NCERT Solutions Class 7 Science Chapter 7 Heat Transfer in Nature

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 7: Heat Transfer in
Nature

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

89 – 104 19 48 English

Solutions, notes, sample papers & more at 43 pages

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

CLASS 7 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 7: Heat Transfer in Nature
Complete NCERT Solutions for Class 7 Science Chapter 7 Heat Transfer in Nature from the NCERT textbook
Curiosity. Every question is answered — Activities 7.1 to 7.5 with Tables 7.1 to 7.5 filled in, the Dive Deeper,
Know a Scientist and Science and Society boxes, every in-text question from pages 89 to 100, all ten Let Us
Enhance Our Learning exercises and the three Exploratory Projects — written in the book's own words and
register.

TEXTBOOK BOOK PAGES

Curiosity (Class 7) 89 – 104

SECTIONS QUESTIONS

19 48

MEDIUM

English

In-text Questions — Pages 89–90
Chapter opener

Q1 Why are cooking utensils generally made of metals?

Because metals are good conductors of heat. A material that lets heat pass through it easily is
called a good conductor, and all metals belong to this group — a fact you met in the chapter
‘The World of Metals and Non-metals’.

Why it happens: when a metal pan is put on the flame, the particles of the metal
touching the flame get heated first. Each heated particle passes the heat on to the
particle next to it, and in this way heat travels quickly from the bottom of the pan
into the food. This process is called conduction. A pan made of wood or clay would
hardly let the heat through, and Pema's grandmother could never cook thukpa in it.

Tip: notice that the handle of a good pan is made of wood, plastic or bakelite — poor
conductors — so that the heat does not reach your hand.

Page 1 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Q2 How does heat get transferred in these materials?

In a solid metal, heat is transferred by conduction — from the hotter part of the object to its
colder part.

Hot end of the metal → particle gets heated

Heated particle → passes heat to its neighbour

Neighbour → passes it on further

… and so on, until the cold end also becomes hot

Why it happens: the particles of a solid are tightly packed and cannot move from
their places. So they cannot carry the heat along with them. What they can do is
vibrate faster when heated and shake up the particle next to them. Heat therefore
travels through the material while the particles themselves stay where they are.
Activity 7.1, with the pins falling one after the other, shows exactly this.

Activity 7.1: Let us experiment — Page 90
7.1 Conduction of Heat

ACTIVITY

Q1 What will happen to the pins? Will they remain attached to the strip or will they
fall?

The pins will not remain attached — all four pins will fall off, one after the other.

Why it happens: the pins are stuck to the metal strip with wax. The end of the strip
away from the stand is being heated, and heat travels along the strip by conduction.
When the heat reaches a pin, the wax holding that pin melts, the pin loses its grip
and drops.

Page 2 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Check it yourself: the pins do not all fall together. That single observation is the
proof that heat needs time to travel from the hot end to the cold end.

Q2 Predict the order in which the pins will fall from the strip.

The candle is kept at the end of the strip that is away from the stand, and the pins are labelled I,
II, III and IV starting from that heated end (Fig. 7.1). So the correct prediction is:

Pin I falls first (closest to the flame)

then pin II

then pin III
and pin IV falls last (farthest from the flame)

Why this order: heat spreads outwards from the heated end. The nearer a pin is to
the flame, the sooner the strip under it becomes hot enough to melt the wax.
Distance, therefore, decides the order — nearest falls first.

Q3 Record your observations in Table 7.1.

TABLE 7.1: FALLING OF PINS

PIN FALLING FIRST REASONS FOR WHAT YOU OBSERVED

PREDICTION OBSERVATION

Table 7.1 as printed on page 90 — copy it into your notebook and fill it in.

This is what the completed Table 7.1 looks like after the activity is performed.

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Class 7 Science Chapter 7 Heat Transfer in Nature
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TABLE 7.1: FALLING OF PINS

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PIN FALLING FIRST REASONS FOR WHAT YOU OBSERVED
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PREDICTION OBSERVATION

Pin I — because it is Pin I fell first, followed Heat travels along the metal strip from the heated end

. comso the wax under pin I melts first. The heat then ag
nearest to the burning in order by pins II, III towards the cold end by conduction. It reaches pin I

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candle and IV first,

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which are progressively farther away.

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Did you know? If you repeat the activity with the candle in the middle of the strip,
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the two pins on either side of the flame — equally distant from it — will fall almost
m . That is the idea tested in question 1(ii) of the exercise.
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together.

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In-text Questions — Page 91
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7.1 Conduction of Heat

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Why does pin I fall before pin II? Why did all the pins not fall together?
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PinemI falls first because it is the nearest to the candle flame, and the pins do not fall together
agl because heat takes time to travel along the strip.
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Why it happens: conduction is a step-by-step process. The particle at the heated
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end gets hot, passes heat to the next particle, that one to the next, and so on. So the

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strip becomes hot gradually, first near the flame and only later at the far end. The
wax under pin I therefore reaches its melting point before the wax under pin II, and
so on down the line. If heat could jump instantly from one end to the other, all four
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pins would indeed fall at the same moment — they do not, and that is the evidence
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for conduction.
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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Q2 From your observations, what can you infer? Do you think that heat is being
transferred along the metal strip from the end that is being heated?

Yes. The observations show clearly that heat is being transferred along the metal strip, from the
heated end towards the colder end.
The inference is:

Heat travels from the hotter part of an object to its colder part. This process is called
conduction.
The transfer is not instant — it moves along the strip step by step, which is why the pins fall
in order.
The metal itself does not move, and neither do its particles. A heated particle simply passes
the heat on to its neighbour.
Metals allow this to happen easily, so they are good conductors of heat.

Tip: the falling pins are only an indicator. The real event you are watching is the
melting of wax, and wax melts only when the strip beneath it becomes hot.

Q3 If we use a strip made of a material like wood or glass in place of a metal strip to
perform Activity 7.1, the pins will not fall. Can you think of the reason for this based
on our learning from the chapter ‘The World of Metals and Non-Metals’?

Because wood and glass are non-metals, and non-metals such as these do not allow heat to
pass through them easily. They are poor conductors (insulators) of heat.

Why it happens: in a metal, heat is passed on rapidly from particle to particle, so the
whole strip warms up quickly. In wood or glass the transfer is extremely slow. The
heat stays bunched up near the flame, and the part of the strip under the pins never
becomes hot enough to melt the wax. So the pins stay stuck.

Did you know? Clay and porcelain are poor conductors too. That is exactly why tea
or coffee served in a kulhad or a porcelain cup stays hot much longer than in a steel
glass — and why the steel glass burns your fingers while the clay cup does not.

Page 5 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Q4 List some materials around you and classify them as good or poor conductors of
heat in Table 7.2.

TABLE 7.2: LIST OF GOOD OR POOR CONDUCTORS OF HEAT

S.NO. MATERIAL GOOD OR POOR CONDUCTOR OF HEAT

1. Steel Good conductor

2. Wood

3.

Table 7.2 as printed on page 91 — the first row is filled in for you, the rest are blank.

Here is Table 7.2 filled with materials found in any home or classroom.

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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

TABLE 7.2: LIST OF GOOD OR POOR CONDUCTORS OF HEAT

S.NO. MATERIAL GOOD OR POOR CONDUCTOR OF HEAT

1. Steel Good conductor

2. Wood Poor conductor

3. Aluminium (pressure cooker, tawa) Good conductor

4. Copper (wire, bottom of a vessel) Good conductor

5. Iron (nail, ladle) Good conductor

6. Glass (tumbler, window pane) Poor conductor

7. Clay / porcelain (kulhad, cup) Poor conductor

8. Plastic (mug, bucket) Poor conductor

9. Wool (sweater, blanket) Poor conductor

10. Paper and cardboard Poor conductor

11. Rubber (chappal sole, handle grip) Poor conductor

12. Air Poor conductor

Tip: a quick test — touch a metal object and a wooden object lying in the same
room. The metal feels colder although both are at the same temperature. It feels
colder only because it is a good conductor and carries heat away from your hand
quickly.

Q5 Does your list include air? If it is there on the list, where have you placed it?

Yes — and air must be placed in the column of poor conductors (insulators) of heat.

Why it happens: the particles of a gas are far apart, so heat cannot be passed easily
from particle to particle. Trapped air is therefore one of the best insulators we have,
and we use it everywhere:

Page 7 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Woollen clothes trap air in their pores; the trapped air cuts down the flow of heat from our
body to the surroundings, so we feel warm.
Two thin blankets keep us warmer than one thick blanket, because a layer of air is trapped
between them (Fig. 7.2).
Hollow bricks in outer walls hold air inside them, keeping houses warm in winter and cool in
summer.

Did you know? Air is a poor conductor, but it is an excellent convector — when it is
free to move it carries heat very well. It insulates only when it is trapped and cannot
flow.

In-text Questions — Page 92
7.1 Conduction of Heat / 7.2 Convection

Q1 Is it possible to construct houses that are not affected much by the outside heat
and cold?

Yes, it is. Houses built in places with a very hot or a very cold climate use the idea of heat
transfer to stay cool or warm — mainly by putting a poor conductor of heat between the inside
and the outside.

Hollow bricks in the outer walls: the air trapped inside the hollow space is a poor conductor,
so it keeps the house warm in winter and cool in summer.
Double wooden walls filled with cow dung and mud, as in the Mori block of Uttarkashi,
Uttarakhand. Wood and mud are poor conductors, so they prevent heat loss and keep the
house warm through heavy snowfall.
Thick mud walls and thatched roofs in hot, dry parts of India, and light-coloured outer
walls that reflect the Sun's heat.

Why it works: heat always flows from the hotter side to the colder side. If the wall
between them is a poor conductor, that flow is slowed down almost to a stop — heat
cannot get in during summer and cannot escape during winter.

Q2 Why is the smoke going up?

Because the smoke is hotter and lighter than the air around it.

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Class 7 Science Chapter 7 Heat Transfer in Nature
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Wood burns → hot gases and tiny solid particles are released (this mixture is smoke)
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Smoke is warmer than the surrounding air
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Warm air/gas expands → occupies more space → becomes lighter
a⇒ the smoke rises up

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Why it happens: when a gas is heated it expands. The same amount of matter now
ag lighter than the cooler air around it, and the
fills a bigger space, so it becomes
cooler, heavier air pushes it upward. Cooler air moves in from the sides to take its

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place — this movement of heated matter is called convection.

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Try This: light an agarbatti in a still room and watch the thin thread of smoke. It

a gl always climbs first, then bends when it cools down and mixes with the room air.
m a s
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ase— Pages 92–93
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Activity 7.2: Let us investigate
7.2 Convection

ACTIVITY
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Observe what happens to the cup.
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Q1

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The paper cup that has the burning candle below it rises up, and the wooden stick tilts — the
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other cup goes down [Fig. 7.3(b)].
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Why it happens: the candle heats the air around its flame. This hot air enters the
inverted cup and warms the air inside it. Warm air expands, occupies more space
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and becomes lighter than the surrounding air, so it pushes up against the inside of
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the cup and the cup is lifted. The cup on the other side has only ordinary, cooler air
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Check it yourself: the stick must be balanced horizontally before the candle is lit.
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Only then can you be sure that the tilt is caused by the heated air and not by

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unequal weights.
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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Q2 Record your observations in Table 7.3 and think of probable reasons.

TABLE 7.3: RECORDING OBSERVATIONS AND PROBABLE REASONS

OBSERVATION ABOUT THE PROBABLE REASONS FOR THE
CUPS OBSERVATION

Table 7.3 as printed on page 93 — copy it into your notebook and fill it in.

Table 7.3, completed:

TABLE 7.3: RECORDING OBSERVATIONS AND PROBABLE REASONS

OBSERVATION ABOUT THE PROBABLE REASONS FOR THE OBSERVATION
CUPS

The cup kept above the burning candle The air near the candle flame gets heated. As the air inside that cup
rises up; the other cup comes down and the warms up, it expands and occupies more space, so it becomes lighter
wooden stick no longer stays horizontal. and rises up, carrying the cup with it. The air under the second cup is
not heated, so that side stays down.

Why it matters: this is the first direct evidence in the chapter that a gas transfers
heat by actually moving — the beginning of convection. Compare it with Activity 7.1,
where nothing moved at all.

In-text Questions — Page 93

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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

7.2 Convection

Q1 You observed that the cup under which the candle was placed, rises up (Fig. 7.3b).
Why is it so?

Wooden
stick

Thread

Paper cup

Burning
candle

Redrawn sketch of Fig. 7.3(b), page 93 — the set-up after a burning candle is placed
below one of the two paper cups.

Because the air inside that cup becomes hot, expands and turns lighter, and rising hot air lifts
the cup.

Candle flame heats the air around it

Air inside the cup warms up

Warm air expands → occupies more space

Same air in a bigger space → becomes lighter

⇒ it rises up and the cup goes up

Try This: place a partially inflated balloon in the Sun (Fig. 7.4). After some time the
air inside gets heated, expands, and the balloon becomes visibly larger — the same
expansion, made easy to see.

Page 11 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Q2 How does heat transfer take place in liquids? Do liquids also rise up when heated
like air?

Heat travels through liquids mainly by convection — and yes, a liquid also rises when it is
heated, exactly as air does.

Water at the bottom gets heated

It expands → becomes lighter → rises

Cooler, heavier water at the sides comes down to take its place

This water is heated in turn and also rises

⇒ a convection current is set up until all the water is hot

Why it happens: unlike a solid, the particles of a liquid can move about freely. So
they do not merely pass heat to their neighbours — they carry the heat with them
from one place to another. That is the whole difference between conduction and
convection.

Check it yourself: Activity 7.3, with a grain of potassium permanganate at the
bottom of a beaker, makes this invisible current visible as a coloured streak.

Activity 7.3: Let us find out — Pages 93–94
7.2 Convection

ACTIVITY

Q1 Observe the movement of the coloured streak in the water.

As soon as the candle is lit, a streak of purple colour rises straight up from the centre of the
beaker's base, spreads out near the top, and then comes down along the sides of the beaker
[Fig. 7.5(b)]. The streak keeps going round and round until the whole beaker of water is
coloured.

Page 12 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Beaker

Water
hot water rises

cool water
sinks

candle

The convection current in Activity 7.3 — the coloured streak goes up in the middle, above the flame,
and comes down along the cooler sides.

Why it happens: the potassium permanganate simply colours the water so that its
movement becomes visible. The water at the bottom, right above the flame, gets
heated, expands, becomes lighter and rises. The comparatively cooler and heavier
water at the sides sinks to take its place, gets heated in turn, and rises again. This
continuous circulation is convection, and the coloured streak is nothing but the path
of the moving water particles.

Caution: handle potassium permanganate and the burning candle only under the
supervision of your teacher or an adult, and keep the beaker on a tripod stand with
a wire gauze.

In-text Question — Page 94

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Class 7 Science Chapter 7 Heat Transfer in Nature
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7.2 Convection
co m
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m l as
Q1
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Why does the streak of coloured water go up in the middle and come down from the
m a g
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sides?

a g

co m
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Because a convection current has been set up in the beaker — hot water rises in the middle
e m
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and cooler water sinks at the sides.

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Water at the bottom (above the flame) gets heated

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It expands, becomes lighter and rises up the middle
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Water at the sides is cooler and heavier
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⇒seit comes down the sides to take the place of the rising water
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This water now gets heated and rises in turn

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The cycle repeats until the entire volume of water is heated

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Why it happens: the candle is placed right below the centre of the base, so only the
water in the middle is heated directly. Heating makes water expand — the same
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mass now occupies more volume, so it becomes lighter than the water around it and

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g therefore gets heated
.csides flows down and inwards. The entire volume of water
floats upward. Nature does not leave a gap behind, so the cooler, heavier water at
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the

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by the actual movement of water particles, which is exactly what convection

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means.

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Tip: this is also why we heat a vessel of water from below and never from the top.

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Heating from the top gives you no convection current at all — the point tested in
question 6 of the exercise.

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. 7.4: Let us investigate — Pages 94–95 a g l
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Activity

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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

7.2.1 Land and Sea Breeze

ACTIVITY

Q1 Measure the temperature of soil and water every 5 minutes and record the data in
Table 7.4.

TABLE 7.4: TEMPERATURE OF SOIL AND WATER WHEN HEATED

S.NO. TIME TEMPERATURE OF SOIL TEMPERATURE OF WATER
(MIN) (°C) (°C)

1. 0

2. 5

3. 10

4. 15

5. 20

Table 7.4 as printed on page 95 — the times are given, the two temperature columns are
blank.

Two identical bowls, one half filled with soil and one half filled with water, are kept side by side
in bright sunlight with a laboratory thermometer in each (Fig. 7.6). A typical set of readings
taken on a clear, sunny day looks like this.

TABLE 7.4: TEMPERATURE OF SOIL AND WATER WHEN HEATED

S.NO. TIME (MIN) TEMPERATURE OF SOIL (°C) TEMPERATURE OF WATER (°C)

1. 0 30 30

2. 5 34 31

3. 10 38 32

4. 15 41 33

5. 20 44 34

Page 15 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Check it yourself: your own numbers will be different — they depend on the day,
the place and the time. What must come out the same is the pattern: the soil column
climbs much faster than the water column. Record your own readings; do not copy
these.

Care while measuring: keep the bulb of each thermometer fully buried in the soil
and fully dipped in the water, and make sure it does not touch the bottom or the
sides of the bowl — otherwise you measure the temperature of the bowl, not of the
soil or the water.

Q2 Did the temperature rise by the same amount for both the soil and the water at the
same time?

No. In the same interval of time, the temperature of the soil rises much more than the
temperature of the water.
With the readings above, after 20 minutes in the same sunlight:

Rise in temperature of soil = 44 °C − 30 °C = 14 °C

Rise in temperature of water = 34 °C − 30 °C = 4 °C

Why it happens: both bowls receive the same amount of heat from the Sun, but
water needs far more heat than soil to raise its temperature by one degree. So the
same heat produces a big rise in the soil and only a small rise in the water.

Q3 If not, which one got heated faster?

The soil got heated faster.

Why it matters: this single fact explains the whole of the next section. Land (soil,
sand, rock) heats up faster than water during the day — and, as the book goes on to
show, it also cools down faster at night. That difference is what creates sea breezes
and land breezes along a coast.

Page 16 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Did you know? The same property makes a beach at noon unbearable to walk on
barefoot while the sea water just a few steps away still feels cool.

Q4 How much was the rise in temperature of the soil and the water in 20 minutes?

Using the sample readings of Table 7.4 (both started at 30 °C):

Soil: 44 °C − 30 °C = 14 °C
Water: 34 °C − 30 °C = 4 °C

Difference = 14 °C − 4 °C = 10 °C

So in the same 20 minutes and the same sunshine, the soil became about 10 °C hotter than the
water did — the soil heated up roughly three to four times as much.

Tip: always subtract the reading at 0 minutes from the reading at 20 minutes. The
rise in temperature is what matters here, not the final temperature.

In-text Question — Page 95
7.2.1 Land and Sea Breeze

Q1 Does the soil also cool faster than water?

Yes. If the heated set-up is brought indoors and left for 20 minutes, the thermometer readings
show that the soil cools faster than water — just as it got heated faster.

TIME AFTER BRINGING TEMPERATURE OF SOIL TEMPERATURE OF WATER
INDOORS (°C) (°C)

0 min 44 34

10 min 36 32

20 min 32 31

Page 17 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Why it happens: water both takes in and gives out a large amount of heat for a
small change in temperature; soil does not. So soil warms quickly and loses that
warmth quickly, while water changes its temperature slowly in either direction.

Did you know? This is why the air over the sea is warmer than the air over the land
at night, and why a land breeze blows from the land towards the sea after dark.

In-text Questions — Page 96
7.3 Radiation

Q1 I wonder how heat from the fire reaches us?

The heat of the fire reaches us by radiation — it travels directly from the hot object (the fire) to
us, and no medium is needed for this transfer.

PROCESS HOW HEAT MOVES MEDIUM EXAMPLE HERE
NEEDED?

Conduction Particle passes heat to its Yes Flame → metal pan
neighbour; particles stay in place

Convection Heated particles themselves move Yes Smoke and hot air rising
from place to place from the fire

Radiation Heat travels straight from the hot No Warmth felt by Pema and
object to us Palden facing the fire

Why it must be radiation: the hot air of the fire rises straight up, so convection
cannot bring warmth sideways to the children's faces. Nor are they touching the fire,
so conduction is ruled out. The warmth reaches them directly across the gap — that
is radiation. The heat of the Sun reaches the Earth in the very same way, crossing
empty space where there are no particles at all.

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Class 7 Science Chapter 7 Heat Transfer in Nature
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co m
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You must have observed that a hot utensil kept away from the flame cools down
e
Q2

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after some time. What is the reason for it?

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The hot utensil cools down by radiating heat to its surroundings.

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Why it happens: all objects radiate heat. An object that is hotter than its ag
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surroundings gives out more heat than it receives, so its temperature falls until it
a
becomes as warm as the room. The hot utensil keeps sending out heat to the cooler

m
air, walls and table around it, and cools down. (Some heat also leaves by conduction
co
m.
into the platform it rests on and by convection into the air above it, but the utensil

m as e
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would cool down by radiation alone.)
a g
a s em
a gl Did you know? The exchange works both ways. A cold glass of water kept in a warm
room gains heat by radiation from its surroundings and slowly becomes warm.
m a s
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l a se
Dive Deeper — Page 96 ag

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7.3 Radiation

m .
DIVE DEEPER

m as e
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a s em Why is it more comfortable to wear white or light-coloured clothes during summers
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Q1
a and dark-coloured clothes during winters?

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Because light and dark surfaces behave differently towards the heat that falls on them.

Light-coloured clothes reflect most of the heat that falls on them. Less heat enters the

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body, so we feel cooler and more comfortable in summer.

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Dark surfaces absorb more heat. More of the Sun's heat is taken in and kept close to the

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body, so we feel warmer and more comfortable in winter.
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WHAT IT DOES TO HEAT FALLING HOW WE BEST
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CLOTHING ON IT FEEL SEASON

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White / light Reflects most of it Cooler Summer

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Black / dark
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Absorbs most of it Warmer Winter

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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Try This: on a sunny day, put one sheet of white paper and one sheet of black paper
side by side in the Sun for ten minutes, then touch them. The black sheet is clearly
warmer.

Know a Scientist — Page 99
7.4 Water Cycle

KNOW A SCIENTIST

Q1 Who was Varahamihira, and what did he say about rainfall in his work
Brihatsamhita?

Varahamihira was an astronomer and mathematician of the sixth century CE who lived in
Ujjaini — modern-day Ujjain, in Madhya Pradesh.
In his work Brihatsamhita he gave methods for predicting seasonal rainfall. His predictions were
based on natural factors that he observed carefully, such as:

cloud formation — the shape, colour and behaviour of clouds;
wind patterns — the direction and nature of the winds;
the position of the stars and the moon;
and other natural phenomena.

Why this matters: the winds and clouds Varahamihira watched are themselves
produced by heat transfer — the Sun heats land and water unevenly, air rises by
convection, water evaporates and condenses into clouds. Long before modern
instruments existed, he was reading the same water cycle you are studying in this
chapter, and using it to help farmers plan their sowing.

Did you know? Today the India Meteorological Department does this work with
satellites, radars and computer models — but the basic idea is the same: watch the
clouds, the winds and the moisture, and predict the rain.

In-text Question — Page 99

Page 20 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

7.4.1 Seepage of water beneath the Earth

Q1 How does water seep through the surface of the Earth?

Rainwater that falls on the ground does not all flow away. Part of it sinks into the tiny spaces
between the particles of soil and the cracks in rocks and moves slowly downwards. This process
of surface water seeping through soil and rocks is called infiltration.

Rain falls on the ground

↓ enters the spaces between soil and rock particles (infiltration)

↓ moves deeper through the pore spaces

↓ collects in the pore spaces of sediments and openings in rocks
= groundwater, stored in layers called aquifers

What decides how fast: water infiltrates more readily if the spaces between the soil
and rock particles are wider, open and interconnected (Fig. 7.11). Activity 7.5 tests
exactly this with clay, sand and gravel.

Tip: the water people draw from wells and handpumps is this same seeped water,
brought back up to the surface.

Activity 7.5: Let us investigate — Page 99
7.4.1 Seepage of water beneath the Earth

ACTIVITY

Q1 Predict the amount of water flowing out of each bottle.

200 mL of water is poured into each inverted bottle — one holding clay, one sand and one
gravel — and the water is collected for 10 minutes (Fig. 7.10). A sensible prediction is:

Page 21 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Bottle 3 (gravel) → most water collected, and fastest

Bottle 2 (sand) → a moderate amount

Bottle 1 (clay) → least water collected, and slowest

Reason for the prediction: the bigger the spaces between the particles, the more
easily water can pass through. Gravel particles are large, with wide gaps between
them; sand particles are much smaller; clay particles are the finest of all and pack
together very tightly, leaving almost no open path for water.

Check it yourself: use the same bottle size, the same 200 mL of water and the same
10 minutes for all three, otherwise the comparison is not fair.

Q2 Compare the amount of water that comes through each bottle.

On performing the activity, the water comes through in this order: gravel > sand > clay. Here is
Table 7.5, completed.

BOTTLES PREDICTION OBSERVATION
FILLED WITH
SEEPAGE OF WATER (VERY SEEPAGE OF WATER (VERY
SLOW/SLOW/FAST) SLOW/SLOW/FAST)

Bottle 1 (Clay) Very slow Very slow — only a little water trickles into the
beaker; most of it stays above the clay

Bottle 2 (Sand) Slow Slow — a fair amount of water collects, but it
takes time

Bottle 3 (Gravel) Fast Fast — almost all the water runs through quickly
and fills the beaker

Why it happens: the spaces between gravel particles are wider than those in sand
and clay, so water can seep through gravel more easily. In clay the particles are
extremely fine and closely packed, so the water is held back.

Page 22 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Did you know? This is why a pond dug in clayey soil holds water for months, while
the same pond dug in sandy soil empties within days.

In-text Questions — Page 100
7.4.1 Seepage of water beneath the Earth

Q1 Do your findings match with your predictions?

Yes — for most students the findings match the prediction exactly: water seeps fastest through
gravel, slower through sand and slowest through clay.

If they do not match: that is useful information, not a failure. Check whether the
clay was packed too loosely, whether the hole in one cap was bigger than the others,
or whether the gravel was full of fine dust. In science, a prediction that fails sends
you back to look for the reason — which is how the activity teaches you to keep
every other condition the same.

Tip: write down your prediction before pouring the water. Comparing an honest
prediction with the observation is the whole point of the Prediction and Observation
columns in Table 7.5.

Q2 You may have observed that water seeps fastest through gravel, slower through
sand, and slowest through clay. Why is it so?

Because of the size of the spaces between the particles.

Gravel — large particles, wide, open, connected spaces → water seeps fastest

Sand — smaller particles, narrower spaces → water seeps slower
Clay — very fine, closely packed particles, tiny spaces → water seeps slowest

Page 23 of 43

as e
Class 7 Science Chapter 7 Heat Transfer in Nature
a g l AglaSem · NCERT Solutions

co m
m.
Why it happens: water can only move through the gaps between particles, not

m l a se
through the particles themselves. The spaces between gravel particles are wider
o
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m
when compared to those in sand and clay, so water can seep through

l a seeasily. Water can infiltrate more readily if the spaces between soil and rock
g
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aparticles are wider, open and interconnected (Fig. 7.11). In clay the gaps are so fine
and so poorly connected that the water is almost stopped.
co m
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Did you know? The same rule works underground. Layers of sand and gravel below
the surface make good aquifers, because they can both take in water and hold a lot
of it in their pore spaces.
co m
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m l as
m .co a g
ase and Society — Page 101
agl
Science
7.4.1 Seepage of water beneath the Earth

m a s
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SCIENCE AND SOCIETY

a s em
Q1
aglis it made, and how does it help the people of Ladakh?
What is an ice stupa? How

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An ice stupa is a tall, cone-shaped structure of ice built in Ladakh in winter to store water for
g
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the dry

a s
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How
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1. During winter, water from the mountain streams is channelled downhill through
underground pipes.
com g l a
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2. The water is then sprayed into the cold air.

g l as low temperature makes it freeze.
a layer, growing into a tall cone — the ice stupa.
3. As the spray falls, the extremely
4. The ice builds up layer by

How it helps: in spring the streams of Ladakh often dry up, because the heat from the Sun's
co m
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radiation is not yet enough to melt the snow high on the mountains. The ice stupa melts slowly
m
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through the spring, releasing water for farming and other needs right through the summer.
a
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a The science inside it: the cone shape is the clever part. A cone has a small surface
c
area for the volume of ice it holds, and its own shadow shields much of it, so the
m .
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Sun's radiation melts it slowly instead of all at once — exactly when the farmers need

m .
the water.
as e
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co m
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.co


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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Did you know? The same thinking is behind rainwater harvesting structures and
recharge pits used elsewhere in India — catch the water when it is plentiful, store it,
and use it when it is scarce.

Let Us Enhance Our Learning — Pages 102–104
Chapter exercises

Q1 (i) Choose the correct option in each case. Your father bought a saucepan made of
two different materials, A and B, as shown in Fig. 7.14. The materials A and B have
the following properties — (a) Both A and B are good conductors of heat (b) Both
A and B are poor conductors of heat (c) A is a good conductor and B is a poor
conductor of heat (d) A is a poor conductor and B is a good conductor of heat

B

A

Redrawn sketch of the saucepan in Fig. 7.14, page 102. A and B mark the two different
materials the pan is made of.

(c) A is a good conductor and B is a poor conductor of heat.
In Fig. 7.14, A is the body of the saucepan — the part that sits on the flame and holds the food
— and B is the handle.

Page 25 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

PART WHAT IT MUST DO PROPERTY USUAL
NEEDED MATERIAL

A — body of the Carry heat from the flame quickly Good conductor of Aluminium, steel,
pan into the food heat iron

B — handle Stop heat from reaching the hand Poor conductor of Wood, plastic,
that holds it heat bakelite

Why the other options are wrong: if both were good conductors (a), the handle
would become too hot to hold. If both were poor conductors (b), the food would
never cook. Option (d) is the arrangement turned upside down — a pan that refuses
to heat but a handle that burns you.

Page 26 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Q1 (ii) Pins are stuck to a metal strip with wax and a burning candle is kept below the
rod, as shown in Fig. 7.15. Which of the following will happen? (a) All the pins will
fall almost at the same time (b) Pins I and II will fall earlier than pins III and IV
(c) Pins I and II will fall later than pins III and IV (d) Pins II and III will fall almost
at the same time

Metal
strip
Pin
Stand

IV III II I
Burning
candle

Redrawn sketch of Fig. 7.15, page 102 — four pins stuck with wax under a metal strip,
with the burning candle in the position shown in the book.

(d) Pins II and III will fall almost at the same time.
Look carefully at Fig. 7.15. The pins are in the order IV, III, II, I from the stand outwards, and the
candle is placed midway between pins II and III — not at the end of the strip as in Activity 7.1.

Distance of pin II from the flame = distance of pin III from the flame

⇒ heat reaches both at the same time

⇒ the wax under both melts together
⇒ pins II and III fall almost together, then pins I and IV

Page 27 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Why it happens: in conduction, heat spreads out from the heated point in both
directions along the strip. What decides when a pin falls is only its distance from
the flame. Pins that are equally distant fall together.

Tip: option (b) and option (c) would be tempting if the candle were at one end.
Always check where the flame is before answering.

Q1 (iii) A smoke detector is a device that detects smoke and sounds an alarm. Suppose
you are fitting a smoke detector in your room. The most suitable place for this
device will be: (a) Near the floor (b) In the middle of a wall (c) On the ceiling (d)
Anywhere in the room

(c) On the ceiling.

Why it happens: smoke is a mixture of hot gases and tiny solid particles. Being
warmer than the surrounding air, it expands, becomes lighter and rises up by
convection. So the very first place smoke collects in a room is right below the ceiling.
A detector fixed there catches the smoke earliest and sounds the alarm while there
is still time to act.

Why not the others: near the floor (a) the smoke would arrive only after the whole
room had filled up; in the middle of a wall (b) it would still be late; and (d) is simply
not true — position matters a great deal.

Q2 A shopkeeper serves you cold lassi in a tumbler. By chance, the tumbler had a small
leak. You were given another tumbler by the shopkeeper to put the leaky tumbler
in it. Will this arrangement help to keep the lassi cold for a longer time? Explain.

Yes, it will. The double tumbler keeps the lassi cold for a longer time.

Page 28 of 43

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Class 7 Science Chapter 7 Heat Transfer in Nature
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co m
m.
Why it happens: when one tumbler is placed inside another, a thin layer of air gets

m l a se
trapped between the two. Air is a poor conductor of heat. Heat from the warm
o
m
room now .chas to cross the outer tumbler, then the trapped layeraofg air, and only then
se tumbler before it can reach the lassi. The air layer slows that flow down
theainner
l
g so the lassi warms up much more slowly than it would in a single tumbler.
abadly,

. c om ag
a s em air (poor conductor) → inner tumbler → cold
Warm room air → outer tumbler → trapped

lassi agl
⇒ heat flows in slowly ⇒ lassi stays cold longer
co m
se m.
o m l a
g only better — it has
m .c know? A thermos flask works on exactly this principle,
a
se walls with the air removed from the space between them, so there are almost no
Did you

g l a
two
a particles left to conduct heat at all.

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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Q3 State with reason(s) whether the following statements are True [T] or False [F]. (i)
Heat transfer takes place in solids through convection. [ ] (ii) Heat transfer through
convection takes place by the actual movement of particles. [ ] (iii) Areas with clay
materials allow more seepage of water than those with sandy materials. [ ] (iv) The
movement of cooler air from land to sea is called land breeze. [ ]

STATEMENT T/ REASON
F

(i) Heat transfer takes place in False In solids, heat is transferred mainly by conduction. Convection
solids through convection. needs the particles themselves to move from one place to
another, and the particles of a solid are fixed in their positions —
they can only vibrate. Convection therefore takes place in liquids
and gases, not in solids.

(ii) Heat transfer through True That is the definition of convection. Heated liquid or gas
convection takes place by the expands, becomes lighter and rises, while cooler, heavier matter
actual movement of particles. sinks to take its place — the particles carry the heat with them,
as the coloured streak in Activity 7.3 shows.

(iii) Areas with clay materials False It is the other way round. The spaces between sand particles are
allow more seepage of water wider than those between the very fine, closely packed clay
than those with sandy particles, so water seeps faster through sand and slowest
materials. through clay (Activity 7.5).

(iv) The movement of cooler True At night the land cools faster than the sea. The warmer air above
air from land to sea is called the sea rises, and cooler air from the land moves towards the sea
land breeze. to take its place. This movement of cooler air from land to sea is
the land breeze (Fig. 7.7b).

Tip: remember the breezes by where the air comes from — a sea breeze blows from
the sea (during the day), a land breeze blows from the land (at night).

Q4 Some ice cubes placed in a dish melt into water after sometime. Where do the ice
cubes get heat for this transformation?

The ice cubes get this heat from their surroundings — from the dish they are kept in, from the
air around them, and from the table or surface below.

Page 30 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Surroundings (air, dish, table) are warmer than the ice

⇒ heat flows from the warmer surroundings to the colder ice

⇒ ice absorbs this heat and melts into water

Why it happens: heat always flows from a hotter body to a colder one. Ice is far
colder than a room, so it keeps absorbing heat until it has melted. The heat reaches
it in all three ways at once — by conduction from the dish it touches, by convection
of the air currents around it, and by radiation from the warmer objects nearby.

Check it yourself: put one ice cube in a steel bowl and another in a thick woollen
cloth. The cube in the steel bowl melts much faster, because steel is a good
conductor and supplies heat to it quickly.

Q5 A burning incense stick is fixed, pointing downwards. In which direction would the
smoke from the incense stick move? Show the movement of smoke with a diagram.

The smoke will still move upwards — no matter which way the incense stick points.

support / stand

incense stick
(pointing down)

burning tip

smoke curls around the stick
and rises upwards

floor

Page 31 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

The incense stick points downwards, but the smoke curls around it and always travels upwards.

Why it happens: smoke is a mixture of hot gases and tiny solid particles. Being
hotter than the air around it, it expands, becomes lighter, and is pushed up by the
cooler, heavier air that sinks around it. This is convection, and it depends only on
the smoke being hot — not on the direction in which the stick is held. So the smoke
leaves the tip, curls around the stick and climbs.

Check it yourself: hold a burning agarbatti sideways in a still room. The smoke
bends upward within a centimetre or two of the tip, every single time.

Q6 Two test tubes with water are heated by a candle flame as shown in Fig. 7.16. Which
thermometers (Fig. 7.16a or Fig. 7.16b) will record a higher temperature? Explain.

Thermometer Thermometer

Water Water

Burning
candle

Burning
candle

Fig. 7.16(a) Fig. 7.16(b)
Fig. 7.16: Two thermometers dipped in two test tubes

Redrawn sketch of Fig. 7.16, page 103 — the same test tube of water in two set-ups, with
the candle and the thermometer bulb in different places.

The thermometer in Fig. 7.16(a) will record the higher temperature.

Page 32 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

SET- WHERE THE WHERE THE WHAT HAPPENS
UP CANDLE IS THERMOMETER
BULB IS

Fig. Below the closed In the water, above the Heated water rises past the bulb, cooler
7.16(a) bottom of the test heated part water sinks to be heated in turn — a
tube convection current heats the whole tube.
Higher reading.

Fig. At the side, near In the water, near the The hot water stays at the top because it is
7.16(b) the upper part of bottom of the tube lighter. Very little heat reaches the bottom.
the water Lower reading.

Why it happens: water is heated mainly by convection, and convection currents can
only carry heat upward. In (a) the water is heated from below, so the warm water
rises through the whole tube and passes the thermometer bulb — the entire tube of
water gets hot. In (b) the heat is given near the top; the warm water is already at the
top, so it has nowhere to rise to and simply stays there. The bulb at the bottom can
only be reached by the very slow conduction through water, which is a poor
conductor of heat.

Did you know? This is why a kettle, a pressure cooker and a kadhai are always
heated at the base. Heating a vessel near its rim would waste almost all the fuel.

Q7 Why are hollow bricks used to construct the outer walls of houses in hot regions?

Because a hollow brick has air trapped inside it, and air is a poor conductor of heat.

Hot outside air → outer face of the wall

→ air trapped in the hollow brick (poor conductor)

→ inner face of the wall

⇒ very little heat gets through ⇒ the rooms stay cool

Page 33 of 43

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Class 7 Science Chapter 7 Heat Transfer in Nature
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co m
m.
Why it happens: the particles of a gas are far apart, so heat cannot be passed on

m l a se
easily from one to the next. The trapped air therefore acts as a barrier and slows
o
down the.c a gbrick has no such
m
flow of heat from the hot outside into the house. A solid
se and lets heat through much more readily.
airapocket
l
ag
o m
c ag
Did you know? The same wall works in reverse in winter — it stops the heat inside
the house from escaping. That is why m .
s e hollow bricks keep houses cool in summers
ablock of Uttarkashi, the same idea is used with
agl
and warm in winters. In the Mori
walls of two wooden layers filled with cow dung and mud.

co m
se m.
o m l a
g in areas around
m .c how large water bodies prevent extreme temperature
Explain
a
se them.
Q8

l a
ag
s
m a
m.co
Because water heats up and cools down much more slowly than land, a large lake, sea or ocean
agl
l a se
acts like a huge cushion that smooths out the temperature of the region around it.

ag
By day and in summer: the Sun's heat falls on both land and water. The land becomes very

m
hot quickly, but the water warms up only a little — it absorbs a great deal of heat for a small
rise in temperature. The cooler air over the water then moves inland as a sea breeze and
. co
se m
m a
brings the temperature of the coast down.
o l
g cold. The water, having
By.cnight and in winter: the land loses its heat quickly and becomes
a
m
se stored a lot of heat, cools very slowly and keeps giving heat to the air above it. This warm air,
a
agl and the land breeze pattern it sets up, prevents the coast from becoming bitterly cold.

se m
com g l a
. a
The evidence: Activity 7.4 shows it directly — in the same sunlight for 20 minutes,
m
ase
the temperature of the soil rose much more than that of the water, and on cooling
the soil again lost its heat faster.
agl

co m
Did you know? This is exactly what Pema and Palden's grandfather meant. Kerala
m .
o m l a se
has a long coastline, so its winters are warm and humid, while Gangtok, far from any
ag and Nagpur on a May
.clarge water body, becomes bitterly cold. Compare Mumbai
m afternoon — same state, very different heat.
l a se
ag
.c
s e m
m a
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g l as
a

com
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.co


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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Q9 Explain how water seeps through the surface of the Earth and gets stored as
groundwater.

Rainwater that falls on the ground partly flows into ponds, lakes, rivers and oceans, and partly
seeps into the ground. The seeping happens in these steps:

1. Water enters the spaces between the particles of soil and the cracks and openings in the
rocks below. This process of surface water seeping through soil and rocks is called
infiltration.
2. Water infiltrates more readily where these spaces are wider, open and interconnected (Fig.
7.11) — fastest through gravel, slower through sand, slowest through clay.
3. Moving downward, the water is finally held in the pore spaces of sediments and the
openings in rocks beneath the surface. Water stored like this is called groundwater.
4. The underground layers of sediments and rocks that store water in their pore spaces are
called aquifers (Fig. 7.12).

Rain falling on the ground

Soil and rock — water moves down
through the open, connected spaces = INFILTRATION

AQUIFER — water held in pore spaces
= GROUNDWATER, reached by wells and bore
wells

Rain → infiltration through soil and rock → storage in an aquifer as groundwater.

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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Did you know? This groundwater may lie anywhere from a few metres to hundreds
of metres below the ground, depending on the location. It is the water we bring up
through wells, handpumps and bore wells — and it is not unlimited. Rainwater
harvesting and recharge pits are used to put it back.

Q10 The water cycle helps in the redistribution and replenishment of water on the
Earth. Justify the statement.

The statement is fully justified, because the water cycle keeps moving water from where there is
plenty of it to where it is needed, and keeps putting back what is used up.

Sun heats oceans, rivers and lakes → water evaporates as water vapour

Plants and trees add vapour by transpiration

Vapour rises, cools and condenses into clouds

Clouds travel over land and give rain, snow and hail (precipitation)

Water flows into ponds, lakes, rivers and oceans, or infiltrates into the ground

⇒ back to the start of the cycle

Redistribution — the vapour rises mostly over the oceans but the clouds carry it far inland, so
rain falls on hills, plains, forests and fields thousands of kilometres from the sea. Snow that falls
on the mountains melts in summer and flows down as rivers, feeding places that get very little
rain of their own.
Replenishment — the rivers, lakes and wells we draw water from are refilled every year by the
same cycle. Rainwater that seeps into the ground recharges the aquifers, so groundwater
sources are made good again.

The bigger point: the water cycle also conserves the total amount of water on the
Earth. Not a drop is created or destroyed — the same water is used, returned and
used again. What we can lose is water at a particular place, which is why rainwater
harvesting and recharge pits matter so much.

Tip: in an exam, name all four processes — evaporation, transpiration, condensation
and precipitation — and then say clearly what is redistributed and what is
replenished.

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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Exploratory Projects — Page 104
Chapter projects

EXPLORATORY PROJECTS

Q1 Society: Visit a site of water harvesting or a recharge pit. Find out from people how
they are constructed and how they work. Prepare a report with illustrations.

This is a field project, so the report must be built out of what you see and hear. Here is how to do
it and what a good report must contain.
How to go about it

1. Find a site nearby — a rooftop rainwater harvesting system in a school or apartment block, a
recharge pit or soak pit in a colony, a village tank or johad, a check dam, or a stepwell.
2. Go with a teacher or an elder. Carry a notebook, a measuring tape and a camera or pencil for
sketching.
3. Ask the people who built it or look after it: Who made it, and when? What does it cost?
How much water does it collect? Where does the water go? Who cleans it, and how
often? Has the water level in the nearby well risen since it was built?
4. Sketch or photograph the parts — catchment, gutter, down pipe, first-flush device, filter, pit
or tank.

What a good report must contain

Name and location of the site, date of visit.
A labelled diagram of the structure.
How it is constructed — materials, depth, filter layers.
How it works — the path of the water from the roof or the ground to the pit.
Benefits reported by the people, and problems they face.
Your own conclusion linking it to the chapter — infiltration, aquifers, groundwater recharge.

Sample answer: “We visited the recharge pit in the corner of our school ground. It is a square
pit about 1.5 m wide and 3 m deep. Rainwater from the school roof comes down a pipe into a
small chamber that traps leaves and dust, and then enters the pit. Inside, the pit is filled in
layers — large boulders at the bottom, then gravel, then coarse sand at the top. The caretaker
told us the sand layer is cleaned before every monsoon. The water passing through these layers
seeps into the ground by infiltration and reaches the aquifer below. Since the pit was built four
years ago, the school borewell has not run dry in May, which it used to do earlier. We learnt that
gravel and coarse sand are used because water seeps fastest through wide, open, connected
spaces — exactly what we saw in Activity 7.5.”

Page 37 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Tip: a recharge pit is filled with gravel and sand, never clay. After Activity 7.5 you
know exactly why.

Q2 Activity: Tightly wrap a thin paper strip around a metallic rod. Try to burn the paper
with a candle while rotating the rod continuously. Does the paper burn? Explain
your observations.

No — the paper does not burn (it may darken a little, but it does not catch fire), as long as it is
wrapped tightly around the metal rod and the rod is rotated continuously.

Why it happens: the metal rod is a good conductor of heat. Paper catches fire only
when its own temperature reaches its ignition temperature. Here, the moment the
flame heats the paper, the heat passes straight through the thin paper into the
metal touching it, and the metal carries it away rapidly along the rod by conduction.
The paper therefore never gets hot enough to burn. Rotating the rod keeps moving
fresh, cooler metal under the flame and stops any one spot from becoming too hot.

Flame gives heat to the paper

Paper is thin and pressed tightly against the metal

⇒ heat is conducted away into the rod almost at once

⇒ paper stays below its ignition temperature ⇒ it does not burn

Check it yourself: try the same thing with a wooden or a glass rod, or leave the
paper loosely wrapped so that a layer of air lies between paper and metal. Now the
paper burns — because wood, glass and air are poor conductors and cannot carry
the heat away.

Caution: do this only under the supervision of a teacher or an adult, and hold the
rod at its far end.

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Class 7 Science Chapter 7 Heat Transfer in Nature
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co m
m.
Activity: Take a sheet of paper. Draw a spiral on it, as shown in Fig. 7.17a. Cut the
se
Q3

o m l a
paper along the spiral. Suspend the paper as shown in the Fig. 7.17b above a
g for your
m .c candle. Observe what happens. Provide an explanation
burning a
l a se
observation.
a g

co m
e m . ag
g l as
a

co m
e m.
m l as
m .co a g
l a se
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m a s
m .co agl
l a se
ag
Fig. 7.17(a): Cutting paper Fig. 7.17(b): Spiral paper

com
in spiral above a burning candle

.
m a s em and cut out, then
co hung above a burning candle.gl
Redrawn sketch of Fig. 7.17, page 104 — the spiral drawn on the paper

. a
a sem
agl
se m
m l a
o
.crotate steadily above the candle, and it keeps turning ag
m
e Take the candle away and it slows down and stops.
Observation: the paper spiral begins to
as long as the candle burns belowsit.
l a
ag
Candle heats the air above it
co m
m .
m heavier air moves in from the sides to take its place glas
Hot air expands → becomes lighter → rises
e
. c o a
se m Cooler,

g l a
a
= a steady upward stream of air, a convection current
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The rising air pushes against the slanting turns of the spiral
s e m
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⇒ the spiral spins
e m . co agl
g l as
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m .
m ase
.co


a g l Page 39 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Why it happens: the spiral is cut so that each turn is tilted, like the blades of a fan or
a windmill. When the rising column of hot air strikes these tilted surfaces, it pushes
each one sideways as it passes, and the paper turns on its thread. The spiral is, in
effect, a tiny turbine driven by convection.

Try This: hold the spiral over a hot cup of tea, or over a room heater. It turns there
too — proof that any source of rising warm air will do. Keep the paper well above the
flame so that it does not catch fire.

Chapter at a glance
Heat travels from a hotter place to a colder place in three ways — conduction, convection
and radiation. Pema and Palden meet all three in one evening: the metal pan on the fire,
the smoke rising from the burning wood, and the warmth they feel sitting around the
fireplace.
In conduction, heat passes from the hotter part of an object to its colder part. A heated
particle passes heat on to its neighbour, and so on, but the particles themselves do not
move from their positions. This is how heat travels in solids.
Materials that let heat pass through them easily — metals such as aluminium, iron and steel
— are good conductors of heat. Wood, glass, clay, porcelain, plastic and air are poor
conductors (insulators). Woollen clothes, two thin blankets and hollow bricks all work by
trapping air.
In convection, heat is carried by the actual movement of particles of a liquid or a gas. On
being heated, the fluid expands, becomes lighter and rises; cooler, heavier fluid comes
down to take its place, setting up a convection current — seen as the coloured streak in
Activity 7.3.
Land and sea breezes are convection on a large scale. Land heats up and cools down faster
than water, so warm air rises over the land by day and cool air blows in from the sea (sea
breeze); at night the flow reverses and a land breeze blows from land to sea.
In radiation, heat travels directly from a hot object to us and no medium is needed. That is
how the Sun's heat reaches the Earth and how a hot utensil kept away from the flame cools
down. Light surfaces reflect heat; dark surfaces absorb it.
The Sun drives the water cycle — evaporation and transpiration lift water as vapour,
condensation forms clouds, and precipitation brings it back as rain, snow and hail. The cycle
redistributes and replenishes water and conserves the Earth's total water.
Water that soaks into the ground by infiltration is stored as groundwater in aquifers.
Seepage is fastest through gravel and slowest through clay. Excessive extraction and

Page 40 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

concrete surfaces deplete groundwater; rainwater harvesting, recharge pits and Ladakh's
ice stupas help conserve it.

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Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS EXAMPLE FROM THE POINT TO REMEMBER
CHAPTER

Conduction Transfer of heat from the hotter Pins I, II, III, IV falling one Particles pass heat on but
part of an object to its colder after another from a heated do not leave their
part metal strip (Activity 7.1) positions

Good conductor A material that allows heat to Aluminium, iron, steel — All metals are good
of heat pass through it easily used for cooking utensils conductors of heat

Poor conductor A material that does not allow Wood, glass, clay and Air is one of the best
(insulator) heat to pass through it easily porcelain cups, plastic, air insulators — wool and
hollow bricks use it

Convection Transfer of heat by the actual The coloured streak rising in Works only in liquids and
movement of particles of a the middle and coming gases, never in solids
liquid or a gas down at the sides (Activity
7.3)

Sea breeze Movement of cooler air from Blows during the day, when Windows of coastal
the sea towards the land land is warmer than the sea houses face the sea to
(Fig. 7.7a) catch it

Land breeze Movement of cooler air from Blows at night, when the sea The wind direction
the land towards the sea is warmer than the land (Fig. reverses between day
7.7b) and night

Radiation Transfer of heat that does not Heat of the Sun reaching the All objects radiate heat to
require any medium Earth; warmth felt around a their surroundings
fireplace

Water cycle The continuous upward Evaporation, transpiration, It redistributes water and
movement of water as vapour condensation and conserves the Earth's
and downward movement precipitation (Fig. 7.9) total water
through precipitation

Evaporation Change of water into water Wet clothes drying faster on The Sun's heat makes
vapour a sunny day evaporation faster

Transpiration Loss of water as vapour from Shown by the arrow rising It adds to the water
trees and plants from the tree in Fig. 7.9 vapour in the air along
with evaporation

Precipitation Water falling from clouds as Clouds bring rain, snow and It is the downward half of
rain, snow or hail hail the water cycle

Page 42 of 43

Class 7 Science Chapter 7 Heat Transfer in Nature AglaSem · NCERT Solutions

TERM WHAT IT MEANS EXAMPLE FROM THE POINT TO REMEMBER
CHAPTER

Infiltration The seeping of surface water Water passing fastest Wider, open and
through soil and rocks through gravel and slowest interconnected spaces
through clay (Activity 7.5) allow faster infiltration

Groundwater Water stored in the pore spaces The water drawn from wells It is not unlimited — it is
of sediments and openings in and handpumps getting depleted
rocks beneath the surface

Aquifer An underground layer of Fig. 7.12 — a well drawing May lie a few metres to
sediments and rocks that stores water from an aquifer hundreds of metres
water in pore spaces below the ground

Page 43 of 43