NCERT Solutions Class 8 Science Chapter 9 the Amazing World of Solutes Solvents and Solutions

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 9: The Amazing World of
Solutes, Solvents, and Solutions

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

134 – 151 21 52 English

Solutions, notes, sample papers & more at 51 pages

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 9: The Amazing World of
Solutes, Solvents, and Solutions
Salt vanishes into water, chalk powder does not; a spoon of sugar dissolves, the tenth spoon settles at the
bottom. This chapter follows that one everyday observation to two big ideas — solubility, the limit a solvent
has at a given temperature, and density, the mass packed into unit volume, which decides much of what
floats and what sinks.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 134 – 151

SECTIONS QUESTIONS

21 52

MEDIUM

English

Probe and ponder — Page 134

Page 1 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Chapter opener — people gathering salt at the seashore

PROBE AND PONDER

Q1 What do you think is happening in the picture above?

Chapter-opening picture, page 134. A wide sea-shore scene. On the left, blue-
green sea water washes over a flat stretch of wet sand; on the right the sand
rises into a dry, pale bank. Along the water's edge the wet sand is crusted with
rough white patches. In the middle foreground an elderly, balding man in a
white dhoti and shawl, wearing round spectacles, bends right down; a long
bamboo staff is in his left hand and he is picking up a little of the white crust
with his right hand. Two people crouch beside him on the left, also reaching
down towards the white patches. Behind them a large crowd stands watching
him — men in white kurtas and white caps, women in white and pale saris with
the end drawn over the head, one man in a dark waistcoat leaning in, another
with a white towel over his shoulder.

Chapter-opening picture, page 134 — described here; see the textbook for the
illustration itself.

People are collecting natural salt from the seashore. Behind them is the sea; in front of them the
wet sand is crusted with white salt, and a man in a dhoti is bending down to pick up a handful of
it.
The science of the picture is the science of this whole chapter. Sea water is not pure water — it is
a solution in which common salt and other salts are the solutes and water is the solvent.
Sunshine and wind evaporate the water, but the salt cannot evaporate. As water leaves, the
solution left behind becomes more and more concentrated, until it is saturated. Beyond that
point every further litre of water that evaporates leaves solid salt behind, and the salt appears
as white crystals on the sand. The people are simply picking up what the sea left.

Why it happens: Evaporation removes only the solvent. The mass of dissolved salt
stays the same while the volume of water falls, so the concentration keeps rising —
and once the solution is saturated, the extra salt has nowhere to go except out of the
solution as crystals.

Page 2 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Did you know? The scene is drawn from the Dandi march of April 1930, when
Mahatma Gandhi walked to the sea at Dandi and picked up salt from the shore to
break the salt law. The same evaporation is still used today in the salt pans of
Gujarat and Tamil Nadu — and in Ningel village of Manipur, described in the Our
scientific heritage box at the end of this chapter.

Q2 What happens when you add too much sugar to your tea and it stops dissolving?
How can you solve this problem?

The tea has become a saturated solution of sugar at that temperature. It cannot hold any more,
so the extra sugar sinks and stays as grains at the bottom of the cup.
Two things will fix it, and one will not:

Heat the tea (or add hot tea). Solubility of a solid rises with temperature, so a saturated
solution behaves as an unsaturated one once it is warmer, and the settled sugar dissolves.
Add more tea — that is, more solvent. The same sugar now has more water to spread
through, so the solution is no longer saturated.
Stirring alone will not help. Stirring only makes dissolving faster; it does not raise the limit.
Once the limit is reached, you can stir all evening and the grains will stay.

Why it happens: Dissolving means the water particles pull sugar particles off the
crystal and keep them apart. At a fixed temperature the water particles have a fixed
amount of energy, so only a fixed number of sugar particles can be held apart in a
given amount of water — that is the solubility. Heating gives the particles more
energy and more room, so more sugar can be held; adding water gives more water
particles to do the holding.

Q3 Why do sugar and salt dissolve in water but not in oil? Why is water considered a
good solvent?

Because dissolving is a tug-of-war between attractions, and water wins it while oil does not.
In a grain of salt or sugar the particles are held tightly to one another in a regular arrangement.
For the grain to dissolve, the particles of the liquid must pull those particles away one by one
and then keep them apart and evenly spread. Water particles can do this: each water particle
has a slightly positive side and a slightly negative side, so it grips the particles at the surface of

Page 3 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

the crystal, tugs them loose and surrounds them. Oil particles have no such pull. They attract
co m
one another well enough, but they hardly attract salt or sugar particles at all — so the crystal is
e m.
m l as
.co g
never pulled apart and simply lies in the oil unchanged.

em a
s
Why water is called a good solvent:
a
a gItldissolves a very wide range of substances — salts, sugars, ORS powder, many medicines,
and even gases such as oxygen and carbon dioxide.

co m
. ag
It is liquid over the whole range of everyday temperatures, is cheap, is not poisonous and

e m
as
does not catch fire.

a g l
It carries dissolved substances through living things — sap in a plant, blood in your body,
digested food in your intestine.

co m
Did you know? Ayurveda, Siddha and other Indian systems of medicine have used
em.
m l as
.co g
water as the main solvent for preparing formulations for centuries, and also oil,
m a
l a se
ghee and milk where a substance dissolves better in those.

a g
m a s
m .co
Why are water bottles usually tall and cylindrical in shape instead of spherical?
agl
se
Q4

g l a
ANSWER a
Because a tall cylinder is easy to stand, easy to hold and easy to pack, and a sphere is none of
com
these.
m .
a se
. com l
agSPHERE
m
ase
REQUIREMENT TALL CYLINDER

agl Stands on a table or in a Flat circular base — stands upright Rolls away; needs a stand
bag
se m
commeet round it
Small diameter, .fingers g l a
Grip in one hand
em
Same volume forces a wide
a
a s
gl
diameter — hard to hold

Packing on a shelf or in a
a
Cylinders sit side by side with little space Spheres leave large empty gaps

om
carton wasted between them

. c
m a s
Theem
level tells you very little

agl
Judging how much is left Volume = πr²h, so the height of the water is a

. co near the widest part
m
direct measure of the amount

ase Pouring and filling
a g l A narrow neck at one end controls the flow Awkward to pour without
spilling
.c
s e m
m a
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 4 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why it happens: A sphere does have one advantage — for a given volume it uses
the least material — but a bottle must also stand, be gripped, be stacked and be
poured. Shape is chosen for the way an object is used, not for material alone. This is
exactly why a measuring cylinder is also made narrow and tall (see page 144).

In-text Questions — Page 135

Page 5 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

9.1 What Are Solute, Solvent, and Solution?

Q1 Can you predict whether this mixture is uniform or not (Fig. 9.1)?

Fig. 9.1, page 135 — redrawn sketch: a glass tumbler holding the mixture of sugar, salt
and water.

It is a uniform mixture. The sugar and salt have dissolved completely in the water, so the liquid
is clear and you cannot see the sugar or the salt separately anywhere in the glass.

Page 6 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why it happens: Water particles pull the sugar and salt particles off their crystals
and spread them evenly through the whole liquid. Every drop of the mixture
therefore has the same composition — which is why every sip of homemade ORS
tastes the same and never salty in one sip and sweet in the next. A uniform mixture
like this is called a solution.

Q2 What happens when chalk powder is mixed with water—does it form a uniform
mixture?

No. Chalk powder does not dissolve. It stays as tiny solid grains that make the water look milky,
and if the glass is left standing, the grains slowly settle at the bottom. This is a non-uniform
mixture, like sand in water or sawdust in water (Fig. 9.2).

Why it happens: Water particles cannot pull the chalk particles apart from one
another — the attraction inside the chalk is much stronger than anything water can
offer. So the chalk stays in clumps of many particles, large enough to see and heavy
enough to settle. In a solution the solute is broken down to single particles that are
far too small to see or to settle.

Check it yourself: Hold the glass up to the light. A solution of sugar in water is clear
right through; chalk in water is cloudy. That difference in appearance is the easiest
test for uniform against non-uniform.

Q3 We know air is a mixture. Would a mixture of gases also be considered a solution?

Yes. Air is a uniform mixture of nitrogen, oxygen, carbon dioxide, water vapour and other gases,
and a uniform mixture is a solution. The gases are spread evenly, no component can be seen or
picked out separately, and a sample of air taken from anywhere in this room has the same
composition.
Following the chapter's own rule for two substances in the same state — the one present in the
larger amount is the solvent — nitrogen (about four-fifths of air) is the solvent and oxygen and
the rest are the solutes.

Page 7 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why it happens: Gas particles are very far apart and move about at high speed in
every direction. Different gases mixed in one container therefore intermingle
completely on their own and stay mixed; they never settle into layers the way sand
settles in water.

Tip: A solution does not have to be a liquid. Air is a gas–gas solution, sea water is a
solid–liquid solution, and an alloy such as brass is a solid–solid solution.

Activity 9.1: Let us investigate — Page 136
9.2 How Much Solute Can a Fixed Amount of Solvent Dissolve?

ACTIVITY

Q1 Gradually add a spoonful of salt into the glass tumbler and stir. Observe how many
spoons of salt you can add before it stops dissolving completely. Record your
observations in Table 9.1.

AMOUNT OF SALT TAKEN OBSERVATION (SALT DISSOLVES / SALT DOES
(TEASPOON) NOT DISSOLVE)

One

Two

Three

Four

…

Table 9.1, page 136: Dissolution of salt in water — copy the table out and fill in the
observation column as you work.

The first few spoons disappear completely. Somewhere around the sixth to the eighth spoon in
half a tumbler of water (roughly 100 mL), the grains stop vanishing and a layer of salt stays at
the bottom however long you stir. A typical filled Table 9.1 looks like this.

Page 8 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

co m
m.
AMOUNT OF SALT TAKEN OBSERVATION (SALT DISSOLVES / SALT DOES NOT

ase
com
(TEASPOON) DISSOLVE)

. a g l
m
ase
One Salt dissolves completely — solution is unsaturated

gl
aTwo Salt dissolves completely — unsaturated

co m
. ag
Three Salt dissolves completely — unsaturated

e m
Four
g l as Salt dissolves, but takes longer stirring

Five
a Salt dissolves with long stirring

co m
em.
Six Most dissolves; a few grains remain

m l as
Seven
m .co a g
Salt does not dissolve — it settles at the bottom. Solution is now

l a se saturated

g
a Your own number may differ from seven, and it should. It depends on how much water you
m a s
.co agl
took, how big your spoon is, and how warm the room is — so record what you observe.

se m
g l a
a
Why it happens: The water can hold only a fixed number of salt particles apart from
one another at that temperature. Until that limit is reached each new spoonful is
pulled apart and spread through the water. Once the limit is reached, any further salt
co m
m .
e
simply stays as crystals — the solution has become saturated.
m l as
.co
m Use the same spoon, levelled off each time, and stir for the same length of time a g
a s eTip:
agl after every addition. Otherwise you are changing two things at once and the count
means nothing.
se m
com g l a
m . a
gl ase
a — Page 136
Some discussion points
Activity 9.1
co m
m .
m
LET US DISCUSS
as e
.co a g l
a s emQ1
gl
How many spoons of salt were you able to dissolve before some of it remained
a c
.
undissolved?

s e m
m a
m . co agl
l a se
ag staying at the bottom. Write down your own count, because it is the
In half a glass tumbler of water — about 100 mL — you will usually manage six to eight level
teaspoons before salt starts
count for your volume of water, your spoon and your room temperature.

co m
m .
m as e
.co


a g l Page 9 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why it happens: Common salt has a solubility of roughly 36 g in 100 mL of water at
room temperature, and a level teaspoon holds about 5 g. So about seven spoons is
what you should expect — and this is why the number is not a matter of opinion: it is
a fixed property of salt and water at that temperature.

Try this: Repeat with only a quarter tumbler of water. You will find you can dissolve
about half as many spoons — showing that the limit is set by how much water you
took, not by the salt.

Q2 What does this indicate about the capacity of water to dissolve salt?

It shows that a fixed amount of water has a definite limit — it cannot dissolve salt endlessly. Up
to that limit the solution is unsaturated; at the limit it is saturated at that temperature, and
any extra salt stays undissolved at the bottom.
Three things follow from this one observation:

The limit depends on how much solvent there is. Twice the water dissolves about twice the
salt.
The limit depends on the temperature. Warm the water and it will take more salt (Activity
9.2 shows this for baking soda).
The limit is a property of that solute in that solvent. Sugar, baking soda and salt each have
their own limit in water — this maximum amount that dissolves in a fixed quantity of solvent
is called the solubility.

Why it happens: A water particle can hold apart only a limited number of salt
particles. Once every water particle is already busy surrounding dissolved particles,
there is nothing left to pull the next crystal apart — so it stays a crystal.

In-text Questions — Page 136

Page 10 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

9.2 How Much Solute Can a Fixed Amount of Solvent Dissolve?

Q1 What will happen if we keep on adding more salt in a given amount of water?

At first every spoonful dissolves and the solution gets steadily more concentrated. Then a stage
arrives when the added salt stops dissolving and settles at the bottom. From that moment the
solution is saturated at that temperature, and no matter how much more salt you pour in, none
of it will dissolve — the extra simply piles up as solid.

Why it happens: A fixed amount of water has a fixed capacity to keep solute
particles apart and spread out. That capacity is used up gradually as you add salt.
Once it is used up, the water cannot pull any more particles off the crystals, so the
crystals stay whole and sink.

Tip: The solution above the settled salt is not weak — it is at its strongest possible
for that temperature. Adding salt to a saturated solution changes the amount of
solid at the bottom, not the strength of the liquid.

In-text Questions — Page 137
9.2 How Much Solute Can a Fixed Amount of Solvent Dissolve?

Q1 Can you now reflect — which solution is more concentrated; 2 spoons of salt in 100
mL of water or 4 spoons of salt in 50 mL of water?

The 4 spoons in 50 mL — and by a factor of four.
Concentration means the amount of solute in a fixed quantity of solution, so bring both to the
same volume before comparing.

First solution: 2 spoons in 100 mL

Second solution: 4 spoons in 50 mL = 4 × 2 spoons in 2 × 50 mL = 8 spoons in 100 mL

8 spoons per 100 mL ÷ 2 spoons per 100 mL = 4 times as concentrated

Page 11 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why it happens: You cannot compare 2 with 4 directly, because the two amounts of
salt are shared among different amounts of water. The second solution has twice as
much salt spread through half as much water — 2 × 2 = 4 times as crowded with salt
particles. The first is the dilute one, the second the concentrated one.

Tip: Dilute and concentrated are relative terms — they only tell you which of two
solutions has more solute per unit volume. They say nothing about whether either
one is saturated.

Q2 Does temperature affect the solubility of a solute?

Yes — strongly. For most solids, solubility increases as temperature increases. Activity 9.2
shows it directly: baking soda that would not dissolve at 20 °C dissolves at 50 °C, and more still
dissolves at 70 °C.
Two consequences worth remembering:

A solution that is saturated at one temperature behaves as an unsaturated solution when it
is heated — its limit has moved up.
Because the limit is different at every temperature, a solubility value is meaningless unless
the temperature is stated with it.

Why it happens: Heating makes the particles of both the solvent and the solid move
faster. Faster-moving water particles strike the crystal harder and can tear more
particles loose, and the faster movement also keeps the freed particles from settling
back on to the crystal. So the balance shifts in favour of dissolving.

Did you know? Gases behave in the opposite way — their solubility falls as the
water warms up (Section 9.3).

Activity 9.2: Let us experiment (Demonstration activity) — Page 137

Page 12 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

9.2.1 How does temperature affect the solubility of a solute?

ACTIVITY SAFETY FIRST

Q1 Now, heat the contents to 50 °C while stirring (Fig. 9.7). What happens to the
undissolved baking soda?

Laboratory stand

Laboratory thermometer

Glass rod

Beaker

Water containing baking soda

Wire gauze

Tripod stand

Spirit lamp

Fig. 9.7, page 138 — redrawn sketch of the apparatus for dissolving baking soda in
water.

It dissolves. The layer of solid baking soda lying at the bottom of the beaker disappears as the
water warms from 20 °C to 50 °C, and the liquid becomes clear again.

Why it happens: At 20 °C the water had already reached its limit — it was a
saturated solution, and the extra baking soda had nowhere to go. Heating raises
that limit. The hotter water particles move faster, knock particles off the solid more
effectively and hold more of them apart, so the solution that was saturated at 20 °C
is unsaturated at 50 °C and the settled solid goes into solution.

Page 13 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

co m
m.
Safety first: Be careful while using the heating device. Keep the thermometer bulb

m l a se
in the liquid but not touching the bottom of the beaker, stir with the glass rod and
o
.cthe thermometer, and switch off the spirit lamp before
a gremoving the
m
never with

l a se
ag
beaker.

o m
. c
Again, heat the contents further tom70 °C while continuing to stir. What do you ag
a se
Q2
observe?
ag l
c o m
.
s etom70 °C clears that
The same thing happens again — the undissolved baking soda dissolves. After the solution was
m a
. co
made saturated
a gl
once more at 50 °C by adding fresh baking soda, heating it

s em
solid too.
a
l the three stages side by side and the pattern is unmistakable:
agPut
m a s
.co agl
TEMPERATURE OF STATE OF THE SOLUTION BEFORE AMOUNT OF BAKING

m
ase
WATER HEATING SODA DISSOLVED

20 °C
agl
Saturated; solid at the bottom Least

50 °C Becomes unsaturated, then saturated More
co m
again after more is added
m .
m as e
.co a g l
m
70 °C Becomes unsaturated again Most

l a se
ag
se m
com g l a
.
least dissolved most dissolved

m a
ase
agl

co m
m .
m as e
.co a g l
se m
g l a 20 °C — saturated, 50 °C — more 70 °C — no solid
a solid left over dissolves left at all
c
m .
The same beaker at three temperatures. Blue dots are dissolved baking soda particles spread
m a s e
. c o agl
through the water; the orange layer is baking soda that has not dissolved. As the water is heated, the

s e m shrinks and finally disappears.
layer
a
agl

co m
m .
m ase
.co


a g l Page 14 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why it happens: Every temperature has its own solubility value for baking soda.
Raising the temperature raises that value, so each time you heat the beaker the
same water can hold more dissolved solid than before.

Q3 What do you infer from this experiment?

For most solids, solubility in a liquid increases as the temperature rises. Water at 70 °C
dissolves more baking soda than water at 50 °C, and water at 20 °C dissolves the least of the
three.
Two statements follow, and both are worth writing down:

A saturated solution at one temperature behaves as an unsaturated solution if the
temperature is raised — the leftover solid then dissolves.
Any figure quoted for solubility must carry a temperature with it, otherwise it does not say
anything definite.

Why it happens: Two opposite processes go on at the crystal's surface — particles
leaving it and going into the water, and dissolved particles returning and sticking
back on. A saturated solution is simply the state where these two happen at the
same rate. Heating speeds up the leaving far more than the returning, so the
balance shifts and more solid ends up dissolved before the two rates match again.

Check it yourself: Let the 70 °C solution cool back to room temperature without
disturbing it. Solid baking soda will reappear at the bottom — proof that you crossed
the limit going up and are crossing it again coming down.

Be a scientist — Page 138

Page 15 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Our scientific heritage

BE A SCIENTIST

Q1 What inspired Asima Chatterjee to work on medicinal plants?

India's own long tradition of plant-based medicine. Ayurveda, Siddha and the other Indian
systems had used herbs to treat disease for centuries, and Asima Chatterjee set out to find
which chemical compound in each plant actually does the healing — turning traditional
knowledge into tested chemistry.
Her method is the chemistry of this chapter used as a tool. To pull a useful compound out of a
plant you dissolve the crushed herb in a chosen solvent — water, alcohol, or a hydro-alcoholic
mixture — so that the compound you want passes into the solution while much of the rest of
the plant does not. Choosing the right solvent, and then separating and purifying what
dissolved, is how she isolated compounds and developed anti-epileptic and anti-malarial
drugs.

ACHIEVEMENT WHAT IT WAS

Doctorate of Science The second Indian woman to earn it, after Janaki Ammal

Shanti Swarup Bhatnagar Award First woman to receive it in the field of chemical science

Padma Bhushan Honoured by the nation for her contribution to science

Why the solvent matters: A compound dissolves only in a solvent whose particles
attract it. Using water alone leaves behind compounds that water cannot dissolve;
adding alcohol picks up a different set. By choosing the solvent, a chemist chooses
what comes out of the plant.

In-text Questions — Page 139
9.3 Solubility of Gases

Q1 Do gases also dissolve in water?

Yes. Many gases, including oxygen, dissolve in water — though oxygen dissolves only to a small
extent.

Page 16 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Small as that amount is, everything living in a pond, a river or the sea depends on it. Fishes,
aquatic plants and other water organisms take their oxygen from the water itself, not from the
air above it, and it is this dissolved oxygen that keeps them alive.

Why it happens: Gas particles at the surface of the water are constantly striking it,
and some of them slip in between the water particles and are held there. Because
gas particles are already far apart and are not tightly bound to one another, they
need no tearing apart the way a salt crystal does — but water can hold only a small
number of them, so the solubility of a gas is low.

Did you know? The fizz in a cold drink is carbon dioxide dissolved in water under
pressure. Open the bottle, the pressure drops, and the gas comes out of solution as
bubbles.

Q2 Is the mixture of gases in water a uniform or non-uniform mixture?

It is a uniform mixture — that is, a solution. The gases dissolve evenly through the water; you
cannot see them, they do not float up as visible bubbles, and a sample taken from any depth
has the same dissolved gas in it.

Why it happens: A dissolved gas is present as separate particles fitted into the gaps
between water particles, exactly as dissolved salt is. Particles that small and that well
spread cannot be seen even under a magnifying device, which is the test for a
uniform mixture.

Tip: The bubbles you sometimes see clinging to the inside of a glass of water left
standing are gas that has come out of solution as the water warmed up — they are
not the dissolved gas itself.

Q3 Does temperature affect the solubility of gases in liquids also? If so, how?

Yes, and in the opposite way to solids: the solubility of gases generally decreases as the
temperature increases. Cold water holds more dissolved oxygen; as water warms up, the
dissolved oxygen escapes.

Page 17 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

SOLUTE EFFECT OF RAISING THE TEMPERATURE EXAMPLE FROM THE CHAPTER

Most solids Solubility increases Baking soda in water, 20 °C → 70 °C

Gases Solubility decreases Oxygen in pond water in summer

Why it happens: A dissolved gas particle is only loosely held in the gaps between
water particles. Heating gives it more energy — enough to break free and escape
into the air above. For a solid the heat is used to tear particles off a crystal, which
helps dissolving; for a gas there is nothing to tear apart, so the extra energy only
helps it leave.

Did you know? This is why fish suffer in a shallow pond during a summer afternoon,
and why warm water discharged from a factory into a river can harm aquatic life
even if it carries no pollutant at all.

Q4 I observed that in some non-uniform mixtures, such as sawdust in water, the
sawdust floats, whereas in the mixture of sand and water, the sand sinks. I wonder
why that happens?

Because of density — the mass packed into unit volume. Sawdust is less dense than water, so it
floats; sand is much denser than water, so it sinks.

Density = Mass / Volume

Water ≈ 1 g/cm³ Wood and sawdust: less than 1 g/cm³ Sand: about 2.6 g/cm³

Why it happens: Wood is not solid all through — it is built of tiny hollow cells full of
air, so a piece of wood packs very little mass into its volume. Sand is grains of rock
with no air trapped inside, so the same volume of sand carries far more mass.
Neither one dissolves, so both stay as visible particles and the mixture is non-
uniform in each case; what differs is only whether the particles sit on top or at the
bottom.

Page 18 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

co m
m.
Tip: The book adds an important caution on page 140 — density is not the only

m l a se
factor that decides whether something floats. A steel ship floats although steel is far
o g
.c water, because of the shape it is given and the air itaencloses.
m
denser than

l a se
ag
In-text Questions — Page 140
. com ag
m Is Density?
9.4 Why Do Objects Float or Sink in Water? • 9.5eWhat
s
a
agl
You may have noticed that, while washing rice, husk particles present in the rice
m
Q1

co
m.
float on the surface of water while rice sinks to the bottom of the container. Why
does this happen?
m as e
.co a g l
se m
g l a

a The husk is less dense than water and the rice grain is denser, so the husk stays on the surface
s
while the rice settles.
m a
em
.co agl
a s
Why it happens: Husk is the dry papery outer covering of the grain. It is thin, hollow
glfair volume while carrying almost no mass — its
and traps air, so it occupiesaa
density comes out below 1 g/cm³. The rice grain inside is packed solid with starch

co m
.
and has no air spaces, so the same volume carries much more mass and its density

e m
as
is above 1 g/cm³. Water simply sorts them by density.
m l
m .co a g
a s eDid
a gl you know? Farmers have used this for centuries. Winnowing separates husk
from grain in air for the same reason, and floating off the husk in water is the wet
se m
version of the same trick.
com g l a
m . a
gl ase
a
How do scientists define density?
Q2

co m
m .
m is the mass present in a unit volume of a substance.glas e
. c o a
s e m
Density

a
agl c
Density = Mass / Volume
m .
m a s e
e m . co agl
l as
SI unit: kilogram per cubic metre, kg/m³
g
a
Convenient units for liquids and small solids: g/mL and g/cm³

co m
m .
m ase
.co


a g l Page 19 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Two properties of density are worth fixing in your mind straight away:

It does not depend on the shape or the size of the piece you have. A gram of iron and a
kilogram of iron have exactly the same density.
It does depend on temperature and pressure. Pressure matters mainly for gases; for solids
and liquids its effect is negligible.

Why it is defined this way: Saying "iron is heavier than wood" is loose talk — a nail
is lighter than a log. Fixing the volume at one unit removes the size of the piece from
the comparison and leaves only the property of the material itself. That is what
makes density a fair way to compare an iron rod with a wooden stick of the same
size.

Tip: The chapter's own picture is a good one to remember — a crowded bus is high
density, the same bus with a few passengers is low density. The bus (volume) has not
changed; the number of people (mass) has.

Think like a scientist — Page 141

Page 20 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

9.5 What Is Density?

THINK LIKE A SCIENTIST

Q1 Have you noticed that some packets of ghee or oil are labelled with a volume of 1
litre but a weight of only say 910 grams (Fig. 9.11)? What does this tell us about the
density of the oil, and is it less or more than that of water?

Page 21 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

1 litre (910 g)
Batch No.: 01
Packed on: Jan. 2025

Fig. 9.11, page 141 — redrawn sketch of the packed-oil bottle, with the label as printed
on it.

Page 22 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

It tells us the oil's density directly — and it is less than that of water.

Volume of oil = 1 litre = 1000 mL

Mass of oil = 910 g

Density = Mass / Volume

Density = 910 g ÷ 1000 mL

Density of oil = 0.91 g/mL

Density of water ≈ 1 g/mL, so relative density of oil = 0.91 ÷ 1 = 0.91 (no unit)

0.91 g/mL is less than 1 g/mL, so the same 1 litre of water would weigh about 1000 g while the
oil weighs only 910 g. Since the oil is lighter for the same volume, oil floats on water — exactly
what you see when a drop of oil is added to a glass of water.

Why it happens: The label is not saying the packet is short of oil. It is a full litre; oil
simply packs less mass into each millilitre than water does, because its particles are
bulkier and cannot be as closely packed. Mass and volume are two different
quantities, and it is their ratio — the density — that decides floating and sinking.

Check it yourself: Look at a 1 litre milk packet. It will say about 1030 g, a little more
than a litre of water, because milk carries dissolved sugar, salts and proteins. Milk
therefore sinks in water; oil floats on it.

In-text Questions — Page 143

Page 23 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

9.5.1 Determination of density — How to measure volume?
co m
e m.
m l as
Q1
.co
A tetra pack says it contains 200 mL buttermilk (chach) (Fig. 9.14). What does that
m a g
l a se
mean?

a g

co m
e m . ag
g l as
a

co m
em.
m l as
m .co a g
l a se
a g
m a s
m.co agl
l a se
a g

co m
m .
m as e
.co a g l
se m
g l a
a
se m
com g l a
m . a
ase
agl

co m
m .
m as e
.co a g l
g l a se m 200 mL
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 24 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Fig. 9.14, page 143 — redrawn sketch of the tetra pack of buttermilk, marked 200 mL.

It means the buttermilk inside occupies a space of 200 millilitres. The number on the pack is a
volume, not a mass — it tells you how much room the liquid fills, not how heavy it is.

1 mL = 1 cm³, so 200 mL = 200 cm³

1 L = 1000 mL = 1 dm³, so 200 mL = 0.2 L

Buttermilk is mostly water, and 1 mL of water is close to 1 g,

so 200 mL of buttermilk weighs roughly 200 g

Why volume is stated and not mass: A drink is sold by the space it fills because
that is what the container has to hold and what you actually drink. For a liquid the
two are easy to convert into one another only if you know the density — which is
precisely the quantity this chapter is building towards.

Tip: Volume of solids is usually quoted in cm³ and of liquids in mL, but they are the
same size of unit — 1 mL is exactly 1 cm³. That is why Table 9.2 records the water
displaced in mL and the volume of the stone in cm³.

Q2 How accurately can these measuring cylinders measure?

Only as accurately as their smallest division allows, and that depends on the size of the cylinder
— a bigger cylinder has a coarser scale.

CAPACITY OF THE MEASURING CYLINDER SMALLEST VOLUME IT CAN READ

10 mL or 25 mL 0.1 mL

100 mL 1 mL

250 mL 2 mL

500 mL 5 mL

Page 25 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

So choosing a cylinder is a choice between two faults. To measure 70 mL of water:

a 50 mL cylinder cannot do it in one go — you must measure 50 mL and then 20 mL, and
measuring in two steps is inconvenient and adds error twice;
a 250 mL or 500 mL cylinder does it in one step but reads only to 2 mL or 5 mL, so the
accuracy drops;
a 100 mL cylinder does it in one step and reads to 1 mL — it is the best choice.

Why bigger means coarser: A wider cylinder needs a wider tube to hold the same
height of liquid, so each millilitre poured in raises the level by less. The marks would
come impossibly close together if they were drawn every millilitre, so they are drawn
further apart in volume instead.

Activity 9.4: Let us observe and calculate — Page 143
9.5.1 Determination of density — How to measure volume?

ACTIVITY

Q1 What is the maximum volume it can measure?

100 mL. The cylinder in Fig. 9.16 is marked 100 mL at the top of its scale, and that top mark is
the largest volume it can measure in one filling.

Why it is fixed: The capacity is decided by the length of the graduated part of the
tube. Filling past the last mark tells you nothing, because there are no divisions
above it to read.

Tip: Always check this number before you start. Trying to measure 150 mL in a 100
mL cylinder means two fillings — and two chances to make a reading error.

Q2 How much is the volume difference indicated between the two bigger marks (for
example, between 10 mL and 20 mL)?

10 mL. On the cylinder in Fig. 9.16 the numbered marks run 10, 20, 30 … up to 100, so the step
from one numbered mark to the next — 10 mL to 20 mL, or 40 mL to 50 mL — is 10 mL every
time.

Page 26 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why the marks are evenly spaced: The cylinder has the same width all the way up,
so the same volume of water always raises the level by the same height. That is what
makes a straight, evenly divided scale possible — and it is one more reason the
vessel is made a cylinder and not a cone or a flask.

Q3 How many smaller divisions are there between the two bigger marks?

10. Count the fine lines between 10 mL and 20 mL on Fig. 9.16 — there are ten equal spaces
from one numbered mark to the next.

Tip: Count the spaces, not the lines. Between two numbered marks you will see nine
short lines, which cut the gap into ten spaces. Counting lines instead of spaces is the
commonest mistake in finding a least count.

Q4 How much volume does one small division indicate?

1 mL. Ten millilitres are shared equally among ten divisions.

Volume between two bigger marks = 10 mL

Number of divisions between them = 10

One small division = 10 ÷ 10 = 1 mL

Why this works: This is exactly the method you used for the thermometer in
Curiosity, Grade 6 — take the value of one full step of the numbered scale and divide
it by the number of equal parts that step is cut into. It works for any linear scale: a
ruler, a thermometer or a measuring cylinder.

Page 27 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Q5 The smallest volume that the measuring cylinder can read is__________.

1 mL — for the 100 mL measuring cylinder shown in Fig. 9.16.
This smallest readable value is called the least count of the instrument. It sets the limit of what
the cylinder can tell you: with a least count of 1 mL you can honestly report 47 mL, but not 47.3
mL.

CYLINDER VOLUME BETWEEN BIG MARKS DIVISIONS SMALLEST READING

100 mL (Fig. 9.16) 10 mL 10 1 mL

10 mL or 25 mL 1 mL 10 0.1 mL

250 mL 10 mL 5 2 mL

500 mL 50 mL 10 5 mL

Tip: If the cylinder in your school lab is a different size, do not copy 1 mL. Work out
its own least count from its own scale.

In-text Questions — Page 144
9.5.1 Determination of density — How to measure volume?

Q1 Why are measuring cylinders always designed narrow and tall instead of wider and
short like a beaker?

Because in a narrow tube the same small volume of liquid produces a much larger rise in level,
so the marks can be spaced far enough apart to be read accurately.
For a cylinder, volume = π r² h, so the rise in level for a poured-in volume is

Page 28 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

co m
e m.
h = Volume ÷ (π r²)
m l as
m .co a g
l a se
g
Narrow cylinder, r = 1.5 cm: area = π × 1.5² ≈ 7.1 cm²
a1 mL raises the level by 1 ÷ 7.1 ≈ 0.14 cm = 1.4 mm — easy to see and mark

co m
e m . ag
Wide beaker, r = 4 cm: area = π × 4² ≈ 50.3 cm²
g l as
a
1 mL raises the level by 1 ÷ 50.3 ≈ 0.02 cm = 0.2 mm — far too small to read

co m
em.
m l as
m .co a g
l a se Same volume of water
a g added to each

m a s
em
.co agl
a s
l rise
a gbig

co m
tiny rise
m .
m as e
.co a g l
se m
g l a
a
narrow cylinder wide beaker
se m
com g l a
m . a
e
as millimetres; in the wide one it barely moves, so no useful scale
The same small volume of water is poured into a narrow cylinder and into a wide beaker. In the

g l
narrow vessel the level climbs several
a can be marked on it.

c o m
Why it happens: The rise in level is the volume divided by the m .
s e cross-sectional area.

. om the vessel narrow makes that area small, whichagmakes
cMaking la the rise large for the
a s em same volume. A tall shape is then needed simply to fit the required capacity into a
agl narrow tube.
c
m .
m a s e
m . co
Tip: There is a second gain. A narrow tube gives a small, sharply curved meniscus,
agl
l a se
which is much easier to line your eye up with than the wide, flat surface in a beaker
isgsmaller too.
— so parallax error a

co m
m .
m ase
.co


a g l Page 29 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

In-text Questions — Page 145
Activity 9.5: Let us measure 50 mL of water

Q1 I wonder how the level of a coloured liquid is measured?

For a coloured liquid you read the mark that coincides with the top of the meniscus — the
opposite of what you do for water.

LIQUID WHICH PART OF THE REASON
MENISCUS TO READ

Water and other colourless Bottom of the meniscus The curve is clearly visible through the
liquids transparent liquid

Coloured liquids (ink, potassium Top of the meniscus The colour hides the bottom of the
permanganate solution, milk) curve; only the upper edge can be seen
sharply

In both cases the rule that never changes is: keep your eye exactly level with the surface you
are reading.

Why it happens: A liquid in a narrow tube curves at the surface because it is
attracted to the glass. In a colourless liquid you can look right through and see the
lowest point of that curve. In a coloured liquid, light cannot pass through, so all you
see is a solid band of colour whose sharp upper edge is the only line you can fix on.

Tip: If your eye is above the surface you will read too high, and if it is below you will
read too low. This error, called parallax, is far bigger than the least count of the
cylinder — so bend down and bring your eye to the level of the liquid.

Activity 9.7: Let us measure — Page 146

Page 30 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Determining volume of objects with irregular shapes

ACTIVITY

Q1 Tie the object, say a stone, with a thread and slowly lower it into the measuring
cylinder. What do you notice?

The water level rises — from 50 mL to 55 mL in Fig. 9.19. The stone has pushed aside, or
displaced, exactly its own volume of water, so the rise in level is the volume of the stone.

S.NO. OBJECT INITIAL FINAL WATER VOLUME OF
VOLUME VOLUME DISPLACED THE OBJECT
(ML) (A) (ML) (B) (ML) (B–A) (CM³)

1. Stone 50 mL 55 mL 5 mL 5 cm³

2. Metal key 50 mL 52 mL 2 mL 2 cm³

3. Any other 50 mL 53 mL 3 mL 3 cm³
(glass
marble)

Rows 2 and 3 are sample readings — fill in whatever your own objects give. With the mass from
Activity 9.3 you can now finish the calculation the chapter sets out:

Density = Mass / Volume

Density = 16.400 g ÷ 5 cm³

Density of the stone = 3.28 g/cm³

Why displacement works: Two things cannot occupy the same space at the same
time. When the stone goes in, the water that used to be in that space has to move
somewhere, and the only place it can go is upward. So the extra height of water
holds exactly the same volume as the stone — which is how you find the volume of a
body with no regular shape and no formula.

Check it yourself: Lower the stone gently and keep it fully under water without
letting it touch the sides. If part of the stone stays above the surface, you measure
only the part that is submerged; if water splashes out, your final reading is too low.
The volume of the thread is taken as negligible.

Page 31 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

In-text Questions — Page 147
Let us dig deeper!

Q1 Did you know that our planet, Earth, is composed of several layers, such as crust,
upper mantle, lower mantle, outer core, and inner core, each with its particular
range of density?

Yes — and the layers are arranged in a clear order: the density increases as you go from the
surface towards the centre. The outermost layer, the crust, is the lightest of them all.

LAYER (FROM OUTSIDE INWARDS) DENSITY COMPARED WITH THE OTHERS

Crust Lightest — least dense

Upper mantle Denser than the crust

Lower mantle Denser still

Outer core (liquid) Much denser

Inner core (solid) Densest of all

Why it happens: As you go deeper, both the pressure and the temperature rise
sharply. The enormous pressure of everything lying above squeezes the material
into a smaller volume without changing its mass, and Density = Mass/Volume, so the
density goes up. The deeper layers are also made of heavier materials to begin with.
Compressing matter like this is exactly the effect described in Section 9.5.3 — only
here it is happening on the scale of a planet.

Tip: Notice that temperature and pressure pull in opposite directions here. Heat
alone would expand the material and lower its density; the pressure inside the Earth
is so great that it wins comfortably.

Ever heard of ... — Page 148

Page 32 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

9.5.3 Effect of pressure on density

EVER HEARD OF ...

Q1 Why does ice float on water?

Because ice is less dense than liquid water — the same mass of water takes up more space
after it freezes.
The steps are these:

Water is densest at 4 °C. Cool it below that and it starts behaving unusually.
At 0 °C it turns into ice, and its structure changes — the particles arrange themselves in a
pattern that takes up more space. This is called expansion.
The mass has not changed, but the volume has increased. Since Density = Mass/Volume, the
density falls.
Being lighter for the same volume, ice floats on the water below it.

Why it happens: In almost every other substance, cooling brings the particles closer
and freezing makes the solid denser than its liquid, so the solid sinks. Water is the
exception: when it freezes, its particles lock into an open, roomy arrangement with
gaps built into it, so the solid ends up bulkier than the liquid it came from.

Did you know? Life in cold lakes depends on this exception. Because ice floats, it
forms a lid on the surface and keeps the water beneath warm enough for fish and
other creatures to survive the winter. If ice sank, lakes would freeze solid from the
bottom up.

Think like a scientist — Page 148
9.5.3 Effect of pressure on density

THINK LIKE A SCIENTIST

Q1 What change can you make to this setup to make the egg float in water instead of
sinking?

Dissolve plenty of common salt in the water and stir until no more will dissolve. The egg
that sank in tap water will lift off the bottom and float in the salt solution.

Page 33 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

What to do, step by step:
co m
se m.
m a
Take the egg out and keep the same tumbler of tap water.
o l
g to saturated.
Add salt a cspoonful at a time, stirring after each, until the solution is close
. a
m
sethe egg in gently. It will rise and float with a small part of it above the surface.
l a
Lower
ag
Why it happens: You have not changed the egg at all — you have changed the

. com
liquid. Dissolving salt adds a large amount of mass to the water, while the volume
ag
a s emparticles slip into the gaps between the water
increases only slightly because the salt

agl the same, so Density = Mass/Volume goes up. Tap
particles. Mass up, volume almost
water is a little less dense than a fresh egg, so the egg sinks; once enough salt is

co m
m.
dissolved, the solution becomes denser than the egg and the egg floats.

o m l a se
g plain water very
.c Fill the tumbler half with strong salt solution, then pour
a
m
se down the side so that it rests on top without mixing. Lower the egg in and it
Try this:

g l a
gently
a will come to rest in the middle — floating on the salt solution and sunk in the plain

m
water at the same time. This is also the reason a person floats so easily in the Dead
a s
Sea.
m.co agl
l a se
a g
Keep the curiosity alive — Page 149
com
m .
m as e
.co a g l
se m
g l a
a
se m
com g l a
m . a
ase
agl

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 34 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

End-of-chapter question set (pages 149–151)

KEEP THE CURIOSITY ALIVE

Q1 State whether the statements given below are True [T] or False [F]. Correct the false
statement(s). (i) Oxygen gas is more soluble in hot water rather than in cold water.
(ii) A mixture of sand and water is a solution. (iii) The amount of space occupied by
any object is called its mass. (iv) An unsaturated solution has more solute dissolved
than a saturated solution. (v) The presence of different gases in the atmosphere is
also a uniform mixture.

STATEMENT T/ CORRECTED STATEMENT
F

(i) Oxygen gas is more soluble in hot water False Oxygen gas is more soluble in cold water than in hot
rather than in cold water. water.

(ii) A mixture of sand and water is a False A mixture of sand and water is a non-uniform
solution. mixture, not a solution.

(iii) The amount of space occupied by any False The amount of space occupied by any object is called
object is called its mass. its volume.

(iv) An unsaturated solution has more solute False An unsaturated solution has less solute dissolved
dissolved than a saturated solution. than a saturated solution at the same temperature.

(v) The presence of different gases in the True —
atmosphere is also a uniform mixture.

Page 35 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why each one is what it is:

(i) A dissolved gas particle is loosely held between the water particles. Heating
gives it enough energy to escape, so warm water holds less oxygen — which is
why fish struggle in warm shallow water.
(ii) Sand does not dissolve. Its grains stay visible and settle at the bottom, so the
components are not evenly distributed.
(iii) Mass is the quantity of matter (grams, kilograms); volume is the space
occupied (cm³, mL, m³). Two different quantities — and their ratio is density.
(iv) Saturated means the maximum possible amount has already dissolved.
Unsaturated means the solution is still below that maximum, so it must hold less.
(v) Air is a solution of gases — nitrogen, oxygen, carbon dioxide and others
spread evenly, with no component visible separately.

Q2 Fill in the blanks. (i) The volume of a solid can be measured by the method of
displacement, where the solid is __________ in water and the ____________ in water level
is measured. (ii) The maximum amount of _______________ dissolved in _______________ at a
particular temperature is called solubility at that temperature. (iii) Generally, the
density ____________ with increase in temperature. (iv) The solution in which glucose
has completely dissolved in water, and no more glucose can dissolve at a given
temperature, is called a __________ solution of glucose.

BLANK ANSWER

(i) the solid is _____ in water and the _____ in water level is measured immersed (dipped) … rise (increase)

(ii) The maximum amount of _____ dissolved in _____ at a particular solute … a fixed quantity (100 mL) of
temperature solvent

(iii) Generally, the density _____ with increase in temperature decreases

(iv) … is called a _____ solution of glucose saturated

Page 36 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why these are the answers:

(i) The submerged solid pushes aside its own volume of water, and that water has
nowhere to go but upward — so the rise in level equals the volume of the solid.
(ii) Solubility is a maximum, it is a property of a solute in a particular solvent, and it
is quoted for a fixed quantity of solvent (100 mL) and a stated temperature. Leave
any one of these out and the number means nothing.
(iii) On heating, the particles move apart and the volume increases while the mass
stays the same. Density = Mass/Volume, so the density falls.
(iv) "Completely dissolved and no more can dissolve at that temperature" is the
definition of a saturated solution.

Q3 You pour oil into a glass containing some water. The oil floats on top. What does
this tell you? (i) Oil is denser than water (ii) Water is denser than oil (iii) Oil and
water have the same density (iv) Oil dissolves in water

(ii) Water is denser than oil.

Density of water ≈ 1 g/mL

Density of cooking oil ≈ 0.91 g/mL (from the 1 litre / 910 g packet on page 141)

0.91 g/mL < 1 g/mL, so oil rises above water

Why the other options fail:

(i) If oil were denser it would sink and settle below the water, which is not what happens.
(iii) If the densities were equal there would be no reason for one to rise above the other;
they would simply stay wherever they were put.
(iv) Oil does not dissolve in water at all — that is why you can still see two separate layers. If
it dissolved you would get one clear uniform liquid.

Why it happens: When two liquids that do not mix are put together, the one that
packs less mass into each millilitre ends up on top. Water particles attract one
another strongly and pull together, squeezing the oil layer above them.

Page 37 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Q4 A stone sculpture weighs 225 g and has a volume of 90 cm3. Calculate its density
and predict whether it will float or sink in water.

Mass = 225 g, Volume = 90 cm³

Density = Mass / Volume

Density = 225 g ÷ 90 cm³

Density = 2.5 g/cm³

Density of water = 1 g/cm³

2.5 g/cm³ > 1 g/cm³ → the sculpture will sink

Its relative density with respect to water is 2.5 ÷ 1 = 2.5, a number with no unit — the sculpture
is two and a half times as dense as water.

Why it sinks: Take 90 cm³ of water and it weighs 90 g; the same 90 cm³ of this stone
weighs 225 g. For the space it occupies the stone is far too heavy, so it cannot stay
up in the water.

Q5 Which one of the following is the most appropriate statement, and why are the
other statements not appropriate? (i) A saturated solution can still dissolve more
solute at a given temperature. (ii) An unsaturated solution has dissolved the
maximum amount of solute possible at a given temperature. (iii) No more solute
can be dissolved into the saturated solution at that temperature. (iv) A saturated
solution forms only at high temperatures.

The most appropriate statement is (iii) No more solute can be dissolved into the saturated
solution at that temperature. That is exactly the definition of saturation.

Page 38 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

co m
m.
STATEMENT WHY IT IS NOT APPROPRIATE

ase
com
dissolve more .solute a g l
(i) A saturated solution can still This describes an unsaturated solution. A saturated one has already reached

a s em its limit; extra solute settles at the bottom.

a(ii)glAn unsaturated solution has The two words have been swapped. Holding the maximum is what makes a
dissolved the maximum amount solution saturated; unsaturated means it is still below the maximum.

co m
. ag
possible

se m
(iv) A saturated solution forms only
l a
lessgsolute, hot water with more — Activity 9.1 makes a saturated salt
A solution can be saturated at any temperature. Cold water saturates with
at high temperatures
a
solution at room temperature.

co m
se m.
m l a
Why (iii) is worded carefully: Notice the phrase "at that temperature". Saturation is
o g liquid becomes
m .c
not a permanent a
state of a solution — heat it and the very same

l a se
g
unsaturated, because its limit has moved up. Any statement about saturation that
a leaves out the temperature is incomplete.

m a s
m.co agl
l a se
Q6
a g
You have a bottle with a volume of 2 litres. You pour 500 mL of water into it. How
much more water can the bottle hold?

co m
m .
e

m l as
m .co a g
l a se Capacity of the bottle = 2 L

ag 1 L = 1000 mL, so 2 L = 2 × 1000 = 2000 mL

se m
com g l a
m . a
ase
Water already poured in = 500 mL

agl
Space still empty = 2000 mL − 500 mL
co m
m .
Space still empty = 1500 mL = 1.5 L
m as e
.co a g l
se m
g l a
a Why you must convert first: You cannot subtract 500 from 2 — the two numbers
c
are in different units. Bring both to the same unit (millilitres here) and the
m .
m a s e
co agl
subtraction becomes meaningful. This is the first thing to check in any volume

m .
e
problem.

g l as
a

co m
m .
m as e
.co


a g l Page 39 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Tip: 1500 mL is also 1500 cm³, since 1 mL = 1 cm³. And since water has a density
close to 1 g/mL, that much water would weigh about 1500 g, or 1.5 kg.

Q7 An object has a mass of 400 g and a volume of 40 cm³. What is its density?

Mass = 400 g, Volume = 40 cm³

Density = Mass / Volume

Density = 400 g ÷ 40 cm³

Density = 10 g/cm³

Its relative density with respect to water is 10 ÷ 1 = 10. The object is ten times as dense as water,
so it will sink in water.

What the number means: Every single cubic centimetre of this object carries 10 g
of matter, while 1 cm³ of water carries only about 1 g. Densities as high as this
belong to heavy metals — lead is about 11.3 g/cm³ and silver about 10.5 g/cm³.

Page 40 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Q8 Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled
one sinks? Explain.

(a) (b)

Fig. 9.25 (a) and (b), page 150 — redrawn sketch: (a) an unpeeled orange in a glass of
water, (b) a peeled orange in a glass of water.

The peel is full of tiny air pockets. It adds a great deal of volume to the orange while adding very
little mass, so the whole unpeeled fruit has an average density below 1 g/cm³ and floats (Fig.
9.25a). Remove the peel and you throw away that light, air-filled layer; what is left is dense juicy
pulp with a density a little above 1 g/cm³, so it sinks (Fig. 9.25b).

UNPEELED ORANGE (FIG. PEELED ORANGE (FIG.
9.25A) 9.25B)

Mass A little more (peel included) A little less

Volume Much more — the thick spongy peel Much less

Average density = Less than 1 g/cm³ More than 1 g/cm³
Mass/Volume

Result in water Floats Sinks

Page 41 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Why it happens: This looks like a paradox — removing mass makes the orange sink
— but density is a ratio. Peeling removes a little mass and a lot of volume, and
taking away more from the bottom of the fraction than from the top makes the ratio
bigger. The air trapped in the peel is doing exactly what the hollow cells do for
sawdust and what a hollow bamboo pole does for a raft.

Try this: Do it yourself with two oranges of the same size, one peeled and one not.
Then remember the chapter's caution on page 140 — density is not the only factor
that decides floating, but in this pair it is the deciding one.

Q9 Object A has a mass of 200 g and a volume of 40 cm³. Object B has a mass of 240 g
and a volume of 60 cm³. Which object is denser?

Object A: Density = 200 g ÷ 40 cm³ = 5 g/cm³

Object B: Density = 240 g ÷ 60 cm³ = 4 g/cm³

5 g/cm³ > 4 g/cm³ → Object A is denser

Why the heavier object is not the denser one: B has the greater mass — 240 g
against 200 g — yet it is the less dense of the two, because that mass is spread
through 60 cm³ instead of 40 cm³. Density asks a different question from mass: not
"how much matter is there?" but "how much matter is packed into each unit of
space?" To compare fairly you must always divide by the volume.

Check it yourself: Bring both to the same volume. In 120 cm³ you would have 600 g
of A but only 480 g of B — the same conclusion, reached without dividing.

Page 42 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Q10 Reema has a piece of modeling clay that weighs 120 g. She first moulds it into a
compact cube that has a volume of 60 cm3. Later, she flattens it into a thin sheet.
Predict what will happen to its density.

Nothing at all — the density stays exactly the same, 2 g/cm³.

As a cube: Density = 120 g ÷ 60 cm³ = 2 g/cm³

Flattened into a sheet:

Mass = still 120 g (no clay has been added or removed)

Volume = still 60 cm³ (the clay has only been rearranged, not squeezed or stretched)

Density = 120 g ÷ 60 cm³ = 2 g/cm³

Why it happens: Flattening changes the shape, not the amount of matter and not
the space it occupies. As the chapter states, the density of a substance is
independent of its shape or size; it depends only on temperature and pressure.
Since neither of those has changed, neither does the density.

Tip: A thin sheet of clay may well float on water while the cube sinks — but that is
not a change of density. It is a change of shape, and it is exactly the reason a steel
ship floats though a steel nail does not.

Q11 A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?

Rearrange the density formula to make volume the subject.

Density = Mass / Volume → Volume = Mass / Density

Volume = 600 g ÷ 7.9 g/cm³

Volume = 75.95 cm³ (about 76 cm³)

Page 43 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Check it the other way round: 75.95 cm³ × 7.9 g/cm³ = 600.0 g. ✔
co m
e m.
m l as
.co a g
Why the answer is so small: 600 g of water would fill 600 cm³ — nearly half a large

s em
water bottle. The same 600 g of iron fills less than 76 cm³, a lump roughly 4 cm on a
a
gl because iron packs 7.9 g into every cubic centimetre. That is what a high
aside,
density means in practice.

co m
m . ag
l a se
Tip: Watch the units as you divide: g ÷ (g/cm³) = cm³. If your units do not come out as
ag upside down.
a volume, you have used the formula

co m
em.
m l as
m .co a g
l a se
a g
m a s
m.co agl
l a se
a g

co m
m .
m as e
.co a g l
se m
g l a
a
se m
com g l a
m . a
ase
agl

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 44 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Q12 You are provided with an experimental setup as shown in Fig. 9.26a and 9.26b. On
keeping the test tube (Fig 9.26b) in a beaker containing hot water (~70 °C), the
water level in the glass tube rises. How does it affect the density?

Glass tube

Test tube

Water

Test tube stand

(a)

Water level

Test tube

Beaker containing hot
water

(b)

Fig. 9.26 (a) and (b), page 151 — redrawn sketch of the setup: (a) a corked test tube of
water with a glass tube through the cork, standing in a test-tube stand; (b) the same
test tube clamped to a stand and lowered into a beaker of hot water.

The density of the water decreases. The rise in the glass tube is the visible sign that the water
has expanded.

Page 45 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Mass of water in the test tube: unchanged — the tube is closed by the cork, nothing gets in

or out

Volume of that water: increases — which is why the level climbs up the glass tube

Density = Mass / Volume

Same mass ÷ larger volume = smaller density

Why it happens: Heating gives the water particles more energy, so they move faster
and push a little further apart from one another. The same number of particles —
the same mass — now needs more room, and the narrow glass tube makes even a
small expansion easy to see as a tall rise in level. This is the general rule of Section
9.5.2: the density of a substance decreases on heating and increases on cooling.

Did you know? The same effect explains why hot air rises and how a hot-air balloon
lifts off (Fig. 9.22), and it is also the working principle of the liquid thermometer you
used in Activity 9.2 — the liquid in the bulb expands up a narrow bore exactly as the
water does here.

Discover, design, and debate — Page 151
Science • Society • Interdisciplinary Projects

DISCOVER, DESIGN, AND DEBATE PROJECTS

Q1 Research project on Dead Sea: Why is there no aquatic life in the Dead Sea? Try to
find out if there are any other similar water bodies.

How to do the project: collect the salt content, the density and the temperature of the Dead
Sea from an atlas or an encyclopaedia, compare them with ordinary sea water, and then explain
the biology from the chemistry. Present it as a one-page report with a comparison table and a
map.
Sample answer:
The Dead Sea is not really a sea but a landlocked lake between Israel and Jordan. Rivers bring
dissolved salts into it, but it has no outlet, and the desert sun evaporates the water fast.
Evaporation removes the solvent and leaves the solute behind, so over thousands of years the

Page 46 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

water has become an extremely concentrated salt solution — close to saturated, and roughly
ten times as salty as ordinary sea water.
Almost nothing can live in it, for three reasons:

The solution is too concentrated for living cells. A fish or a plant placed in it loses water
from its cells into the surrounding solution and cannot survive.
Very little oxygen dissolves in it. A liquid already crowded with dissolved salt has little
room left for dissolved gas, and the water is warm — and warm water holds less oxygen
anyway.
No plants can grow in it, so there is no food chain to support animals.

Only some microbes survive there, which is why the lake is called "dead". Its very high density —
noticeably greater than that of the human body — is also why a swimmer floats on it without
effort.

WATER WHERE WHY IT IS SIMILAR
BODY

Great Salt Lake Utah, USA Landlocked, no outlet, very high salt content; only brine shrimp and
algae survive

Lake Assal Djibouti, Africa One of the saltiest lakes on Earth; salt is mined from its shores

Sambhar Salt Rajasthan, India India's largest inland salt lake; salt is produced by evaporating its
Lake water

Lonar Lake Maharashtra, A crater lake, both salty and alkaline; very few organisms live in it
India

Pangong Tso Ladakh, India A high-altitude salt lake with almost no fish

The common cause: every one of these is a closed basin — water flows in, carrying
dissolved salts, but leaves only by evaporation, which carries no salt away. The
concentration can therefore only go up.

Q2 Investigate how well common salt dissolves in different solvents, such as water,
vinegar, and oil. Compare the solubility of salt in each solvent and record your
observations.

Method — keep everything the same except the solvent. Take three clean transparent
glasses and put 50 mL of water in the first, 50 mL of vinegar in the second and 50 mL of cooking
oil in the third, all at room temperature. Add one level teaspoon of common salt to each, stir for

Page 47 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

the same length of time with a clean spoon, and observe. Keep adding spoonfuls until salt stops
dissolving, counting as you go. Record everything in a table.

SOLVENT WHAT YOU OBSERVE ROUGHLY HOW CONCLUSION
(50 ML) MANY SPOONS
DISSOLVE

Water Salt disappears; liquid stays 3 to 4 Dissolves very well
clear

Vinegar Salt disappears, slightly more About 3 Dissolves well —
slowly vinegar is mostly water

Cooking oil Grains stay whole and settle None Does not dissolve at all
at the bottom, however long
you stir

Why the results come out this way: Salt dissolves only when the solvent's particles
can pull the salt particles off the crystal and hold them apart. Water particles can do
this. Vinegar is a solution of acetic acid in water, so it behaves almost like water,
though the dissolved acid takes up a little of the water's capacity. Oil particles have
no such pull at all, so the crystal is never broken up — the salt simply lies in the oil
unchanged.

Check it yourself: Use the same spoon, level it off each time, and stir for the same
count in every glass. If you change two things at once you cannot say which one
caused the difference — that is the whole point of a fair test.

Q3 Debate in class — Is water truly the most versatile solvent?

How to run the debate: divide the class into two sides, give each ten minutes to collect
examples, and insist that every point be supported by an example rather than an opinion. Judge
on evidence, not on volume.

Page 48 of 51

as e
a g l
Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

co m
m.
YES — WATER IS THE MOST VERSATILE NO — IT IS NOT THE MOST VERSATILE
SOLVENT
m as e
. c o a g l
s em
It dissolves an enormous range of substances — salts,
a
It cannot dissolve oils, fats, wax, plastics, or most paints

gl and carbon dioxide.
aoxygen
sugars, ORS powder, many medicines, and gases such as and varnishes — which is why oil paint has to be thinned
with turpentine, not water.

om
— sap in plants, blood in animals, digested food in your .c alcohol or with a hydro-alcoholic mixture, as the
It carries dissolved substances through every living thing Some drug compounds come out of a herb only with
ag
a s em chapter's own heritage box notes.
agl
intestine.

It is cheap, plentiful, non-poisonous and does not catch Traditional Indian medicine also uses oil, ghee and milk
fire — an industrial solvent that is also safe to drink.
co
as solvents where water will not do the job.
m
se m.
m l a
.co best solvent is the one thatgdissolves the substance you
Life itself began and still runs in water because of this. "Most versatile" cannot be settled in the abstract — the

m a
l a se actually need to dissolve.

a g
The strongest position: Water is rightly called the universal solvent, and no other single liquid

om a s
not a fact — grease on a plate needs soap,.c agl
dissolves anything like the same range of everyday substances. But "universal" is a compliment,

s m
eversatile
oil paint needs turpentine, and a chemist chooses a

g l a
solvent to fit the job. Water is the most solvent we have, and it is still not enough on its
own. a

co m
Tip: The best debating point on either side is a concrete example the other side
m .
m as e
.co
cannot explain away. Bring one from your own kitchen.
a g l
se m
g l a
a
se m
com g l a
m . a
ase
Chapter at a glance

a gl
A uniform mixture — one in which the components are spread evenly and cannot be
picked out — is a solution. Salt or sugar in water is a solution; chalk powder, sand or
sawdust in water is a non-uniform mixture.
co m
m .
se
When a solid dissolves in a liquid, the solid is the solute and the liquid is the solvent. When

o m l a
g is unsaturated; once the
.cA solution that can still dissolve more solute at that temperature
two liquids mix, the one present in the smaller amount is the solute.

e m a
a s
agl
solute stops dissolving and settles at the bottom, the solution is saturated at that
temperature. The amount of solute in a fixed quantity of solution is its concentration
.c
s e m
m a
agl
(dilute or concentrated); the maximum that dissolves in a fixed quantity (100 mL) of solvent

. co
m
is its solubility.

l a se
Raising the temperature generally increases the solubility of solids in liquids — a saturated
ag
solution becomes unsaturated on heating — but decreases the solubility of gases. That is
why cold water holds more dissolved oxygen for aquatic life.

co m
m .
m as e
.co


a g l Page 49 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Density = Mass / Volume. Its SI unit is kg/m³; for liquids and small solids g/mL and g/cm³
are convenient. Relative density compares a substance with water and has no unit. Density
does not depend on the shape or size of a piece, only on temperature and pressure.
Mass is measured with a balance, liquid volume with a measuring cylinder (read the
bottom of the meniscus), regular solids by l × w × h, and irregular solids by the
displacement of water.
Heating a substance spreads its particles apart: volume rises, mass does not, so density falls
— hot air rises, and a hot-air balloon flies. Pressure squeezes gases strongly, liquids slightly
and solids almost not at all. Water is odd: it is densest at 4 °C, and on freezing its particles
take an open arrangement, so ice is less dense and floats.

Page 50 of 51

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents, and Solutions AglaSem · NCERT Solutions

Quick revision

TERM WHAT IT MEANS EXAMPLE FROM THE VALUE /
CHAPTER UNIT

Solution A uniform mixture whose components Salt and sugar in water (ORS); —
cannot be seen separately air; sugar syrup of gulab
jamun

Solute / Solvent The dissolved substance / the substance Sugar (solute) in water —
that dissolves it; between two liquids, the (solvent) in chashni
smaller amount is the solute

Saturated solution No more solute can dissolve in it at that Salt added spoon by spoon in —
temperature; extra solute settles at the Activity 9.1
bottom

Unsaturated More solute can still be dissolved at that The same salt solution after —
solution temperature one or two spoons

Concentration Amount of solute in a fixed quantity of 4 spoons in 50 mL is 4 times spoons per
solution — dilute or concentrated as concentrated as 2 spoons 100 mL
in 100 mL

Solubility Maximum solute that dissolves in a fixed Baking soda: more dissolves g per 100
quantity (100 mL) of solvent at a stated at 70 °C than at 50 °C or 20 °C mL
temperature

Mass The quantity of matter in an object; Stone weighed on a digital 16.400 g
measured on a balance balance

Weight The force with which the Earth pulls an Most balances really measure newton (N)
object — not the same as mass weight but are marked in
mass units

Volume The space an object occupies Notebook 25 cm × 18 cm × 2 900 cm³; 5
cm; stone by displacement cm³

Least count of a The smallest volume it can read 100 mL cylinder: 10 mL 1 mL
measuring cylinder between marks ÷ 10 divisions

Density Mass present in unit volume, Mass / Aluminium block 27 g in 10 2.7 g/cm³;
Volume cm³; stone 16.400 g in 5 cm³ 3.28 g/cm³

Relative density Density of a substance ÷ density of water Aluminium with respect to 2.7 (no
at that temperature water unit)

Page 51 of 51