Undesirable Milk to curd (desirable) · Rusting of iron
(undesirable)

How fast is it? Fast / Slow Burning of magnesium (fast) · Weathering of
rocks (very slow)

What made it happen? Natural / Man-made Ripening of fruits (natural) · Chopping
vegetables (man-made)

The grouping this chapter uses is the first one: physical change and chemical change.

Page 5 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Activity 5.2: Let us create and discuss — Page 59
5.1 A Substance May Change in Appearance but Remain the Same!

ACTIVITY

Q1 Take a few sheets of paper and fold them to create new objects (Fig. 5.1). Do you get
the same paper back when you unfold these objects?

boat bird (crane) butterfly paper dart

frog cup / flower paper hat

Fig. 5.1, page 59 — objects made by folding sheets of paper (redrawn sketch of the
textbook photograph).

Yes. When the boat, the aeroplane, the bird or the butterfly is unfolded, we get back the same
sheet of paper — the same size, the same colour, the same weight. Only some crease lines
remain.

Why it happens: Folding only changes the shape of the sheet. The material —
paper — is untouched. Since no new substance is formed, this is a physical change,
and because we can undo it, it is also a reversible change.

Did you know? This art of folding paper without cutting or pasting is called origami.
Every origami model is a beautiful demonstration that a physical change can alter
appearance completely without altering the material at all.

Page 6 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Q2 Take a balloon and inflate it. Now, loosen your grip and let the air escape out. Do
you get the uninflated balloon back?

Yes. As soon as the air rushes out, the rubber shrinks back and we get the same uninflated
balloon in our hand.

Inflating: rubber sheet stretches → balloon becomes big

Letting the air out: rubber relaxes → balloon becomes small again

Material before = material after = rubber

Why it happens: Blowing air in only stretches the rubber and changes the balloon's
size and shape. No new substance is made, so it is a physical change — and a
reversible one.

Q3 Take another balloon; inflate it and grip the opening tightly. Now, prick it with a
pin. What happens? Will you be able to get the uninflated balloon back?

The balloon bursts with a loud bang and tears into pieces of rubber. No — we cannot get the
whole uninflated balloon back.

Why it happens: The stretched rubber is under great strain. The pin makes a tiny
cut, the tear runs across the whole sheet in an instant, and the balloon splits. Even
so, the pieces are still rubber — nothing new has been made. So bursting a balloon
is still a physical change, only it is an irreversible one.

Tip: Do not mix up “physical” with “reversible”. Bursting a balloon, crushing chalk and
tearing paper are all physical changes that cannot be reversed. The test for a
physical change is only this — no new substance is formed.

Page 7 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Q4 Crush a small piece of chalk into powder. Can you get the chalk piece back from the
powder?

No — the powder cannot be pressed back into the original stick of chalk. But the powder is still
chalk: it is the same white colour, it still writes on the blackboard, and it still tastes and feels the
same.

Why it happens: Crushing breaks one big piece into millions of tiny particles. Only
the size has changed — the substance has not. So this is a physical change that
cannot be reversed at home.

Try This: Rub the powder on the blackboard with your finger. It writes just as well as
the stick did. That is the proof that no new substance was formed.

Q5 Is there any similarity in the changes listed in A, B, and C?

Yes — in all three cases only the appearance changed, while the material stayed exactly
the same.

CHANGE WHAT CHANGED MATERIAL BEFORE MATERIAL AFTER

A. Folding paper into objects Shape Paper Paper

B. Inflating / bursting a balloon Size and shape Rubber Rubber

C. Crushing chalk Size Chalk Chalk

Since no new substance is formed in any of them, all three are physical changes. This is
exactly the definition the book gives: changes in which only physical properties like shape, size
and state change are called physical changes.

Activity 5.3: Let us explore — Page 60

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

5.2 A Substance May Change in Appearance and Not Remain the Same!
co m
e m.
m as
ACTIVITY

.co a g l
a s em blow air (exhale) into each glass tumbler, one at a time, using separate straws
gl
Now,
a
Q1
(Fig. 5.2) and observe them. Do you notice any changes?

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Straw Straw

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a s em
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A B

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Glass tumbler Glass tumbler

.
containing tap water containing lime water

m
ase
agl (a) (b)

m a s
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Fig. 5.2, page 60 — blowing air (exhaling) through a straw into (a) tap water and (b) lime
agl
l a se water.

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Yes — but only in tumbler B.
a g l
s m
eTUMBLER
gl a CONTAINS WHAT IS SEEN ON BLOWING AIR KIND OF
a CHANGE

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A Tap water Only bubbles rise. The water looks exactly as it did No chemical

m
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before. change

B Lime water
l
agBubbles rise and the clear lime water turns milky Chemical
(cloudy). Left standing, a white substance settles at change
the bottom.
co m
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.c Because one or more new substances are formed, thisaisga chemical change, brought
The white solid that settles is a new substance — calcium carbonate — which is insoluble in

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about by a chemical reaction:

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com a
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Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
.
a s em air) (insoluble, white)
agl
(Lime water) (from exhaled

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Why it happens: The air we breathe out is rich in carbon dioxide. In tumbler A it
simply escapes as bubbles, because it does not react with plain water in any visible
way. In tumbler B the carbon dioxide reacts with the calcium hydroxide dissolved in
the lime water and makes tiny white grains of calcium carbonate. These grains do
not dissolve, so they hang in the liquid and make it look milky.

Caution: Blow gently through the straw and never suck. Lime water is harmful if
swallowed.

Activity 5.4: Let us experiment — Page 61
5.2 A Substance May Change in Appearance and Not Remain the Same!

ACTIVITY

Q1 Take a teaspoonful of vinegar or lemon juice in a test tube. Add a pinch of baking
soda (sodium hydrogen carbonate) to it. What do you observe?

The mixture fizzes at once. We hear a fizzing, bubbling sound and see a froth of gas bubbles
rising and filling the upper part of the test tube. The test tube also feels slightly cool.

Why it happens: Vinegar (and lemon juice) is acidic, baking soda is a hydrogen
carbonate. The moment they meet they react and a gas is set free. A gas coming out
of a mixture of two substances that had no gas in them is one of the clearest signs
that a new substance has been made — that is, of a chemical change.

Tip: Add the baking soda in a small pinch. If you add a spoonful, the froth climbs out
of the test tube and the gas is lost before you can test it.

Page 10 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Q2 Pass this gas through freshly prepared lime water kept in another test tube, as
shown in Fig. 5.3a. What do you observe?

Lime water

Bubbles of carbon
dioxide gas
Vinegar and
baking soda
Fig. 5.3(a), page 61 — the gas from vinegar + baking soda is passed through lime water
in the second test tube.

The clear, colourless lime water turns milky, exactly as it did in Activity 5.3 when we blew air
into it.

Gas from vinegar + baking soda → passed into lime water

Clear lime water → milky white

On standing, a white insoluble solid settles at the bottom

Why it happens: The gas reacts with the calcium hydroxide of the lime water and
forms tiny particles of calcium carbonate, which do not dissolve. Suspended in the
liquid, they scatter light and the lime water looks milky.

Check it yourself: The lime water must be freshly prepared and clear. Old lime
water has already reacted with the carbon dioxide of the air and will not give a sharp
result.

Page 11 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Q3 The lime water turns milky. What do you infer about the gas formed by mixing
vinegar and baking soda?

We infer that the gas is carbon dioxide.

Why it happens: Turning lime water milky is the accepted test for carbon dioxide.
In Activity 5.3, the gas we breathed out was known to contain carbon dioxide and it
turned lime water milky. Here an unknown gas does exactly the same thing, so it
must be the same gas. Since carbon dioxide was not present in either the vinegar or
the baking soda to begin with, it is a new substance — and the reaction is a
chemical change:

Vinegar + Baking soda → Carbon dioxide + Other substances

You can perform the same activity with two small used bottles and a flexible straw instead of
test tubes, as shown in Fig. 5.3b.

Q4 Repeat the above activity using baking soda and water. Do you observe any bubble
formation? Is this a physical or a chemical change?

No fizzing and no stream of gas bubbles are seen. The baking soda simply dissolves in the
water and the liquid stays clear. (A few small air bubbles that were clinging to the powder may
float up, but the fizzing sound is absent.) This is a physical change.

BAKING SODA + VINEGAR BAKING SODA + WATER

What is seen Loud fizzing, plenty of gas bubbles Powder dissolves quietly

Lime water test Lime water turns milky Lime water stays clear

New substance formed? Yes — carbon dioxide No

Type of change Chemical change Physical change

Page 12 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Why it happens: Baking soda needs an acid to release carbon dioxide. Vinegar and
lemon juice are acids, plain water is not. In water the particles of baking soda only
spread out among the water particles — that is dissolving, and the baking soda can
be recovered by evaporating the water. Since nothing new is formed, the change is
physical.

Did you know? This is why a recipe for dhokla or cake asks you to add curd, lemon
juice or eno along with the soda. Without the acid there is no gas, and the batter will
not rise.

In-text Questions — Page 62
5.3.2 Combustion

Q1 Let us recall the burning of a magnesium ribbon (Fig. 5.5) learnt in the chapter ‘The
World of Metals and Non-metals’. Can you predict if this is a physical or a chemical
change?

White smoke

Magnesium ribbon
held in tongs
Dazzling white light

Burner flame Watch glass

Fig. 5.5, page 62 — a magnesium ribbon burning with a dazzling white light (redrawn
sketch of the textbook photograph).

It is a chemical change.

Page 13 of 48

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

co m
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Magnesium + Oxygen → Magnesium oxide + Heat + Light
m l as
(Ribbon)
m .co(Air) (White powder)
a g
l a se
ag
Three observations prove it:

m
1. The shiny, silvery, flexible ribbon is gone and a white powder is left behind — a completely
. co ag
m
different substance, magnesium oxide.

l a se
ag— the white ash can never be turned back into a
2. A dazzling white light and a lot of heat are given out.
3. The change cannot be reversed
magnesium ribbon in the laboratory.

co m
se m.
m l a
Why it happens: When magnesium is heated enough, it combines with the oxygen
o
c The new substance, magnesium oxide, has none ofathe
of the.air. g properties of
se m
amagnesium
agl in which a substance combines with oxygen and gives out heat and/or light is called
— it does not shine, it cannot be bent, and it does not burn. A reaction

combustion.
m a s
m.co agl
l a se
a g
Caution: Never look straight at the burning ribbon. The light is bright enough to
hurt your eyes. Also, rusting of iron (Fig. 5.4) is a chemical change for the very same
reason — a new brown substance, iron oxide, is formed.
co m
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m
ase 5.5: Let us investigate — Page 62
agl
Activity

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

5.3.2 Combustion — is oxygen essential?

ACTIVITY

Q1 Place two identical candles on two separate petri dishes and light them. Cover one
of these with a glass tumbler, as shown in Fig. 5.6. What happens to the candle
flames in the two cases?

Flame goes out; smoke inside

Glass tumbler

Burning candle

(a) (b)

Fig. 5.6, page 62 — candle (a) burning in the open air, (b) covered with a glass tumbler.

The open candle keeps burning. The covered candle grows dim, flickers and goes out after
a short while.

CANDLE AIR SUPPLY WHAT THE FLAME DOES

(a) Not covered Fresh air reaches it all the time Burns steadily and goes on burning

(b) Covered with a glass Only the air trapped inside the Becomes smaller and is extinguished after
tumbler tumbler some time

Why it happens: Burning uses up the oxygen of the air. The open candle is supplied
with fresh oxygen continuously, so it keeps burning. Under the tumbler the amount
of oxygen is fixed. As the candle burns, the oxygen is used up and carbon dioxide
takes its place, so the flame is starved and dies out. This shows that oxygen is
essential for combustion.

Page 15 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Try This: Repeat with a big tumbler and a small tumbler over identical candles. The
candle under the bigger tumbler burns longer, because a bigger tumbler traps more
air and therefore more oxygen.

In-text Questions — Page 63
5.3.2 Combustion · Science and Society · Is the Presence of Air Enough for Combustion?

SCIENCE AND SOCIETY

Q1 We observe that the candle that was not covered (Fig. 5.6a) continues to burn,
whereas the candle that was covered with a glass tumbler (Fig. 5.6b) stops burning
after some time. Why does this happen?

Flame goes out; smoke inside

Glass tumbler

Burning candle

(a) (b)

Fig. 5.6, page 62 — candle (a) burning in the open air, (b) covered with a glass tumbler.

Because the covered candle does not have a continuous supply of air. The oxygen inside the
tumbler is soon used up, and without oxygen the flame is extinguished.

Wax + Oxygen (from air) → Carbon dioxide + Water vapour + Heat + Light

Under the tumbler: oxygen finishes → reaction stops → flame goes out

Page 16 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Why it happens: The component of air that supports combustion is oxygen. As the
covered candle burns, it turns the carbon of the wax into carbon dioxide, which
collects inside the tumbler. Carbon dioxide does not support burning, so as oxygen is
replaced by carbon dioxide, the flame weakens and is put out.

Q2 This can be confirmed by the presence of carbon dioxide in the glass tumbler
inverted on the candle. How can you test the presence of carbon dioxide gas?

By the lime water test. Pour a small amount of freshly prepared lime water into the petri dish
(or into the tumbler as soon as the candle goes out) and swirl it gently. The lime water turns
milky, which confirms that carbon dioxide is present.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(Lime water, clear) (white, insoluble → milky)

Why it happens: The carbon dioxide inside the tumbler was formed by the carbon
of the wax joining with the oxygen of the air. Because calcium carbonate does not
dissolve in water, the tiny white grains stay suspended and the lime water looks
milky — a change we can see with our eyes, which is why it makes such a useful test.

Q3 If a person’s clothes catch fire, what is the best way to extinguish the fire?

Wrap a blanket or a thick cloth tightly around the person. This cuts off the supply of air, and
the fire is extinguished.

Make the person stop, drop and roll on the ground — rolling smothers the flame in the
same way.
Water may also be poured over the person, if it is at hand.
Call for help and take the person to a doctor even if the burn looks small.

Why it happens: Fire needs three things — fuel, oxygen and heat. The clothes are
the fuel and cannot be removed instantly, so the quickest thing we can take away is
the oxygen. A blanket forms an airtight cover between the burning cloth and the air,
and the flame dies for the same reason the covered candle did.

Page 17 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Caution: Never use a synthetic blanket or cloth to put out a fire. Synthetic fibres
melt in the heat and stick to the skin, causing far worse injuries. Use cotton, wool or
a jute sack. Also, never let the person run — running fans the flame with fresh air.

Q4 We also know that paper is a combustible substance, but we can keep it in the air
for any length of time without it catching fire. What else is needed to start
combustion?

Heat — enough heat to raise the substance to its ignition temperature, the minimum
temperature at which it catches fire.
So combustion needs three things together, shown as the fire triangle:

Remove any one side and the fire stops

🔥
EN

HE
YG

AT
OX

FUEL (combustible substance)
The fire triangle (Fig. 5.8): a fire needs fuel, oxygen and heat at the same time.

1. A combustible substance, also called fuel — paper, wood, cotton, kerosene.
2. Oxygen — supplied by the air.
3. Heat that allows the fuel to reach its ignition temperature.

Page 18 of 48

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

co m
m.
Why it happens: Paper lying on a table already has two of the three — it is the fuel,

m l a se
and air is all around it. What is missing is the third. Room temperature is far below
o g A lighted
.c temperature of paper, so it never catches fire on its own.
a
m
the ignition

l a se or sunrays focused with a lens, supplies that missing heat.
ag
matchstick,

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c ag
Tip: Every way of putting out a fire removes one side of this triangle — a blanket or
sand removes oxygen, water removesm .
a s e heat, and closing the regulator of a gas

agl
cylinder removes the fuel.

co m
em.
coofmAir Enough for Combustion?
Activity 5.6: Let us investigate — Page 64
g l as
. a
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Is the Presence

a s
agl
ACTIVITY

om a s
. c agl
Q1 Hold a piece of paper with a pair of tongs and bring a lighted matchstick to it. It
quickly catches fire. Do we say m
s e that we need a fire to start the burning process?

a gla

co m
.
No. What we really need is heat, not a flame. A flame is only one convenient way of supplying

e m
as
that heat.
m l
.co a g
m it happens: The flame of the matchstick is already far hotter than the ignition
a s eWhy
agl temperature of paper, so the paper is heated up to that temperature in a fraction of
a second and catches fire at once. But the very next step of this activity proves that a
se m
com g l a
. a
flame is not necessary — sunrays focused by a magnifying glass, with no fire
m
ase
anywhere near, set the paper burning just as surely.

agl
Caution: Perform this activity only under the supervision of your teacher or an adult,
co m
and always hold the paper with tongs.
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m a s e
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a

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Q2 Take another piece of paper. Using a magnifying glass, focus the sunrays to make
the smallest and brightest spot on the paper, as shown in the Fig. 5.7a. Hold it there
for some time. What do you observe?

Sunrays

Sheet of paper

Smallest, brightest spot

Magnifying glass

Fig. 5.7(a), page 64 — focusing the sunrays with a magnifying glass to a small bright
spot on the paper (redrawn sketch of the textbook photograph).

For a while nothing seems to happen. Then the bright spot turns brown, the paper starts to
emit smoke, and finally it catches fire (Fig. 5.7b) — even though no flame was ever brought
near it.

Sunrays spread over the lens → focused into one tiny bright spot

Spot heats the paper → temperature of the paper rises steadily

Temperature reaches the ignition temperature → smoke → fire

Why it happens: A magnifying glass collects the sunlight falling on its whole surface
and brings it to a single small point. All that energy now heats a very small patch of
paper, so the temperature of that patch climbs quickly. Thus a substance can burn
even without fire.

Page 20 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Check it yourself: The nearer the spot is to a sharp, tiny point of light, the faster the
paper burns. A big blurred spot spreads the same energy over a larger area and the
paper only gets warm.

Q3 We observe that the paper starts to emit smoke, and then catches fire (Fig. 5.7b).
Thus, we find that a substance can burn even without fire. How do we explain this
change?

Sunrays
Smoke

Sheet of paper

Paper catches fire

Magnifying glass

Fig. 5.7(b), page 64 — the paper begins to smoke and then catches fire (redrawn sketch
of the textbook photograph).

By the idea of ignition temperature.
Focusing the sunrays heats the paper. Its temperature keeps rising with time. After some time
the paper becomes so hot that it starts burning. The minimum temperature at which a
substance catches fire is called its ignition temperature.

CASE SOURCE OF HOW LONG REASON
HEAT IT TAKES

Lighted matchstick Flame Almost at once The flame is already hotter than the
brought to paper ignition temperature of paper

Sunrays focused by a Concentrated After some time The paper has to be heated gradually
lens sunlight up to its ignition temperature

Page 21 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Why it happens: Every combustible substance has its own ignition temperature.
Below it, the substance simply gets warm; at or above it, the substance begins to
burn. This is why a piece of paper can sit in air for years without catching fire — it
never reaches its ignition temperature.

Activity 5.7: Think, pair, and share — Page 65
5.4 Can Physical and Chemical Changes Occur in the Same Process?

THINK, PAIR, AND SHARE

Q1 Look at the Fig. 5.9. Analyse what students are discussing about the burning candle.
What do you think?

The wax melts and Wax melts, flows, and
evaporates… solidifies in different shapes…

I think the wax burns, and
new substances are formed…

Maybe…

Maybe the wick draws up
the liquid wax…

Fig. 5.9, page 65 — five students discussing what changes occur when a candle burns.

All the students are partly right — and taken together they describe the whole story. The
burning of a candle involves both physical and chemical changes at the same time.

Page 22 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

WHAT THE STUDENT SAYS IS IT CORRECT? TYPE OF
CHANGE

“The wax melts and evaporates…” Correct — solid wax melts and the liquid wax Physical
turns into vapour near the flame

“Wax melts, flows, and solidifies in Correct — the wax that runs down the side cools Physical
different shapes…” and sets again

“Maybe the wick draws up the liquid Correct — the wick soaks up molten wax and Physical
wax…” carries it to the flame

“I think the wax burns, and new Correct — the wax vapour burns in the oxygen of Chemical
substances are formed…” the air

What actually happens: the wax of the candle melts, is carried up the wick, and evaporates due
to the heat of the flame. It is this vapour of wax that burns to produce the flame.

PHYSICAL CHANGES CHEMICAL CHANGE

Solid wax Liquid wax Wax vapour Vapour burns
melts rises up the wick forms → flame

no new substance — wax can be got back new substances
formed
Both happen together in one burning candle

The melting, flowing, solidification and evaporation of wax are physical changes; the burning of the
wax vapour is a chemical change.

Did you know? Michael Faraday found the candle so rich a subject that he gave a
whole series of lectures on it, Chemical History of a Candle, in the nineteenth century.
He used it to show the difference between melting, vapourisation and combustion.

In-text Questions — Page 66

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5.5 Are Changes Permanent?
co m
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Q1
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Once something has undergone a change, can we get it back in its original form?
m a g
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a g

m
Sometimes yes, sometimes no. Changes fall into two groups — those that can be reversed and

. co ag
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those that cannot.

as e
CAN BE REVERSED a g l CANNOT BE REVERSED

Melting of ice — it can be refrozen into ice
co m
Chopping vegetables — the pieces cannot return to their

e m.
original size and shape

m l as
m .co
Evaporation of water — the vapour can be condensed
a g
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Making popcorn — it can never go back to being a corn

l a
back into liquid water kernel

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a Folding a handkerchief, inflating a balloon, dissolving Burning of wood, rusting of iron, curdling of milk

m a s
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sugar in water

m .co
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a g
Why it happens: If the change only rearranged the substance — its shape, size or
state — the substance is still there and can usually be brought back. If the change
made a new substance, the old one has been used up, so it cannot simply be
co m
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brought back. That is why most chemical changes are irreversible.
m l as
.co
m Reversible is not the same as physical. Bursting a balloon and crushing chalk are a g
a s eTip:
agl physical but irreversible. Always test for the new substance separately.
se m
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asPage 66
Activity 5.8: Let us think l—
a g

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a

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Table 5.2: Can changes be reversed?

ACTIVITY

Q1 Think again about all the changes that we have discussed or talked about so far. In
which of these can we get back the object or substance in the form we started
with? Record your observations in Table 5.2.

S.NO. CHANGE THE ORIGINAL STATE CAN BE BROUGHT
BACK (YES/NO)

1. Melting ice cubes Yes

2. Chopping vegetables No

3. Boiling water Yes

4. Making popcorn from No
corn

5.

…

12.

Table 5.2, page 66: Can changes be reversed? The first four rows are filled in the book;
rows 5 to 12 are left blank for you.

Here is Table 5.2 completed with twelve of the changes discussed in this chapter.

Page 25 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

S.NO. CHANGE THE ORIGINAL STATE CAN HOW, OR WHY NOT
BE BROUGHT BACK (YES/NO)

1. Melting ice cubes Yes Put the water back in the freezer

2. Chopping No The pieces cannot be joined back
vegetables

3. Boiling water Yes Condense the steam back into
water

4. Making popcorn No New substances have formed; the
from corn kernel is destroyed

5. Folding paper into Yes Unfold and the same sheet is back
objects

6. Inflating a balloon Yes Let the air out

7. Bursting an inflated No The torn rubber cannot be made
balloon whole again

8. Crushing chalk into No The powder cannot be pressed
powder back into a stick

9. Adding beetroot Yes Evaporate the water and the
extract to water colouring matter is left behind

10. Blowing air into No A new substance, calcium
lime water carbonate, has been formed

11. Burning of a No The white magnesium oxide
magnesium ribbon cannot be turned back into the
ribbon

12. Rusting of an iron No A new substance, iron oxide, has
nail been formed

Why it happens: Read the “No” rows. Rows 4, 10, 11 and 12 are chemical changes
— the old substance no longer exists. Rows 2, 7 and 8 are physical changes that are
still irreversible, because the pieces cannot practically be put back together. Both
kinds of reason can make a change permanent.

In-text Questions — Page 66

Page 26 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

5.6 Are All Changes Desirable?

Q1 Many useful changes happen in our daily life. For example, the changing of milk
into curd, ripening of fruits, cutting of fruits, and cooking of food. All these are
desirable changes. Can you think of some other desirable changes happening
around you?

Yes — many changes around us are wanted and useful.

At home: curd being churned into butter, dough rising with yeast to make bread fluffy, wet
clothes drying in the sun, water boiling for drinking, rice cooking, tea leaves giving colour to
water.
On the farm and in the garden: a seed germinating into a seedling, buds opening into
flowers, grain ripening, kitchen waste and dry leaves rotting into compost.
In the workshop and factory: clay being shaped and baked into pots and bricks, iron being
melted and cast into tools, cotton being spun into thread and woven into cloth, sugarcane
juice becoming jaggery.
In our own body: the digestion of food, the healing of a wound, growth in our height.

Some changes are undesirable — the rusting of iron, the decay of food during storage, the
souring of milk in summer, paint drying and releasing substances that pollute the air, and the
rise of carbon dioxide in the atmosphere because of the fuel burnt by cars, trains and
aeroplanes.

Why it happens: Desirable and undesirable are not properties of the change itself
— they depend on the situation. Decomposition of food is undesirable in the
kitchen, but it is exactly what we want in a compost pit. Rusting is undesirable on a
gate, but the same reaction gives the beautiful red layer on basalt rock that the
chapter shows in Fig. 5.10b.

In-text Questions — Page 67

Page 27 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

5.7.1 Weathering of rocks

Q1 Have you seen heaps of sand, soil, and stones lying at the base of mountains as
shown in Fig. 5.10a? These are called sediments. How have these formed?

Rock face of the cliff

Sediments — sand, soil and stones at the base

Fig. 5.10(a), page 67 — heaps of sand, soil and stones (sediments) lying at the base of a
cliff (redrawn sketch of the textbook photograph).

Yes. These sediments are formed by the slow breaking down of large rocks — a process called
weathering, which involves both physical and chemical changes.

Page 28 of 48

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

co m
m.
KIND OF WHAT CAUSES IT WHAT IT DOES TO THE ROCK
CHANGE
m as e
.co Temperature changes due to a g l
Physical
a s em The rock expands by day and contracts by night until it

ag l climatic conditions cracks

Physical Growing roots of trees Roots push into cracks and widen them until pieces

co m
. ag
break off

m
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l a
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Physical Freezing of water inside
cracks

Chemical Water, or chemicals present in
co m
The composition of the rock itself changes — as when

em.
water, reacting with the rock black basalt, which contains iron, forms a red layer of

m l as
.co
iron oxide (Fig. 5.10b)

a g
a s em
agl Why it happens: Large rocks break into smaller pieces, those pieces break into still
smaller ones, and gravity brings them down to the base of the cliff, where they
m a s
c o agl
collect as heaps of stones, soil and sand. Continued over thousands of years,
weathering eventually leads to them .
a s e formation of soil.

agl
Did you know? The red colour of the sediment layer in Fig. 5.10b is the same iron

co m
.
oxide that makes an iron nail look brown when it rusts. Nature is doing on a
e m
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mountain what rain does to a gate.

.co a g l
sem
g l a
a In-text Questions — Page 68
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5.7.2 Erosion

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Have you noticed fine sand collecting on riverbeds or in lakes?

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Yes. That fine sand is formed when rock pebbles, soil and sediments are broken down and

m .c from one place to another by natural forces like windagand flowing water. This process is
se called erosion.
moved

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a

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Big rock → broken into pebbles and soil (weathering)

Pebbles and soil carried away by wind and flowing water (erosion)

Speed of water or wind falls in a lake or an ocean → material settles down
Sediments harden over thousands of years → new rocks

Why it happens: Fast-moving water and wind can carry loose material along with
them. When they slow down — as they do where a river enters a lake or the sea —
they can no longer hold the load, so the material settles at the bottom. On the way,
the pebbles knock against one another and against the riverbed, which is why river
pebbles look smooth and rounded.

Tip: Erosion, such as during a landslide, is a physical change — the rock is only
broken and moved, not turned into a new substance. Most of these changes take
thousands of years and cannot be reversed.

Let Us Enhance Our Learning — Page 69
Chapter-end exercise, Questions 1–10

LET US ENHANCE OUR LEARNING

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)

(c) (i) and (iii)

Page 30 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

STATEMENT TRUE OF A REASON
PHYSICAL CHANGE?

(i) The state of the substance Yes Melting ice changes state; folding
may or may not change. paper does not. Both are physical.

(ii) A substance with different No That is a chemical change — the new
properties is formed. substance has new properties.

(iii) No new substance is formed. Yes This is the defining test of a physical
change.

(iv) The substance undergoes a No A chemical reaction is what produces a
chemical reaction. 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).

Page 31 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

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

CHANGE CAN IT BE REASON
REVERSED?

(i) Stitching cloth Yes The stitches can be opened and the pieces of cloth come
to a shirt back — only tiny needle holes remain. A physical change.

(ii) Twisting of Yes Untwist it and the string is straight again. Only the shape
straight string changed.

(iii) Making idlis No Fermentation and steaming form new substances. The
from a batter cooked idli can never become batter again.

(iv) Dissolving Yes Evaporate the water and the sugar is recovered. The sugar is
sugar in water only spread out among the water particles.

(v) Drawing water Yes The water can simply be poured back into the well. Only its
from a well position changed.

(vi) Ripening of No New substances give the ripe fruit its colour, softness,
fruits sweetness and smell. A ripe mango cannot become raw
again.

(vii) Boiling water Not sure — see As a change of state it is reversible; but in an open pan the
in an open pan below steam escapes into the room and cannot be collected, so in
practice the water is lost.

(viii) Rolling up a Yes Unroll it and the mat is flat again.
mat

(ix) Grinding No Still wheat, so it is a physical change — but the flour can
wheat grains to never be pressed back into grains.
flour

(x) Forming of soil No Weathering involves chemical changes as well, and it takes
from rocks thousands of years. Soil cannot be turned back into the
parent rock.

Page 32 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

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)

STATEMENT TRUE / CORRECT STATEMENT AND REASON
FALSE

(i) Melting of wax is necessary True Solid wax cannot burn. It must first melt, rise up the wick
for burning a candle. and evaporate; it is the vapour of wax that burns.

(ii) Collecting water vapour by False Correct statement: Collecting water vapour by
condensing involves a condensing involves a physical change. Only the state
chemical change. changes, from gas to liquid; the substance is water
before and after.

(iii) The process of converting True Bacteria and fungi break the leaves down into completely
leaves into compost is a new substances. Compost can never be turned back into
chemical change. leaves.

(iv) Mixing baking soda with True A new gas, carbon dioxide, is formed — it fizzes and
lemon juice is a chemical turns lime water milky.
change.

Page 33 of 48

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

co m
m.
Fill in the blanks in the following statements: (i) Nalini observed that the handle of
se
Q4

o m l a
her cycle has got brown deposits. The brown deposits are due to ________, and this is

m .c change. (ii) Folding a handkerchief is a ________ changeagand can be ________.
a ________
se A chemical process in which a substance reacts with oxygen with evolution of
a(iii)
agl heat is called ________, and this is a ________ change. (iv) Magnesium, when burnt in air,
produces a substance called ________. The substance formed is ________ in nature.

co m
. ag
Burning of magnesium is a ________ change.

e m
g l as
a
STATEMENT ANSWER
co m
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(i) The brown deposits are due to ______, and this is a ______ change. rusting (the brown substance is

a g
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rust, that is iron oxide); chemical

a s
a gl(ii) Folding a handkerchief is a ______ change and can be ______. physical; reversed

a s
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(iii) A chemical process in which a substance reacts with oxygen with

agl
combustion; chemical

.
evolution of heat is called ______, and this is a ______

a s ema substance called ______.
agl
(iv) Magnesium, when burnt in air, produces magnesium oxide; basic;
The substance formed is ______ in nature. Burning of magnesium is a chemical
______ change.

co m
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.c of the air to give a new brown substance — thataisgrusting, and a new
Why it happens: In (i), the iron of the cycle handle joins with the oxygen and
m
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moisture

agl
substance means a chemical change. In (iv), the white powder left after the ribbon

m
burns is magnesium oxide; when it is dissolved in water it turns red litmus blue,

a se
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which shows that it is basic in nature.
. a g
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agl
Q5 Are the changes of water to ice and water to steam, physical or chemical? Explain.

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coare l

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Both physical changes.
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Water (liquid) freezes → Ice (solid) — and ice melts back into water
m a s
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Water (liquid) boils → Steam (gas) — and steam condenses back into water
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Substance before = Substance after = water
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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

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.

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.

Page 35 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Q7 Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is
this change physical or chemical and why?

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

PHYSICAL PART OF WEATHERING CHEMICAL PART OF WEATHERING

Temperature changes make rocks expand and contract Water, and chemicals dissolved in water, react with the
until they crack minerals of the rock

Water freezing inside cracks expands and splits the rock Iron in black basalt turns into red iron oxide (Fig. 5.10b)

Growing roots of trees widen cracks and break pieces off The composition of the rock itself is altered — new
substances are formed

Only size and shape change — no new substance New 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.

Page 36 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

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.

PART OF THE STORY CORRECT REASON
OPTION

He chops vegetables, peels physical Only size and shape change; a chopped potato
potatoes, and cuts fruits changes is still a potato

He collects the seeds, fruits, and physical change Only the position of the material changes;
vegetable peels into a clay pot nothing new is formed

The materials decompose due to chemical Bacteria and fungi break the waste into
bacteria and fungi, forming change completely new substances; compost cannot
compost become peels again

The seeds germinate, plants grow chemical Germination and growth are life processes in
and bloom into colourful flowers change 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.

Page 37 of 48

Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

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.

A — Physical only B — Chemical only

Rusting
Tearing of paper Curdling of milk
C — Both
Melting of ice Ripening of fruits
Process of
Folding of clothes burning a Burning of magnesium
candle
Mixing baking soda
with vinegar

The nine changes sorted into areas A, B and C.

AREA CHANGES REASON

A — Physical Tearing of paper; Melting of ice; Folding Only shape, size or state changes — no new
changes of clothes substance

B — Chemical Rusting; Curdling of milk; Ripening of A new substance is formed in each case — iron
changes fruits; Burning of magnesium; Mixing oxide, curd, the substances of ripening,
baking soda with vinegar magnesium oxide, carbon dioxide

C — Both Process of burning a candle Melting, flowing, solidifying and evaporating
physical and of wax are physical; the burning of wax
chemical vapour is chemical

Page 38 of 48

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co m
m.
The experiments shown in Fig. 5.11a, b, c, and d were performed. Find out in which
e
Q10

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case(s) did lime water turn milky and why?

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Vinegar and Lemon juice and

a g baking soda

Lime water
vinegar

Lime water

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(a) (b)

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Vinegar and Lemon juice and

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common salt baking soda

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Lime water Lime water

(c) agl (d)

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Fig. 5.11, page 71 — four identical set-ups; in each, the gas from the left test tube is
m l
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bubbled through lime water in the right one.

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The lime water turns milky in (a) and (d) only.
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SET-UP
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MIXTURE IN THE FIRST TEST TUBE GAS PRODUCED? LIME WATER

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(a) Vinegar and baking soda Yes — carbon dioxide Turns milky

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(b) Lemon juice and vinegar No Stays clear

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(c) Vinegar and common salt No Stays clear

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(d) Lemon juice and baking soda Yes — carbon dioxide Turns milky

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

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

Exploratory Projects — Page 71
Projects and investigations at the end of the chapter

EXPLORATORY PROJECTS

Q1 Write a message on a piece of paper using lemon juice as ink and let it dry. The
message will become invisible. Now use a warm iron over the paper (or hold the
paper over the flame of a candle, taking care that it does not catch fire). The
invisible letters turn dark brown as the paper gets warm. Can any of these changes
be reversed?

Only the first change can be reversed; the second cannot.

STEP WHAT HAPPENS TYPE OF CAN IT BE REVERSED?
CHANGE

The lemon-juice writing The water of the juice Physical Yes — wet the paper again
dries and becomes evaporates, leaving a thin and the mark reappears
invisible colourless film faintly

The warm iron makes The dried lemon juice is Chemical No — the brown letters can
the letters turn dark charred — new brown never be made invisible again
brown substances are formed

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Why it happens: Lemon juice contains carbon compounds (sugars and citric acid).
These break down and char at a much lower temperature than paper does. So the
written parts turn brown while the rest of the paper is still only warm — which is
exactly why the message appears.

Caution: Perform this activity under the supervision of an adult. If you use a candle,
keep the paper well above the flame and moving, so that it does not catch fire.

Q2 We hear a lot of news about landslides and breaking of rocks in hilly regions these
days, causing a lot of damage to life and property. Discuss what steps we can take
to reduce landslides and rock erosion.

Method for the discussion: first list what makes a hillside slip — loose soil, heavy rain, steep
cutting for roads and houses, and the loss of tree cover — then match one preventive step to
each cause. A good answer should name at least four steps and say how each one works.

STEP HOW IT HELPS

Afforestation — plant trees, grasses and deep-rooted Roots bind the soil together and leaves break the
shrubs on the slopes force of falling rain

Stop deforestation and unregulated grazing Bare slopes wash away far faster than covered
ones

Terrace farming instead of ploughing along a slope Flat steps slow the water down so it cannot carry
soil away

Retaining walls, gabions (wire cages filled with stones) Hold loose rock and soil in place along roads
and wire netting on cut faces

Proper drainage — build drains that carry rain water off Water soaking into the hill makes it heavy and
the slope slippery, which is the commonest trigger

Control blasting, quarrying and steep road cutting; Every cut removes the support at the foot of the
avoid heavy construction on fragile slopes hill

Early-warning systems and rainfall monitoring People can be moved to safety before a slide
happens

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Sample answer: “Landslides in our hills have increased mainly because trees are
being cut and slopes are being cut steeply for roads and hotels. We should plant
deep-rooted trees on bare slopes, build proper drains so that rain water does not
soak into the hillside, protect road cuttings with retaining walls and wire netting,
practise terrace farming, and follow strict rules about construction in landslide-
prone areas.”

Q3 Observe the activities going on in the kitchen and note any changes that can be
reversed. Are these physical or chemical changes?

How to do it: sit in the kitchen for half an hour with a notebook. For every change you see, write
three things — what changed, whether the original can be got back, and whether a new
substance was formed.

CHANGE SEEN IN THE KITCHEN CAN IT BE PHYSICAL OR
REVERSED? CHEMICAL?

Ice cubes melting in a glass Yes Physical

Water boiling; steam condensing on the lid Yes Physical

Sugar or salt dissolving in water Yes (evaporate the water) Physical

Making balls of dough and rolling them into Yes (roll it back into a Physical
chapatis ball)

Butter melting on a hot tawa Yes (it sets again on Physical
cooling)

Grinding soaked rice and dal into a paste No Physical, but irreversible

Milk setting into curd; food being cooked; LPG No Chemical
burning

Conclusion: almost every change in the kitchen that can be reversed turns out to be a physical
change — usually a change of state or of shape. The changes that cannot be reversed, such as
cooking, curdling and burning of fuel, are the chemical ones.

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Q4 Yeast is added while baking bread to make it fluffy and soft. How does yeast work?
Try and find out! Take a small bottle, some sugar, fresh yeast, water, and a balloon.
Make a sugar solution in the bottle by mixing two teaspoons of sugar and a small
amount of water. Now add a spoonful of yeast and cover the mouth of the bottle
with a balloon. Leave it undisturbed for about an hour. What do you observe?
Carefully take off the balloon, holding its mouth tightly closed and attach it to
another small bottle containing freshly prepared lime water. Shake the bottle so
that the contents of the balloon get mixed with lime water. What do you observe?
What can you conclude from this experiment? Identify all the changes occurring in
the experiment and state which of them are physical and chemical changes.

What you observe after an hour: the mixture becomes frothy and bubbly, gives off a faint
sour, bread-like smell, and the balloon slowly inflates and stands up on the mouth of the
bottle. A gas is clearly being produced inside.
What you observe with the lime water: when the gas from the balloon is shaken with freshly
prepared lime water, the lime water turns milky.
What you can conclude: the gas made by the yeast is carbon dioxide. Yeast is a living fungus;
it feeds on the sugar and breaks it down, releasing carbon dioxide. In dough, the same gas is
trapped as thousands of tiny bubbles, which push the dough up and leave holes in it — that is
what makes bread fluffy and soft.

Sugar + Yeast → Carbon dioxide + Other substances

Carbon dioxide + Lime water → Calcium carbonate (white, insoluble) + Water → milky

CHANGE IN THE PHYSICAL OR REASON
EXPERIMENT CHEMICAL?

Sugar dissolving in water Physical The sugar only spreads out; it can be
recovered by evaporation

Yeast acting on the sugar and Chemical A new substance, carbon dioxide, is
producing gas formed

The balloon stretching and Physical Only the size and shape of the rubber
inflating change

Gas moving from the bottle into Physical Only the position of the gas changes
the balloon

Lime water turning milky Chemical A new substance, calcium carbonate, is
formed

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Tip: Use lukewarm water, not hot. Yeast is alive — very hot water kills it and then the

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Why it happens: The colour is not a dye that is made and destroyed. The
chameleon's skin carries special cells containing tiny crystals arranged in a regular
pattern. When the animal is calm, the crystals lie close together and the skin reflects
blue and green light; when it is excited, angry or frightened, the skin is stretched
slightly, the crystals move apart, and now yellow, orange and red light is reflected
instead. Pigment-carrying cells in another layer can also spread out or bunch up.
Since no new substance is formed — only the spacing of the crystals and the
spreading of pigment change — this is a physical and reversible change.

Did you know? Chameleons change colour far more for communication and
temperature control than for hiding. A dark skin absorbs more heat on a cold
morning, and a bright display warns off a rival. Cuttlefish, octopuses and some fish
can change colour in the same reversible way.

Where to look: your school library's encyclopaedia of animals, the NCERT reference books on
animal behaviour, or a trusted website such as a national museum or a zoological society page.
Note down the source you used.

Chapter at a glance
A physical change alters only physical properties such as shape, size or state. No new
substance is formed — folding paper, crushing chalk, melting ice.
A chemical change forms one or more new substances through a chemical reaction,
which can be written in short as a chemical equation. Blowing air into lime water, vinegar +
baking soda, rusting and burning are chemical changes.
Lime water turning milky is the standard test for carbon dioxide. Calcium hydroxide +
Carbon dioxide → Calcium carbonate (insoluble, white) + Water.
Combustion is a reaction with oxygen that gives out heat and/or light. It needs three
things — the fire triangle: a combustible substance (fuel), oxygen, and enough heat to
reach the ignition temperature.
One process can involve both kinds of change. In a burning candle, melting, flowing,
solidifying and evaporating of wax are physical; the burning of wax vapour is chemical.
Some changes can be reversed (melting ice, boiling water) and some cannot (chopping
vegetables, making popcorn). Some are desirable (milk to curd, cooking) and some
undesirable (rusting, food decay) — and the same change can be either, depending on the
situation.

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Slow natural changes shape the Earth: weathering (physical breaking plus chemical attack
on rocks) forms soil, and erosion by flowing water and wind is a physical change that
carries the pieces away.

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS WHERE IT EXAMPLE FROM THE
COMES IN THE BOOK
CHAPTER

Physical change A change in which only physical Section 5.1, page Folding paper into objects;
properties (shape, size, state) change 59 crushing chalk into powder;
and no new substance is formed melting ice

Chemical change A change in which one or more new Section 5.2, page Blowing air into lime water
substances are formed 60 — a white insoluble
substance settles

Chemical reaction The process by which new substances Page 60 Calcium hydroxide reacting
are formed in a chemical change with carbon dioxide

Chemical The short form in which a chemical Page 61 Vinegar + Baking soda →
equation reaction is written Carbon dioxide + Other
substances

Lime water test Lime water turns milky (cloudy) when Activity 5.3 and 5.4, Used to identify the gas
carbon dioxide is passed through it pages 60–61 from vinegar and baking
soda

Rusting Formation of a new brown substance, Section 5.3.1, page Rusted iron nails, Fig. 5.4
iron oxide, on iron 62

Combustion A chemical reaction in which a Section 5.3.2, page Magnesium + Oxygen →
substance reacts with oxygen and 62 Magnesium oxide + Heat +
produces heat and/or light Light

Combustible A substance that undergoes Page 62 Wood, paper, cotton,
substance combustion; also called a fuel kerosene

Ignition The minimum temperature at which a Activity 5.6, page Paper reaches it when
temperature substance catches fire 64 sunrays are focused on it
with a lens

Fire triangle The three requirements for combustion Fig. 5.8, page 64 Covering a candle with a
— fuel, oxygen and heat tumbler removes oxygen,
so it goes out

Reversible / A change in which the original Section 5.5, page Melting ice — yes; making
irreversible substance can / cannot be brought back 66 popcorn — no
change

Weathering The physical and chemical changes Section 5.7.1, page Black basalt turning red
that break down rocks and finally form 67 from iron oxide, Fig. 5.10b

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Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical AglaSem · NCERT Solutions

TERM WHAT IT MEANS WHERE IT EXAMPLE FROM THE
COMES IN THE BOOK
CHAPTER

soil

Erosion Carrying away of broken rock, soil and Section 5.7.2, page Smooth river pebbles; fine
sediment by wind and flowing water 68 sand on riverbeds
— a physical change

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