NCERT Solutions Class 8 Science Chapter 7 Particulate Nature of Matter

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 8 · SCIENCE

NCERT Solutions

Chapter 7: Particulate Nature of
Matter

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

98 – 115 24 54 English

Solutions, notes, sample papers & more at 43 pages

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 7: Particulate Nature of
Matter
Complete NCERT Solutions for Class 8 Science Chapter 7 Particulate Nature of Matter from the NCERT
textbook Curiosity. Every question the chapter asks is answered — the Probe and ponder openers, all the
activity questions from Activity 7.1 to Activity 7.9, the Our scientific heritage and Think like a scientist boxes,
every in-text question from pages 99 to 111, all 10 questions of Keep the curiosity alive and the four
Discover, design, and debate tasks — each answer worked out in terms of the particle model: interparticle
spacing, interparticle attraction and the motion of the constituent particles.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 98 – 115

SECTIONS QUESTIONS

24 54

MEDIUM

English

Probe and ponder — Page 98
Chapter opener — the questions the riverbank raises

PROBE AND PONDER

Q1 Why is it possible to pile up stones or sand, but not a liquid like water?

Because a stone and a grain of sand are solids, and water is a liquid — and the difference lies
entirely in how strongly their constituent particles are held together.
Inside every stone and every grain of sand the particles are closely packed and the interparticle
attractions are very strong. They hold each particle in a fixed position, so a grain keeps its own
definite shape and can sit still on the grains below it without collapsing. Grain rests on grain,
and the heap stands.
In water the interparticle attractions are much weaker. The particles are free to move past one
another, so no layer of water can hold up the layer above it. The moment you try to heap water,
the particles at the bottom simply slide sideways, and the water spreads out until its surface is
level.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Notice what a heap really is: a pile of sand is not one solid object — it is a very
large number of separate solid grains resting on each other, with air-filled gaps in
between. That is why a heap of sand can be poured, even though sand is a solid.

Q2 Why does water take the shape of folded hands but lose that shape when released?

Because a liquid has a definite volume but no fixed shape. Your folded hands act as the
container; open them and there is no container left.
The particles of water are free to move, but only within a limited space. When you cup your
hands, the water cannot escape, so its particles settle along the hollow of your palms and the
water copies that shape exactly — the same thing that happens when 200 mL of water is poured
from Container A into Container B in Activity 7.4.
Open your hands and the walls are gone. The interparticle attraction in water is far too weak to
hold the particles in a bowl shape on their own, so they slide past one another and the water
flows away. Its volume has not changed at all — only its shape has.

Why it happens: shape needs particles fixed in position; only a solid has that. A
liquid keeps its volume because its particles stay close together, and loses its shape
because they can move.

Q3 We cannot see air, so how does it add weight to an inflated balloon?

Because air is matter — it too is made of constituent particles, and every one of those particles
has its own mass.
We cannot see air for two reasons: its particles are extremely small, and in a gas the
interparticle spaces are maximum and the attraction is negligible, so the particles are scattered
far apart and never form a surface for light to bounce off. Not seeing something is not the
same as there being nothing there — exactly the lesson of Activity 7.2, where dissolved sugar
cannot be seen but can certainly be tasted.
When you blow up a balloon you force a very large number of air particles into it. Their masses
add up, the Earth pulls on all of them, and the inflated balloon therefore weighs more than the
same balloon empty.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Check it yourself: Activity 7.6 proves the air is really inside. Seal a syringe full of air
with your thumb and push. The plunger resists, and springs back when you let go.
Something invisible is pushing back.

Q4 Is the air we breathe today the same that existed thousands of years ago?

The particles are largely the same ones; the mixture they make up is not.
Particles of matter are not created or destroyed by ordinary changes — grinding, dissolving,
melting and boiling only rearrange them. So the constituent particles in the air around you have
been in use for a very long time: because gas particles move freely in all directions and never
stop, air is stirred and re-stirred continuously, and a particle breathed out in one place can be
taken up by a plant far away and returned to the air again.
What has changed is how much of each substance the air contains. Burning fuel in vehicles,
factories and homes keeps adding new substances, along with the dust we call Suspended
Particulate Matter (see A step further, page 109). So the air of today is made of the same kinds
of particles as before, but not in the same proportions.

Q5 Share your questions

A good question for this chapter is one that cannot be answered without saying something
about the particles — how far apart they are, how strongly they attract each other, or how they
are moving. Write your questions in your notebook and bring them to class.
Sample questions:

If a solid has no space between its particles, why does a stone crack when it is hammered
rather than squash flat?
A syringe full of air can be squeezed; one full of water cannot. What exactly is being
squeezed out?
Why does a wet floor dry up even on a cold day, when water boils only at 100 °C?
The smell of food reaches the last room of the house. Does the food itself travel there?
Ice floats on water. What does that tell us about the spacing of particles in ice?

Page 3 of 43

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Class 8 Science Chapter 7 Particulate Nature of Matter
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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Keep the scale in mind: a speck of sand you can just see with a magnifying glass is
still gigantic compared with a constituent particle. Constituent particles cannot be
seen even through an ordinary microscope.

Q3 Is every speck of this fine chalk powder still composed of the same substance, or
has it changed into something else on breaking or grinding?

Every speck is still the same substance — chalk. Nothing new has been formed.
Look at the powder through a magnifying glass (Fig. 7.1d): each tiny grain is white, dusty and
chalky, exactly like the stick you started with. Breaking and grinding only separate one group of
chalk's constituent particles from another group. The particles themselves are untouched —
they are not cut, changed or converted into anything else.

Why it happens: the properties of a substance come from the kind of particles it is
made of. As long as the particles are unchanged, the substance is unchanged,
however small you make the pieces. This is what makes grinding a physical change.

Q4 Recall Curiosity, Grade 7 chapter ‘Changes Around Us: Physical and Chemical’— is
grinding chalk a physical change or a chemical change?

It is a physical change.
A change is chemical only when a new substance with new properties is formed. Grinding chalk
forms nothing new: the powder is chalk, it looks like chalk, it feels like chalk, and it behaves like
chalk. The only thing that has changed is the size of each speck, which has become smaller and
smaller.

Test to apply: ask yourself, "Is the material after the change the same material as
before?" For grinding chalk the answer is yes, so the change is physical. Compare
this with burning a candle or rusting iron, where the answer is no.

Activity 7.1: Let us explore — Page 99

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Section 7.1 What Is Matter Composed of?

ACTIVITY

Q1 What do you observe?

The chalk keeps breaking into smaller and smaller pieces, and at no stage does it stop being
chalk.

The stick snaps into two clean pieces (Fig. 7.1b), and those pieces snap again — until the bits
are so small that your fingers can no longer grip them and break them.
The mortar and pestle take over where your hands stop. Grinding turns those bits into a fine
white powder (Fig. 7.1c).
Under a magnifying glass (Fig. 7.1d) the powder is not a smooth paste at all — it is a very
large number of separate tiny grains, each one white and chalky.

What this tells us: the stick of chalk was never a single continuous block of material.
It was a huge number of small units packed together, and breaking it merely
separates them. Grinding harder would only make the specks finer still.

Q2 Each tiny grain you observe is still a speck of chalk.

Yes — and the magnifying glass is what lets you check it for yourself. Every grain in the powder
has the same white colour, the same dusty feel and the same chalky nature as the original stick.
This single observation carries the whole idea of the section. If breaking chalk had produced
something else, the powder would look and behave differently. It does not. So a piece of chalk
must be made up of a large number of identical units, and breaking simply pulls those units
apart from one another.

Try this: rub a little of the powder on a blackboard. It still writes. A substance keeps
its properties right down to the smallest speck you can make of it — which is the first
thing the particle model claims.

In-text Questions — Page 100

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Section 7.1 What Is Matter Composed of?

Q1 Are the units of chalk obtained in this manner considered the smallest units of
chalk?

Not the units in your mortar — but yes, the units the process finally leads to. The two must
be kept apart.

What the mortar and pestle actually give you is very fine chalk powder. Every speck of it is
still made of millions and millions of constituent particles. It is nowhere near the smallest
unit.
The text asks you to imagine the grinding continuing. Eventually a stage would come at
which the pieces cannot be broken down any further. Those tiny units are the basic building
blocks the chalk was made of — its constituent particles.

Why the imagining is needed: no mortar can actually reach that stage, and the
particles are far too small to be seen even through an ordinary microscope. We
reach the idea by reasoning from what we can see — that every speck, however fine,
is still chalk.

What the model does not claim here: the chapter does not say these units can
never be divided by any means at all. It says they are the smallest units of chalk —
split them and you no longer have chalk. What they are made of comes in higher
grades (see A step further, page 115: atoms and molecules).

Q2 Recall the dissolution of sugar into water to form a solution. What happens to sugar
when it is dissolved in water?

The sugar breaks up into its constituent particles, which separate from one another and
spread out into the spaces between the water particles.
Each tiny grain of sugar is made up of millions and millions of such particles. Water pulls them
out of the grain one by one and they slip into the interparticle spaces that already exist in the
water. Because those particles are extremely small and now spread singly through the liquid, no
grain of sugar can be seen any more.
The sugar has not disappeared and it has not been destroyed. Every spoonful of the solution
tastes sweet, so the sugar particles must be everywhere in it.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Q3 Do you observe any sugar particles in the solution?

No — not a single one, anywhere in the tumbler.
There are two reasons together, and both matter:

1. The constituent particles of sugar are extremely small — so small that they cannot be seen
even through an ordinary microscope, let alone the naked eye.
2. They are no longer packed together in a grain. Each one has moved off separately into the
interparticle spaces of the water, so there is nothing left that is big enough to see.

But they are still there: the water tastes sweet at the top layer as well as at the
bottom. Taste is the evidence that seeing cannot give. Not being able to see a
thing is not proof that it is absent — a point worth remembering every time this
chapter talks about invisible particles.

Activity 7.2: Let us perform — Page 100
Section 7.1 What Is Matter Composed of?

ACTIVITY

Q1 Does the water taste sweet?

Before stirring — no, or at most very faintly.
The sugar has just been dropped in and, being denser than water, it lies as a heap at the bottom
of the tumbler. Only a little of it has begun to dissolve, and those particles are still down near
the bottom. A spoonful taken from the top layer therefore tastes like ordinary drinking water.

Wait long enough and this changes. Even without stirring, the water particles are
in constant motion and would slowly carry the sugar particles all the way to the top
— the same spreading you watch in Activity 7.8 with potassium permanganate.
Stirring only makes it fast.

Safety first: perform this activity under the supervision of a teacher or an adult, and
never eat or drink anything in the laboratory unless you are asked to.

Page 8 of 43

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Class 8 Science Chapter 7 Particulate Nature of Matter
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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Sugar solution, magnified

water particle

sugar particle

The sugar particles sit in the
interparticle spaces that were
already there in the water.

Why the level falls below mark B: the dissolved particles do not need fresh room of their own — they
occupy space that already existed between the water particles, so the volume of the solution is less
than the volume of the water plus the volume of the sugar.

Q2 Chalk and sugar can both be broken down into their constituent particles. But how
are the constituent particles held together to form the solid pieces we see?

By forces of attraction between the particles themselves, called interparticle attractions.
The chapter tells you three things about these forces, and each one matters:

They are attractive in nature — they pull the particles towards one another and so hold a
piece of chalk together as one object.
Their strength depends on the nature of the substance and on the interparticle distance.
Even a slight increase in that distance decreases the force drastically.
Their strength ultimately decides the physical state of the substance — solid, liquid or gas.

In solid chalk and solid sugar these forces are strong enough to hold every particle in a fixed
position. That is why a stick of chalk has a definite shape and volume, and why it must be
broken by force rather than simply poured.

Look ahead: this one sentence — the force falls off drastically as the distance grows
— explains the rest of the chapter. Heat the solid, the particles move a little further
apart, the force collapses, and the solid melts.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Our scientific heritage — Page 101
Acharya Kanad and the idea of the Parmanu

OUR SCIENTIFIC HERITAGE

Q1 Do you know that since ancient times, people have been thinking about how far
things could be broken down and what is matter made up of?

Yes. In India this question was raised very long ago by Acharya Kanad, an ancient Indian
philosopher, who first spoke about the idea of a Parmanu (atom).

He held that matter is made up of tiny, indivisible, eternal particles called Parmanu.
This idea is set down in his work, the Vaisheshika Sutras.

Notice how close his question is to yours. Activity 7.1 asks exactly what he asked — go on
breaking a piece of matter, and does the breaking ever have to stop? He answered by reasoning
alone; today the same conclusion is supported by experiment, by the way sugar dissolves, by
the way air can be compressed and by the way a colour spreads through still water.

Where the modern account differs: this chapter is careful to say only that the
constituent particles are the smallest units of that substance. It does not claim that
nothing inside them can ever be found. What those particles are — atoms and
molecules — is taken up in A step further on page 115 and studied properly in higher
grades.

In-text Questions — Page 102
Section 7.2.1 Solid state

Q1 How are constituent particles held together in solids?

Very tightly. In a solid the particles are closely packed and the interparticle attractions are very
strong.
These strong forces hold every particle in a fixed position. A particle can still move — but only
to and fro about its own position, which is called vibration or oscillation. It cannot move past
its neighbours and travel from one place to another.
That single fact explains everything you observe in Activity 7.3:

Each object has a definite shape, because every particle is anchored where it is.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Each object has a definite volume, because the particles are packed as closely as their
attractions can hold them.
Hammering can dent or crack the object, but the solid never flows the way water does.

Q2 In the solid state, is there any way to move these particles apart?

(a) Solid
Fig. 7.4(a), page 102 — magnified schematic picture of a solid: the particles are packed
tightly and held in fixed positions.

Yes — heat the solid. That is the only thing needed.

Page 12 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Heat supplied → thermal energy of particles rises

→ vibrations become more vigorous (Fig. 7.4b)

→ particles begin to leave their fixed positions

→ interparticle distance grows, so the attractions weaken

→ the solid becomes a liquid (Fig. 7.4c)

The temperature at which this happens is the melting point of that solid — more exactly, the
minimum temperature at which a solid melts to become a liquid at atmospheric pressure.

Why heating is enough: the interparticle force weakens drastically for even a slight
increase in distance. So once the vibrations are large enough to push the particles
slightly apart, the attractions collapse quickly and the particles are free to move
around.

What melting does not mean: the particles themselves do not melt, soften, swell or
change in any way. Only their spacing and their motion change. Ice and water are
made of exactly the same particles.

Activity 7.3: Let us find out — Page 102
Section 7.2.1 Solid state

ACTIVITY

Q1 In which of the above six objects do you think particles are strongly held together?

In all six — every one of them is a solid, and in a solid the interparticle attractions are always
very strong. That is why each of them has a definite shape and a definite volume that
hammering does not simply squash away.
If you compare them with one another, the hammer sorts them into three kinds of behaviour:

Page 13 of 43

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Class 8 Science Chapter 7 Particulate Nature of Matter
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OBJECT WHAT HAMMERING WHAT IT SUGGESTS

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

The book proves both of these to you:

Activity 7.4 — the same water poured through Containers A, B and C takes the shape of
each one, yet the level stays at the 200 mL mark every time. No fixed shape, fixed volume.
Activity 7.5 — smoke collected in Gas Jar A spreads on its own until it fills the whole of Gas
Jar B as well. Neither shape nor volume is fixed.

Note about Table 7.1: ice melts at 0 °C, urea at 133 °C and iron at 1538 °C. The same
three solids, the same three states — but the temperature at which each gives way
tells you how strong its interparticle attractions are.

In-text Questions — Page 104
Section 7.2.2 Liquid state

Q1 Are you able to move your finger through the water?

Yes — easily, and without breaking or cutting the water permanently. Try the same with a
stone and you cannot move your finger through it at all.
What your finger actually does is temporarily displace the water. The interparticle attractions in
a liquid are weaker than in a solid, so the particles can be pushed aside and can slide past one
another. The moment you remove your finger, the particles close in again and the position of the
water is restored — there is no cut, no hole and no mark left behind.

But the attractions are not zero: they are still strong enough to keep the particles
close together. That is exactly why the water stays as one body with a definite
volume in the vessel, instead of scattering into the room the way a gas would. A
liquid sits between a solid and a gas, and this activity lets you feel that.

In-text Questions — Page 105
Section 7.2.3 Gaseous state

Q1 Do gases also have a fixed volume?

No. A gas has no fixed volume and no fixed shape — it takes both from its container.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Activity 7.5 shows this directly. Smoke is collected in Gas Jar A only. When Gas Jar B is placed
over it and the glass plate is removed, the smoke spreads on its own until it fills the entire
space in Gas Jar B as well (Fig. 7.7d). Nothing pushed it there; it went by itself.
The reason is that in a gas the particles move freely in all directions and the interparticle
attractions are negligible. Nothing holds the particles back, so they keep spreading until the
whole available space is occupied.

The other half of the proof: Activity 7.6 shows the same thing from the opposite
side. Trapped air in a syringe can be squeezed into a much smaller volume just by
pushing the plunger. A quantity of gas that can be made to fill a larger space, and be
squeezed into a smaller one, plainly has no volume of its own.

Fluids: because both liquids and gases flow and do not keep a fixed shape, they are
grouped together as fluids, and this is what distinguishes them from solids.

In-text Questions — Page 107
Section 7.3 How Does the Interparticle Spacing Differ in the Three States of Matter?

Q1 What role does the interparticle spacing play in determining the properties of each
state (solid, liquid, and gas)?

The spacing decides everything, because the spacing is the interparticle distance — and the
strength of the attraction depends on that distance, falling off drastically as it grows.
So one quantity controls two things at once: how strongly the particles are held, and how freely
they can move. Together these fix the shape, the volume and the compressibility of the
substance.

Solid Liquid Gas

spacing minimum a little more maximum

Page 16 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Fig. 7.12 redrawn: spacing is minimum in a solid, a little more in a liquid, and maximum in a gas. The
gaps hold nothing at all — not even air.

STATE INTERPARTICLE ATTRACTION PROPERTIES THAT FOLLOW
SPACING THAT FOLLOWS

Solid Minimum Maximum Fixed shape and volume; particles only
vibrate; practically incompressible

Liquid A little more than in Slightly weaker than Fixed volume, no fixed shape; particles
solids in solids move within a limited space; practically
incompressible

Gas Maximum Minimum Neither shape nor volume fixed; particles
(negligible) move through all the available space;
easily compressed

One warning the chapter gives: you might assume the space between particles is
filled with air. It is not. It contains nothing at all.

Q2 What can we say about the behaviour of gas in the syringe?

That a gas is compressible — its particles can be forced much closer together — and that it
springs back on its own when released.
Your thumb seals the open end, so not one particle of air can escape. Yet the plunger still moves
in and the volume of air inside decreases. The only way the same quantity of air can occupy less
room is if there was a great deal of empty space between its particles to begin with, and the
external pressure of your push has reduced that space.
Stop pushing and the particles, which move freely in all directions, spread out again and the
plunger returns to its original position.

Now repeat it with water: the plunger will hardly move — water is practically
incompressible. The contrast is the point of the activity. Gas particles have a lot of
room between them; liquid particles have very little.

Activity 7.6: Let us experiment — Page 107

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Section 7.3 How Does the Interparticle Spacing Differ in the Three States of Matter?

ACTIVITY

Q1 What do you observe?

Four things, in this order:

1. With your thumb sealing the open end, the plunger can be pushed inwards — the volume
of air inside the syringe decreases, even though no air is getting out.
2. It gets harder to push as you go. The air resists more and more.
3. The moment you stop pushing, the plunger moves back to its original position by itself.
4. Do the whole thing again with water in the syringe instead of air, and the plunger barely
moves at all — water is practically incompressible.

Why the two behave so differently: in a gas the interparticle spacing is maximum,
so pushing simply brings the particles closer and the gas takes up less room. In a
liquid the particles are already close together, so there is almost no spare space left
to squeeze out.

Activity 7.7: Let us observe — Page 108
Section 7.3 How Does the Interparticle Spacing Differ in the Three States of Matter?

ACTIVITY

Q1 Predict whether the water level will increase or decrease with respect to the mark
B.

Prediction: it will decrease. After the sugar dissolves, the final level C settles below B — and,
as Fig. 7.10d shows, it still stays a little above A.

A = level of the water alone

B = level after the sugar is added but before it dissolves

C = level after the sugar has dissolved

A < C < B, so the level falls from B to C

Page 18 of 43

as e
Class 8 Science Chapter 7 Particulate Nature of Matter
a g l AglaSem · NCERT Solutions

co m
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Why the prediction works: at mark B, the sugar is sitting as a heap at the bottom,

m l a se
taking up its own room in the vessel and pushing the water up. Once it dissolves, its
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particles separate and move into the interparticle spaces that already
se particles — space that was not being counted before. So the volume of the
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sugar, and the level comes down.
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Why C does not fall all the way back to A: there are not enough vacant spaces for
every single sugar particle, so the solution still takes up slightly more room than the
water did alone.
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ase Questions — Page 108
agl
In-text
Section 7.3 How Does the Interparticle Spacing Differ in the Three States of Matter?

om a s
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solid

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agl though still above A.

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a g l Page 19 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Q2 What do you observe in each case? Do the sand particles dissolve? Does the volume
of water in the vessel change after mixing, and why?

SOLID DOES IT WHAT HAPPENS TO THE LEVEL
ADDED DISSOLVE?

Common salt Yes Rises when added, then falls back to some extent after dissolving
— the same B to C fall as with sugar

Glucose Yes Same behaviour: rises, then falls back after dissolving

Sand No Rises when added and stays up; no fall, however long you stir

Stone pieces No Rises and stays up, exactly like sand

No, the sand particles do not dissolve. Sand is a solid that does not dissolve in water. When it
is added, the sand particles simply settle down at the bottom and occupy some space in the
container.
Yes, the volume in the vessel does change — it increases, and stays increased. The sand
keeps its own particles locked together in grains, so instead of slipping into the interparticle
spaces of the water, each grain pushes the water aside and takes up room of its own. The total
volume is therefore the volume of the water plus the volume of the sand.

The contrast is the lesson: a soluble solid vanishes into space that already existed,
so the total volume goes down again. An insoluble solid adds space of its own, so
the total volume only goes up.

Q3 Sugar and sand are both solids. Why does sugar dissolve in water but sand does
not?

Because the two solids differ in how strongly their own constituent particles are held
together, compared with how hard the water particles can pull.

In sugar, the interparticle attractions are weak enough for the moving water particles to pull
the sugar particles out of the grain one by one. Freed, the sugar particles spread into the
interparticle spaces of the water — the sugar dissolves.
In sand, the constituent particles are held together so strongly that the water particles are
unable to pull them out at all. Nothing is freed, so nothing spreads, and the sand stays as
visible grains at the bottom. Substances like sand are insoluble in water.

Page 20 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Read this as a contest of forces: dissolving is not something water simply does to
any solid. It happens only when the pull of the water particles wins against the
interparticle attractions inside the solid. Being a solid is not the deciding factor —
the strength of its attractions is.

In-text Questions — Page 109
Section 7.3 How Does the Interparticle Spacing Differ in the Three States of Matter?

Q1 What do you think about the interparticle spacing in solids?

It is the smallest of the three states — but it is not zero.
The constituent particles of a solid are held together by strong forces of attraction, so they do
not move from one place to another and are closely packed. Even so, despite that close packing,
some space is left between the particles, as Fig. 7.12a shows.

The correction the chapter makes: you might assume that this space is filled with
air. It is not. The interparticle spaces contain nothing at all. Air is itself matter made
of particles; if air were inside the gaps, those particles would need gaps of their own,
and the question would never end.

Why a solid is still practically incompressible: the space is there, but it is far too
little, and the attractions holding the particles in place are far too strong, for any
ordinary push to close it up. Contrast this with the syringe full of air in Activity 7.6.

Activity 7.8: Let us experiment — Page 109
Section 7.4 How Particles Move in Different States of Matter?

ACTIVITY

Q1 What do you observe?

The colour travels through the water on its own, without any stirring.

Page 21 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

At first — thin streaks of pink spread out from the grain of potassium permanganate at the
bottom, reaching upwards and sideways (Fig. 7.13a).
After some time — the streaks blur into one another and the entire bulk of water takes
on a uniform pink colour (Fig. 7.13b).

Safety first: do not touch potassium permanganate with your hands. Use a spoon
or a spatula to handle it.

Q2 Do you know why this happens?

Because the water particles are in constant motion. They do the work in two steps:

1. The moving water particles pull the particles of potassium permanganate out of the
grain, one after another.
2. They then keep hitting those freed particles, knocking them this way and that until they are
spread throughout the liquid.

Nothing is stirred and nothing is poured. The spreading is done entirely by particles that are
moving anyway, all the time, whether you are watching or not.

Why sand behaves differently: in many substances the constituent particles are
held together so strongly that the water particles are unable to pull them out at all.
Such substances — sand is the example the book gives — are insoluble in water.
Step 1 fails, so step 2 never begins.

Think like a scientist — Page 110
Section 7.4 How Particles Move in Different States of Matter?

THINK LIKE A SCIENTIST

Q1 Watch carefully and compare. What do you observe?

The same grain of potassium permanganate spreads at three clearly different speeds, and the
order is always the same.

Page 22 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

TUMBLER SPEED OF SPREADING MOTION OF THE WATER PARTICLES

Hot water Fastest Water particles move faster

Water at room temperature Less quick Slower than in hot water

Ice-cold water Slowest Slower still

The three tumblers differ in only one thing — their temperature — so the temperature must be
what is causing the difference.

The conclusion: the movement of particles increases when heat is provided. Faster
water particles pull the potassium permanganate particles out of the grain more
quickly and knock them further in the same time, so the colour reaches every part of
the hot tumbler first.

Keep the test fair: use tumblers of the same size, the same amount of water in
each, and grains of about the same size, dropped in at the same moment. Then
temperature is the only thing that varies, and the comparison means something.

Q2 Try to depict it by drawing a diagram.

A good diagram here must show three tumblers side by side at one instant of time, so that
the comparison is visible at a glance. Draw and label:

the three tumblers, marked hot water, water at room temperature and ice-cold water;
the grain of potassium permanganate at the bottom of each;
the pink colour spread over a large part of the hot tumbler, a middling part of the room-
temperature one, and only a small region around the grain in the ice-cold one;
a short arrow near a few particles to show that they are moving — longest arrows in the hot
tumbler.

Here is one way to draw it:

Page 23 of 43

as e
Class 8 Science Chapter 7 Particulate Nature of Matter
a g l AglaSem · NCERT Solutions

co m
The same grain, the same length of time
e m.
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Hot water Room temperature Ice-cold water
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spreads fastest spreads less quickly spreads slowest

o m l a se
Purple.c
a g after the same interval.
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se The hotter the water, the faster its particles move, and the further the colour has travelled.
bar = the grain of potassium permanganate; pink dots = its particles

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Label the diagram honestly: the pink dots stand for particles far too small to be
.
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enlarged — never a photograph

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In-text Questions — Page 110
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Section 7.4

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the naked eye?
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Indirectly — by watching something we can detect being carried about by the gas particles we
cannot.
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The chapter uses three such stand-ins:
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m ase
.co


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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

METHOD WHAT WE CAN WHAT IT SHOWS ABOUT THE INVISIBLE
DETECT PARTICLES

Smoke in gas jars The grey smoke The tiny smoke particles suspended in the air are
(Activity 7.5) spreading from Jar A constantly hit by the invisible particles of the gases, and
into Jar B their movement lets us observe the motion of gas
particles

Iodine vapour in a The violet vapour Gas particles move in all directions and occupy the entire
closed gas jar (Fig. filling the whole jar available space
7.8)

A burning incense The fragrance, Air particles are moving constantly and knock the
stick (Activity 7.9) reaching the far side fragrance particles right across the room
of the room

Why this counts as evidence: smoke does not swim and fragrance does not walk.
Something must be pushing them, and it must be everywhere in the room, and it
must never stop. Constantly moving air particles are the explanation that fits.

Activity 7.9: Let us find out — Page 110
Section 7.4 How Particles Move in Different States of Matter?

ACTIVITY

Q1 Do you notice the fragrance from a distance?

Yes — though not at once. Wait a few minutes and the fragrance reaches you even at the far
end of the room.

Immediately after lighting: the fragrance is felt only around the incense stick itself.
Shortly afterwards: you can smell it throughout the room.

This happens because the particles of the fragrance spread out and fill the entire room. They do
not travel of their own accord: the particles of air are moving constantly, and they hit the
fragrance particles and help them spread throughout the room.

Why it takes a few minutes and not an instant: a fragrance particle does not fly
straight to your nose. It is knocked about again and again by air particles coming
from every direction, so it works its way across the room by a long, crooked path.

Page 25 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

In-text Questions — Page 111
Section 7.4 How Particles Move in Different States of Matter? — Ever heard of …

Q1 Can you share a few other real-life situations where you have experienced the
movement of gas particles?

Yes — the same thing happens many times every day. In each case, particles get from one place
to another with nobody carrying them, because the air particles are moving constantly and keep
knocking them along.

EVERYDAY SITUATION WHAT IS MOVING, AND HOW YOU KNOW

The smell of dal or tempering reaching the far room Particles from the food spread through the air of the whole
while food is cooked house

Camphor or an agarbatti lit in one corner of a temple Its particles reach every worshipper, exactly as in Activity
7.9

A mosquito repellent or phenyl used at one end of a The smell is soon present everywhere in the room
room

The smell of an LPG leak noticed from another room The gas spreads on its own — which is why it is given a
strong smell in the first place

Attar or perfume applied on the wrist, noticed by Vapour from the liquid spreads through the air to reach
someone standing away them

A cut onion in the kitchen making eyes water at the Particles from the onion travel across the room through the
doorway air

The smell of hot jalebis reaching you before you turn Particles are carried outward from the shop in every
into the lane direction

Ever heard of … the chapter's own example of the particulate nature of matter at
work: when oil-stained clothes are washed with soap, a large number of soap
particles surround the oil particles on the fabric. One end of a soap particle attaches
to the oil and the other mixes with water, so the oil is lifted off and washed away (Fig.
7.15).

Keep the curiosity alive — Page 113

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

End-of-chapter question set (continues on page 114)

KEEP THE CURIOSITY ALIVE

Q1 Choose the correct option. The primary difference between solids and liquids is that
the constituent particles are: (i) closely packed in solids, while they are stationary in
liquids. (ii) far apart in solids and have fixed position in liquids. (iii) always moving
in solids and have fixed position in liquids. (iv) closely packed in solids and move
past each other in liquids.

(iv) closely packed in solids and move past each other in liquids.
This is the one statement that gets both halves right. In a solid the particles are tightly packed
and held in fixed positions by very strong attractions, so they can only vibrate about those
positions. In a liquid the attractions are a little weaker, the spacing a little more, and the
particles are free to move past one another — which is exactly why you can pass your finger
through water but not through a stone.

Why the others are wrong:

(i) — the first half is right, but liquid particles are certainly not stationary; their
motion is what spreads potassium permanganate through a whole tumbler.
(ii) — both halves are the wrong way round. Solid particles are close together, not
far apart; and it is solids, not liquids, whose particles have fixed positions.
(iii) — solid particles do not move from place to place at all, and liquid particles
do not have fixed positions.

Q2 Which of the following statements are true? Correct the false statements. (i)
Melting ice into water is an example of the transformation of a solid into a liquid.
(ii) Melting process involves a decrease in interparticle attractions during the
transformation. (iii) Solids have a fixed shape and a fixed volume. (iv) The
interparticle interactions in solids are very strong, and the interparticle spaces are
very small. (v) When we heat camphor in one corner of a room, the fragrance
reaches all corners of the room. (vi) On heating, we are adding energy to the
camphor, and the energy is released as a smell.

Statements (i), (ii), (iii), (iv) and (v) are true. Only (vi) is false.

Page 27 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

STATEMENT TRUE / REASON
FALSE

(i) True Ice is a solid and water is a liquid, so melting ice is precisely a solid
turning into a liquid — the change shown in Fig. 7.4

(ii) True On heating, the vibrations become vigorous enough for the particles to
leave their fixed positions; the interparticle distance grows and the
forces of attraction get weakened

(iii) True Strong attractions and minimum spacing hold every particle in place, so
a solid keeps both its shape and its volume

(iv) True These are the two defining features of the solid state; the small spaces
contain nothing at all, not air

(v) True The same thing you observe with an incense stick in Activity 7.9 — the
particles spread through the whole room

(vi) False Energy is not what reaches your nose; camphor particles are

Corrected form of (vi): On heating, we are adding energy to the camphor. This energy increases the
motion of its particles until they escape from the solid and spread through the air of the room; the
smell we notice is caused by these camphor particles reaching our nose, not by energy being released
as a smell.

Why the original is wrong: a smell is not a form of energy. It is our response to the
actual particles of a substance arriving at our nose. The energy supplied does a
different job — it frees the particles from the solid and keeps them moving.

Q3 Choose the correct answer with justification. If we could remove all the constituent
particles from a chair, what would happen? (i) Nothing will change. (ii) The chair will
weigh less due to lost particles. (iii) Nothing of the chair will remain.

(iii) Nothing of the chair will remain.
Justification. A chair is not an object that contains particles, the way a box contains marbles.
The chair simply is a very large number of constituent particles held together by interparticle
attractions, arranged in the shape of a chair. Its wood, its weight, its hardness and its shape are
all properties of those particles and of the forces between them.
Take every particle away and there is no material left to be a chair — no wood, no legs, no seat,
no mass, nothing at all. Even the interparticle spaces would go, because a space between
particles cannot exist where there are no particles.

Page 28 of 43

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Class 8 Science Chapter 7 Particulate Nature of Matter
a g l AglaSem · NCERT Solutions

co m
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Why the other options fail:

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(i) assumes the chair is something extra, over and above its particles, that could
a g l
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survive on its own. The whole chapter shows it is not: break a piece of chalk far

a gl enough and you are left with its particles, and nothing else.
(ii) treats the particles as only a part of the chair, so that removing them leaves a

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lighter chair behind. But they are all of it. Removing all of something does not
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leave a lighter version of it.
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Why do gases mix easily, while solids do not?

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a gBecause in a gas the particles are free to travel, and in a solid they are not.

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In gases. The interparticle attraction is negligible and the interparticle spacing is maximum.
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a s emwide spaces between the particles of the other, hit
particles of each simply wander into the
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them, get knocked further along,gand
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no shaking and no heating is needed — Activity 7.9 shows a fragrance crossing a whole room by
itself.
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In solids. The interparticle attractions are very strong and the spacing is minimum. Each particle

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What about mixing two powders? Grinding turmeric and salt together and shaking
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turmeric or pure salt. In a mixture
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When spilled on the table, milk in a glass tumbler, flows spreads out, but the
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Because the milk is a liquid and the tumbler is a solid, and the two differ in how strongly their

a s emthey can move.
particles are held and how freely

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m ase
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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

The milk flows and spreads. In a liquid the interparticle attractions are slightly weaker than in
a solid, so the particles are free to move past one another — though only within a limited space.
Inside the tumbler, the walls provide that space and the milk takes the shape of the tumbler.
Spilled on the table there are no walls, so nothing holds the particles in place; they slide over
one another and the milk spreads out into a thin layer, taking the shape of the table top. Its
volume, however, does not change — the same amount of milk is now merely wide and
shallow instead of tall and narrow.
The tumbler keeps its shape. In a solid the particles are closely packed and the interparticle
attractions are very strong, holding each particle in a fixed position. They can only vibrate about
those positions and cannot move past each other. So the glass has a definite shape and a
definite volume that a fall does not alter — it will crack if the blow is hard enough, but it will
never flow across the table.

The one sentence to take away: the same event, on the same table, at the same
temperature, gives two completely different results — and the only difference
between the two materials is the strength of their interparticle attractions.

Q6 Represent diagrammatically the changes in the arrangement of particles as ice
melts and transforms into water vapour.

Three arrangements of the same particles, joined by two changes of state:

Ice (solid) Water (liquid) Water vapour (gas)

melting boiling

at 0 °C at 100 °C

fixed positions, only vibration move within a limited space move freely in all directions

Magnified schematic pictures. Left to right the interparticle spacing grows and the interparticle
attraction weakens — but the particles themselves are identical throughout.

What each stage shows, and why:

Ice. Particles closely packed, held in fixed positions by strong attractions. They can only
vibrate. Spacing minimum, so the ice has a definite shape and volume.

Page 30 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Heating to the melting point. The thermal energy supplied is used to overcome the
attractions. The vibrations become vigorous enough for the particles to leave their fixed
positions; the interparticle distance increases slightly, the attractions weaken, and the ice
becomes water.
Water. Particles a little more loosely packed, moving — but still close together and still
within a limited space. Fixed volume, no fixed shape.
Heating to the boiling point. The movement becomes so vigorous that the particles move
far apart, the attractions fall away, and the particles escape from the liquid.
Water vapour. Attraction negligible, spacing maximum, particles moving freely in all
directions. Neither shape nor volume is fixed.

When you draw this in your notebook: use the same number of circles of the same
size in all three boxes. Only their spacing and arrangement should change.
Drawing bigger circles for the gas would say that the particles themselves expand on
heating, which is not what happens.

Did you know? Water does not have to reach 100 °C to become vapour. Vapour
forms at every temperature, slowly and only at the surface — that slow process is
evaporation, and it is why a spilled puddle disappears on an ordinary day.

Q7 Draw a picture representing particles present in the following: (i) Aluminium foil (ii)
Glycerin (iii) Methane gas

Aluminium foil is a solid, glycerin is a liquid and methane is a gas — so the three pictures are
the three standard arrangements, drawn to the same scale.

(i) Aluminium foil (ii) Glycerin (iii) Methane gas

solid — packed and fixed liquid — a little more space gas — far apart, free

Each box is the same size and holds particles of the same size. Only the spacing and the freedom of
movement differ.

Page 31 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

SUBSTANCE STATE HOW TO DRAW IT WHY

Aluminium foil Solid Circles closely packed in a regular Interparticle attraction
arrangement, almost touching, filling maximum, spacing minimum,
the whole box particles only vibrate about fixed
positions

Glycerin Liquid The same circles, a little more loosely Attraction slightly weaker than in
packed and irregularly placed, but still a solid; particles move, but only
close together and filling the box within a limited space

Methane gas Gas Only a few circles, scattered far apart Attraction negligible, spacing
with large empty gaps between them maximum, particles move freely
in all directions

Two mistakes to avoid: do not draw the gas particles larger than the solid ones —
the particles are the same in every state. And do not shade the gaps: the
interparticle spaces contain nothing at all, not air.

Page 32 of 43

Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Q8 Observe Fig. 7.16a which shows the image of a candle that was just extinguished
after burning for some time. Identify the different states of wax in the figure and
match them with Fig. 7.16b showing the arrangement of particles.

(a)

(b)

Fig. 7.16

Fig. 7.16 — (a) redrawn sketch of the textbook photograph of a candle just after it was
blown out: a wisp rising from the blackened wick, a small pool in the hollow around the
wick, wax that has run down the sides, and the standing candle on its holder. (b) three
arrangements of particles.

A candle that has just been blown out shows all three states of the same substance — wax —
at the same moment.

Page 33 of 43

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Class 8 Science Chapter 7 Particulate Nature of Matter
a g l AglaSem · NCERT Solutions

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WHERE YOU SEE IT IN FIG. STATE OF WHICH ARRANGEMENT IN FIG. 7.16B

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7.16A THE WAX

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hardened and fill it completely (the upper left one)

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om packed, with slightly larger gaps but still
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The box in which the particles are a little more
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extinguished wick vapour) of it empty (the lower one)
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Cool it and the whole sequence runs backwards: the vapour condenses, the pool

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Check it yourself: hold a burning matchstick in the white trail a moment after
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blowing the candle out. The flame runs down the trail and relights the wick — proof
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that trail is wax vapour and not smoke.
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Q9 Why does the water in the ocean taste salty, even though the salt is not visible?
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the water and far too small to see.
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When salt dissolves it breaks up into its constituent particles. These particles are extremely

a s
agl into the interparticle spaces that already exist between the water particles. The constant motion
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of the water particles then spreads them evenly through the whole body of water. Once that has
s e m
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happened there are no grains of salt left anywhere for the eye to find — but every drop of the

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sea contains salt particles, and your tongue detects them at once.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

This is exactly Activity 7.2 on a very large scale. Two teaspoons of sugar disappear completely
into a tumbler of water, yet a spoonful taken from the top layer tastes sweet. Activity 7.7 adds
the second piece of evidence: the level falls from B to C, showing that the dissolved particles
have slipped into space that was already there.

The general point: invisible does not mean absent. Seeing is only one way of
detecting matter; taste, smell and a change of volume are others, and in this chapter
they are the ones that work.

Did you know? The salt gets there the same way the pebbles on the beach did.
Rivers wear down rocks as they flow (page 99); along with the sand and clay they
also carry dissolved salts to the sea. Water leaves the sea by evaporation, but the salt
particles stay behind — so the sea keeps getting saltier.

Q10 Grains of rice and rice flour take the shape of the container when placed in
different jars. Are they solids or liquids? Explain.

Both are solids. What takes the shape of the jar is the heap, not the material.
Apply the particle test rather than the eye test:

Look at one grain. A single grain of rice has its own definite shape and its own definite
volume, and it keeps them wherever you put it. Its constituent particles are closely packed,
held in fixed positions by strong attractions, and can only vibrate. That is the definition of a
solid. Rice flour is the same thing ground finer — every speck is still a small piece of solid
rice, exactly as every speck of ground chalk was still chalk in Activity 7.1.
Ask what is actually moving. When you tilt the jar, it is the grains that roll over one another.
Inside each grain nothing moves from its place. In a true liquid it is the particles themselves
that slide past one another, and there is nothing smaller for them to be arranged into.
Pour it out again. Rice comes back as the same countable grains, unchanged. Water poured
out does not come back as separate identical pieces.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

TEST RICE GRAINS / RICE FLOUR A TRUE LIQUID SUCH AS
WATER

Can it be piled into a heap? Yes — a heap of rice or flour keeps a No — it flows until the surface
sloping surface is level

Are there visible gaps in it? Yes, air-filled gaps between the grains No, the liquid is continuous

Does each piece keep its own Yes, every grain does No, the water has no pieces
shape?

Why the question is a good trap: flowing and filling a container look like liquid
behaviour, so it is tempting to judge by that alone. But those are properties of a
heap of small solid pieces. The state of matter is settled by how the constituent
particles are arranged and held — and in rice they are locked in fixed positions.

Link back to page 98: this is the same reasoning as the opening question about
heaping sand. Sand pours, yet sand is undoubtedly a solid.

Discover, design, and debate — Page 115
Society–Science interdisciplinary projects

DISCOVER, DESIGN, AND DEBATE

Q1 Fix a balloon over the neck of a bottle and put the bottle in hot water. Explore what
will happen?

The balloon inflates by itself, standing up over the mouth of the bottle — even though you
have not blown into it at all.
How to do it. Take an empty, dry bottle and stretch the mouth of a balloon tightly over its neck
so that no air can leak out. Stand the bottle in a bowl of hot water so that most of it is under
water. Watch the balloon for a minute or two. Then lift the bottle out and stand it in cold water
instead, and watch again.
What you will observe.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

BOTTLE PLACED IN BALLOON

Hot water Slowly swells up and stands erect

Cold water afterwards Shrinks back down and goes limp again

Why it happens: the bottle is not empty — it is full of air, and air is matter made of
particles. Hot water passes heat to that air, so the thermal energy of its particles
increases and they move faster. In a gas the interparticle attractions are negligible,
so faster particles simply spread further apart and take up more space. A gas has no
fixed volume, so the air expands, and the only place it can expand into is the balloon.
Cool the bottle and the particles slow down, the air occupies less space, and the
balloon collapses.

Keep it a fair test: the balloon must be airtight on the neck, or the air will escape
instead of doing the work. And note that no new air has been made — the same
particles are simply spread over a larger volume.

Safety first: use hot water from a tap or a kettle that has been allowed to stand, not
boiling water, and handle the bowl with an adult's help.

Q2 Design and create simple models to represent particles of solids, liquids, and gases
showing interparticle spacing using clay balls, beads, etc.

Method. Make all the balls the same size — that is the single most important rule, because
the particles do not change from state to state; only their spacing and motion do. Take three
identical shallow trays or box lids so that the three models can be compared fairly.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

MODEL HOW TO BUILD IT WHAT IT MUST SHOW

Solid Fill the tray with clay balls of one size, packed in Minimum spacing, regular packing,
neat rows so that they nearly touch, and stick particles fixed in position
them down so none can move

Liquid Use the same number of balls in the same tray, A little more spacing than the solid,
but loose and irregularly arranged with slightly no fixed arrangement, movement
bigger gaps; leave them free to be nudged about within a limited space

Gas Put only four or five balls in the tray, well Maximum spacing, free movement,
separated, and shake the tray gently so that they particles filling the whole available
roll all over it space

Sample presentation. Label the three trays Solid, Liquid and Gas, and write one sentence under
each: "Attraction strongest, spacing minimum, only vibration"; "Attraction slightly weaker, spacing a
little more, movement within a limited space"; "Attraction negligible, spacing maximum, free
movement in all directions".

Judge your own model honestly. Ask three questions of it: are the balls the same
size in all three trays? Are the gaps empty, with nothing drawn or stuffed into them?
Does the solid tray really allow no movement at all except a small wobble? A model
that fails any of these teaches the wrong idea.

A limit worth stating aloud: real constituent particles are unimaginably smaller
than clay balls, are not coloured, and never come to rest. A model shows
arrangement and spacing well; it cannot show scale or ceaseless motion.

Q3 Pretend to be particles of solids, liquids, and gases, at different temperatures—
create and perform a role-play/dance showing particles in motion.

Method. The class becomes the particles and the floor becomes the container. Mark out the
container with chalk or with a rope, and keep it the same size for all three scenes so that the
spacing is what changes.

Page 38 of 43

as e
Class 8 Science Chapter 7 Particulate Nature of Matter
a g l AglaSem · NCERT Solutions

co m
m.
SCENE WHERE YOU STAND HOW YOU MOVE

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Solid, cold
.corows, filling the marked area
Shoulder
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Feet fixed; only sway gently on the spot — never swap

a s em places with a neighbour

l
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aSolid Same rows, same places Sway harder and harder as a drum beat speeds up, until
heated a few students break away

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Liquid Still close together, but no
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longer in rows
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Gas Spread out, far from each
a Move quickly in straight lines, in all directions, right up
other
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to the walls of the room — the chalk boundary no
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longer applies

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gtightly by our neighbours
m .c for the narrator: "We are the particles of ice. We are helda
e only shiver in place. Heat is coming … we shiver harder … the hold breaks, and we are water
Sample script

l a scan
ag— we can move now, but we stay together. More heat … we are moving so fast that we escape
and

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altogether. We are vapour. We fill the whole room."
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a s
Show the temperature, not just the state: let a drum or a clap set the speed.
l = faster particles. That single device turns the
agenergy
Faster beat = more thermal
dance into a correct statement about the chapter — the movement of particles

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increases when heat is provided.

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property of gases beneficial or harmful?

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Both — the property itself is l
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maximum, and the particles move freely in all directions, so a gas released anywhere spreads

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

BENEFICIAL HARMFUL

A leaking LPG cylinder is smelled from another room in The same leak spreads the cooking gas itself through
time to act — the warning smell spreads faster than the the whole kitchen, so a single spark can set fire to the
danger builds up entire room

Oxygen from a cylinder spreads evenly to reach a Smoke and harmful gases from vehicles and factories
patient; anaesthetic gases fill an operating room mask do not stay near the chimney — they spread over
reliably whole cities

The fragrance of incense, camphor or attar reaches every A gas leak in a factory can spread over the
corner of a room or a temple (Activity 7.9) neighbourhoods around it before anyone can contain
it

Air itself keeps mixing, so oxygen reaches every room of Smoke from burning stubble or garbage spreads far
a house without any pump beyond the field where it was lit

How to run the debate. Divide the class in two. Each side must (a) name a real situation, (b)
explain it using interparticle spacing, attraction and particle motion, and (c) answer one point
made by the other side. Do not allow "gases are dangerous" or "gases are useful" as arguments
on their own — every claim must be tied back to the particle model.

A conclusion worth reaching together: because a gas cannot be persuaded to stay
put, it must be contained. This is why LPG is sold in sealed cylinders, why
laboratories keep iodine in closed jars (Fig. 7.8), why factories fit chimneys and
scrubbers, and why we are told never to look for a gas leak with a lighted
matchstick. We do not change the property; we design around it.

Chapter at a glance
All matter is made up of an extremely large number of constituent particles — the basic
units that a piece of a substance is built from. They are so small that they cannot be seen
even through an ordinary microscope, and breaking or grinding a substance never changes
them; it only separates one group of them from another.
The particles are held together by attractive forces called interparticle attractions. Their
strength depends on the nature of the substance and on the interparticle distance — even
a slight increase in that distance weakens the force drastically.
The strength of these attractions decides the physical state. In a solid the particles are
closely packed, the attraction is strongest, and they can only vibrate about fixed positions —
hence a definite shape and volume. In a liquid the attraction is a little weaker, so the
particles move but only within a limited space — definite volume, no fixed shape. In a gas

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

the attraction is negligible and the particles move freely in all directions — neither a fixed
shape nor a fixed volume.
Between the particles there are interparticle spaces — minimum in solids, a little more in
liquids, maximum in gases. These spaces contain nothing at all — not air. Sugar dissolving
in water and air compressing in a syringe are both direct evidence that such spaces exist.
The particles are always in motion. This is why potassium permanganate colours a whole
tumbler of water without stirring, and why the fragrance of an incense stick reaches every
corner of a room. Heating increases the motion, so the spreading is fastest in hot water and
slowest in ice-cold water.
It is the thermal energy of the particles that ultimately fixes the state. At the melting point
the thermal energy supplied overcomes the attractions and the solid becomes a liquid; at
the boiling point the particles escape the liquid altogether. Vapour also forms slowly at
every temperature, from the surface only — that is evaporation.

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

Quick revision

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

Constituent The basic unit that makes up a larger Section 7.1, page Grinding chalk on and on
particle piece of a substance or material 100 finally leaves the units the
chalk was made of (Activity
7.1)

Interparticle The space between the constituent Sections 7.1 and The level falls from B to C
space particles; it contains nothing at all, not 7.3, pages 101 when sugar dissolves
even air and 109 (Activity 7.7, Fig. 7.10)

Interparticle The attractive force holding the particles Section 7.2, page Strongest in a piece of iron,
attraction together; it weakens sharply as the 101 weaker in water, negligible
interparticle distance grows in air

Solid state Definite shape and volume; particles Section 7.2.1, Iron nail, rock salt, stone,
closely packed, attraction strongest, page 102 key, wooden block,
motion only vibration about fixed aluminium (Fig. 7.3)
positions

Liquid state Definite volume, no fixed shape; Section 7.2.2, The same 200 mL of water
particles free to move but only within a page 104 in containers A, B and C
limited space (Activity 7.4, Fig. 7.5)

Gaseous state Neither fixed shape nor fixed volume; Section 7.2.3, Smoke fills the whole of
attraction negligible, particles move page 106 Gas Jar B (Activity 7.5, Fig.
freely in all directions 7.7)

Melting point The minimum temperature at which a Section 7.2.1, Ice 0 °C, urea 133 °C, iron
solid melts into a liquid at atmospheric page 103 1538 °C (Table 7.1)
pressure

Boiling point The temperature at which a liquid boils Section 7.2.2, Bubbles form inside the
and turns into vapour at atmospheric page 105 liquid, not only at its
pressure surface

Evaporation The slow change of a liquid into vapour, Section 7.2.2, Spilled water disappears
at any temperature and only from the page 105 after some time
surface

Fluids Liquids and gases together — they flow Section 7.2.3, Water poured from one
and do not keep a fixed shape page 106 container to another;
smoke spreading in a gas
jar

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Class 8 Science Chapter 7 Particulate Nature of Matter AglaSem · NCERT Solutions

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

Thermal energy The heat energy of the particles; it Let us wrap up, Low in solids, more in
decides how far apart they stay, and page 112 liquids, highest in gases
hence the state of the substance

Suspended Tiny dust particles suspended in air — A step further, The term used when
Particulate not constituent particles; each dust page 109 talking about air pollution
Matter (SPM) particle is itself made of a huge number
of atoms and molecules

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