NCERT Solutions Class 9 Science Chapter 5 Exploring Mixtures and Their Separation

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 9 · SCIENCE

NCERT Solutions

Chapter 5: Exploring Mixtures
and their Separation

NCERT Textbook — Exploration

BOOK PAGES SECTIONS QUESTIONS MEDIUM

72 – 93 36 81 English

Solutions, notes, sample papers & more at 80 pages

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

CLASS 9 · SCIENCE · EXPLORATION

NCERT Solutions — Chapter 5: Exploring Mixtures and
their Separation
Sugar from sugarcane, salt from seawater, plasma from a drop of blood — every one of them is a mixture
being taken apart. This chapter sorts mixtures into solutions, suspensions and colloids, learns to state how
much solute a solution holds, and then builds the separation toolkit: crystallization, distillation,
chromatography, the separating funnel, sublimation, centrifugation and coagulation.

TEXTBOOK BOOK PAGES

Exploration (Class 9) 72 – 93

SECTIONS QUESTIONS

36 81

MEDIUM

English

Think It Over — Page 72
Chapter opener

THINK IT OVER

Q1 Why do suspended particles settle in muddy water over time but not in milk?

Because the two mixtures hold particles of very different size. Muddy water is a suspension;
milk is a colloid.

Mud particles: diameter > 1000 nm

Milk (fat and protein) particles: diameter 1 – 1000 nm

Page 1 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why it happens: Two effects fight each other inside the liquid. Gravity pulls a
particle down, and the ceaseless random bombardment by water molecules knocks
it about and keeps it up. The downward pull grows with the volume of the particle
(as the cube of its radius), while the jostling depends on its surface. So a big mud
particle is dragged down far faster than the molecules can keep it afloat — it
sediments in minutes. A milk particle is a thousand times smaller; the molecular
bombardment is enough to keep it permanently dispersed. The protein molecules in
milk also coat each fat globule and stop the globules from joining into bigger,
heavier ones.

Check it yourself: Leave a glass of muddy water and a glass of milk side by side
overnight. The mud makes a clear layer at the bottom; the milk stays uniform. That
single observation is enough to call one a suspension and the other a colloid.

Q2 How is evaporation different from boiling?

Evaporation is a surface change that goes on at every temperature; boiling is a bulk change that
happens only at one fixed temperature.

POINT OF EVAPORATION BOILING
COMPARISON

Where it happens Only at the free surface of the liquid Throughout the liquid — bubbles
form inside it

Temperature At all temperatures below the boiling Only at the boiling point (100 °C
point for water at normal pressure)

Speed Slow and quiet Fast and vigorous

Energy source Draws heat from the liquid itself and the Heat must be supplied
surroundings, so the liquid cools continuously from outside

Everyday example Wet clothes drying, sweat cooling you Water bubbling in a kettle

Page 2 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why it happens: In a liquid, molecules move with a spread of speeds. At any
temperature a few surface molecules happen to be fast enough to break free — that
is evaporation, and because only the fastest leave, the liquid left behind cools down.
Boiling begins when the temperature is high enough that the vapour pressure inside
the liquid equals the atmospheric pressure pushing down on it. Only then can a
bubble of vapour form inside the liquid without being crushed, and the whole bulk
turns to vapour.

Q3 Why do you see bright rays of sunlight when it passes through small gaps between
the leaves of a dense tree?

Because the air under the tree carries dust, smoke and tiny water droplets, and these particles
scatter the sunlight sideways into your eye. This is the Tyndall effect.

Why it happens: Light travels in a straight line, so a beam moving away from you
cannot be seen at all — you only see light that actually enters your eye. When the
beam runs through clean air nothing turns it aside. But when it meets suspended
particles of about the size of the wavelength of light, each particle scatters a little of
the beam in all directions, and some of that scattered light reaches you. You then
see the path of the beam itself, standing out as a bright ray against the dark shade
of the tree.

Did you know? The same effect makes the beams from stadium floodlights visible
(Fig. 5.24), shows up the path of a cinema projector in a dusty hall, and is exactly
what you see when you shine a laser through milk in Activity 5.1.

In-text Questions — Page 73
5.1 How Can We Classify Mixtures?

Q1 Is the mixture of oil and water homogeneous or heterogeneous?

Heterogeneous. Oil and water are immiscible — they refuse to mix, and however hard you stir
they settle back into two separate layers with a visible boundary between them.

Page 3 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

co m
m.
Why it happens: A homogeneous mixture must have the same composition in every

m l a se
part. Draw a drop from the top of an oil–water mixture and you get oil; draw one
o
c
from the .bottom a g everywhere, so
m
and you get water. The composition is not the same
se is heterogeneous. At the particle level, water molecules attract one
theamixture
l
g
aanother strongly and oil molecules do not join in, so the two sets of molecules stay
apart instead of spreading through each other.
co m
em . ag
g l as
Q2
a
Can you think of some other heterogeneous mixtures?

co m
se m.
a

o m g l
Yes — anycmixture in which you can point to more than one part is heterogeneous.
m . a
l a se in liquid: sand in water, chalk powder in water, muddy water, tea leaves in water.
ag Liquid in liquid: mustard oil and water, kerosene and water.
Solid

m a s
agl
Solid in solid: iron filings and sulfur, sand and common salt, a handful of mixed dal.

m .co
se
Solid or liquid in gas: smoke (solid particles in air), fog and clouds (water droplets in air),
dust in air.
g l a
a
Tip: Alloys such as brass and bronze look like solid–solid mixtures but are
com
homogeneous — the metals were melted together, so they are mixed right down to
m .
m as e
l
the level of individual atoms.

m .co a g
l a se
ag
Activity 5.1: Let us experiment — Group activity — Page 73
se m
com g l a
. a
5.1 How Can We Classify Mixtures?
m
ase
agl
ACTIVITY

Q1 Are the particles visible in each mixture? Record your observations.
co m
m .
m as e
.co
a g l
a s emOnly in beaker B. The three mixtures are salt in water (A), chalk powder in water (B) and a few
agl drops of milk in water (C).
.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 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

BEAKER MIXTURE ARE PARTICLES APPEARANCE
VISIBLE?

A Common salt + 50 mL No Clear and transparent — you can read
water print through it

B Chalk powder + 50 mL Yes Cloudy; white specks clearly seen
water swirling in the water

C A few drops of milk + No Uniformly milky-white, but no
50 mL water separate specks are seen

Why it happens: Your eye can only resolve particles bigger than about 1000 nm. Salt
has broken up into separate ions smaller than 1 nm, so beaker A looks like pure
water. Chalk particles are far bigger than 1000 nm and scatter enough light to be
seen individually. Milk particles lie in between (1 – 1000 nm), so no single particle can
be picked out, yet together they make the liquid look white.

Q2 Direct the light from a laser pointer through the beakers containing the mixtures
(Fig. 5.3) and observe it from the side of the beaker in a direction perpendicular to
the laser beam. Record your observations.

A B C
laser pointer

(a) salt and water (b) chalk powder and water (c) milk and water

Fig. 5.3, page 73 — redrawn sketch of the set-up: a laser beam sent through each of the
three beakers A, B and C and viewed from the side.

The path of the beam is invisible in A, brightly visible in B and visible in C.

Page 5 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

A laser beam passed through the three mixtures of Activity 5.1
violet block = laser pointer

Solution (A) Suspension (B) Colloid (C)
particles < 1 nm particles > 1000 nm particles 1 – 1000 nm
no visible path bright path; settles path visible; no settling

A laser beam through the three mixtures of Activity 5.1 — invisible in the solution, bright in the
suspension, visible in the colloid.

BEAKER PATH OF THE LASER BEAM SEEN WHAT IT TELLS YOU
FROM THE SIDE

A — salt and Not visible A true solution; particles are far too small
water to scatter light

B — chalk and Bright, sharply visible A suspension; large particles scatter light
water strongly

C — milk and Visible A colloid; the mixture looks uniform yet
water still scatters light

Why it happens: You see a beam only if some light is turned sideways towards your
eye. Particles much smaller than the wavelength of light (about 400 – 700 nm) barely
disturb the wave, so a solution lets the beam pass unseen. Particles comparable to
or larger than the wavelength scatter it in every direction, so the beam path lights
up. This is the Tyndall effect, and it is the neatest way to tell a colloid from a solution
when both look clear.

Safety first: Never look straight into the laser beam. Observe only from the side, at
right angles to the beam.

Page 6 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Q3 Predict what you would observe in each of the beakers if you leave them
undisturbed for a few minutes.

Beaker A (salt and water): no change at all. The solution stays clear and uniformly salty
from top to bottom.
Beaker B (chalk and water): the chalk settles as a white layer at the bottom, leaving clearer
water above. This settling is called sedimentation.
Beaker C (milk and water): no change. It stays evenly milky; nothing settles even after
hours.

Why it happens: Settling needs the weight of a particle to beat the random
molecular jostling that keeps it suspended. Chalk particles (> 1000 nm) are heavy
enough, so they lose and fall. Salt exists as separate ions and milk particles are only
1 – 1000 nm across, so in both A and C the jostling wins and nothing settles.

Q4 Set up a filtration apparatus and filter each mixture separately. Is there any residue
left on the filter paper?

Only beaker B leaves a residue.

BEAKER RESIDUE ON THE FILTER FILTRATE
PAPER

A — salt and water None Clear salt solution (still salty — the salt passes
through)

B — chalk and White chalk residue Clear water
water

C — milk and None Milky liquid — unchanged
water

Why it happens: Filter paper is a sieve with pores of roughly 1000 nm. Anything
bigger is held back; anything smaller goes through with the liquid. Chalk particles
are larger than the pores, so they stay behind. Dissolved salt ions and colloidal milk
particles slip straight through, which is why filtration cannot separate a solution
or a colloid.

Page 7 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Q5 Based on your observations, do you think these are the same types of mixtures or
are they different?

They are three different types of mixture. Put the four tests side by side and each beaker gives
its own pattern of answers.

TEST A — SALT + WATER B — CHALK + WATER C — MILK + WATER

Particles visible? No Yes No

Tyndall effect? No Yes Yes

Settles on standing? No Yes No

Residue on filtering? No Yes No

Type of mixture Solution (homogeneous) Suspension (heterogeneous) Colloid (heterogeneous)

Why it happens: One property — the size of the dispersed particles — decides
every one of these results. Below 1 nm you get a solution, between 1 nm and 1000
nm a colloid, above 1000 nm a suspension. The colloid is the interesting middle case:
it looks homogeneous like A but scatters light like B.

In-text Questions — Page 74
5.2 Solutions; 5.2.1 Concentration of a solution

Q1 In what proportion are a solute and a solvent present in a solution? Can these be
expressed quantitatively?

There is no single fixed proportion — a solution can be dilute or concentrated. But the
proportion actually used can be stated exactly, and that number is called the concentration of
the solution.

Concentration = amount of solute present in a given amount of solvent or of solution

At Class 9 level it is written as a percentage in one of three ways:

Page 8 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

co m
em.
% m/m = (mass of solute ÷ mass of solution) × 100
m l as
.co
% m/v = (mass of solute ÷ volume of solution) × 100
m a g
l a se
g
% v/v = (volume of solute ÷ volume of solution) × 100
a

. com
Why it matters: Saying “salt in water” is not enough for ORS, for a saline drip or for
ag
a s em proportion. A hospital saline drip must be
a pesticide spray — each needs a definite

a l 0.9 g of salt in every 100 mL of solution, because
0.9 % m/v sodium chloride, thatgis
that is the concentration at which the solution matches blood and does not damage
the blood cells.
co m
e m.
m l as
.co a g
a s em
gl
If they do not do so, what is likely to happen?
a
Q2

m a s
.co agl

The spray will fail in one of two ways.
a s em
agl
Too little pesticide: the concentration is below what the pest can withstand, so the insects
survive and the crop is not protected. Repeated weak doses can even help resistant pests to
multiply.
. c om
s e mis washed into ponds
Too much pesticide: the excess scorches the crop, builds up in the soil,
and o
. c rivers, a glawho eat it.
m and leaves residues on the grain that reach the people
em
as Why it happens: A pesticide acts through the number of its molecules that reach
a g l
each pest. That number depends on concentration, not on the total volume sprayed.
se m
com g l a
.
Diluting the same 1 mL of pesticide into 100 mL of water instead of 10 mL cuts the
m a
ase
concentration to one-tenth, and the effect falls with it. This is exactly the reasoning

agl
behind Example 5.3, where 1 mL of pesticide in 100 mL of spray is a 1 % v/v solution.

co m
m .
as e
com l
Can you think of other such examples?
g
Q3

m . a
e
as ANSWER
agl c
Yes — almost every prepared solution has a proportion that must be respected.
m .
m a s e
e m . co agl
g l as
a

com
m .
m ase
.co


a g l Page 9 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

SITUATION WHY THE EXACT PROPORTION MATTERS

ORS for a child with diarrhoea The salt and sugar must match the concentration of body fluids, or
water will not be absorbed properly

Saline drip in a hospital 0.9 % m/v NaCl matches blood; stronger or weaker damages the
blood cells

Bleaching solution for cleaning a Too weak does not disinfect; too strong corrodes the surface and
floor irritates the skin

Sugar syrup for jam or a cold drink Decides taste and how long the food can be preserved

Fluoride in toothpaste, chlorine in A useful amount protects; more than that is harmful
drinking water

Sugar and milk in a cup of tea Same ingredients, different proportion, completely different taste

Pause and Ponder — Page 76
5.2.2 How do we express concentration?

PAUSE AND PONDER

Q1 A common talcum powder contains 4 % m/m zinc oxide, which acts as an antiseptic.
How much zinc oxide is present in 300 g of the talcum powder?

12 g of zinc oxide.

% m/m = (mass of solute ÷ mass of mixture) × 100

4 = (mass of ZnO ÷ 300 g) × 100

mass of ZnO = (4 × 300 g) ÷ 100

mass of ZnO = 12 g

Why it works: “4 % m/m” is a statement about every 100 g of the powder: each 100 g
contains 4 g of zinc oxide. 300 g is three such lots, so it contains 3 × 4 g = 12 g. The
remaining 300 g − 12 g = 288 g is talc and the other ingredients.

Page 10 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Tip: Notice that mass by mass percentage is used here even though talcum powder
is a heterogeneous solid mixture, not a solution. The same formula labels milk
powder, spice mixes and packaged foods.

Q2 Your mother gives you a bottle of orange juice concentrate to mix with water and
serve it to your visiting friends. She asks you to mix two tablespoons of the
concentrate with water in a glass tumbler. If each tablespoon measures 15 mL and
you make 150 mL of juice per person, what is the % v/v of orange juice concentrate
in the mixture you prepared?

20 % v/v.

Volume of concentrate (solute) = 2 × 15 mL = 30 mL

Volume of juice (solution) = 150 mL

% v/v = (volume of solute ÷ volume of solution) × 100

% v/v = (30 mL ÷ 150 mL) × 100

% v/v = 20 % v/v

Why v/v and not m/m here: Both the concentrate and the water are liquids, and in
the kitchen it is far easier to measure a liquid by volume (a tablespoon, a measuring
cup) than to weigh it. That is exactly the situation the volume by volume percentage
was designed for — the same reason vinegar is labelled 5 % v/v acetic acid.

Check it yourself: The water added is 150 mL − 30 mL = 120 mL, so the concentrate
and water are in the ratio 30 : 120 = 1 : 4. One part concentrate in five parts of juice is
one-fifth, and one-fifth of 100 is 20 — the same answer.

Page 11 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Q3 Vinegar, used as a food preservative and additive, contains 5 % v/v acetic acid.
Glacial acetic acid is a liquid, i.e., 100% acetic acid. If you want to make vinegar
from glacial acetic acid, how would you proceed?

Measure 5 mL of glacial acetic acid and add water to it until the total volume of the solution is
100 mL. That gives 5 % v/v vinegar.

Required: % v/v = 5, volume of solution = 100 mL
5 = (volume of acetic acid ÷ 100 mL) × 100

volume of acetic acid = (5 × 100 mL) ÷ 100 = 5 mL

Water to be added ≈ 100 mL − 5 mL = 95 mL

For a larger batch use the dilution relation, since the amount of pure acetic acid does not
change on adding water:

C1V1 = C2V2

100 % × V1 = 5 % × 1000 mL

V1 = 50 mL of glacial acetic acid, made up to 1000 mL with water

Safety first: Glacial acetic acid is corrosive and its vapour stings the eyes. Always
add the acid to the water, never water to the acid, do it slowly with stirring, and
work under an adult’s supervision. Make the volume up to 100 mL at the end — do
not simply add 100 mL of water to 5 mL of acid, because volumes of mixed liquids do
not add up exactly.

Activity 5.2: Let us represent solubility graphically — Page 77

Page 12 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

5.2.3 Solubility of substances

ACTIVITY

Q1 Based on the information from the above graph, predict which of the two
compounds, ‘A’ or ‘B’, will dissolve more in a given amount of water at a given
temperature?

400
Compound ‘B’
350
Solubility (g per 100 g of water)

300
287

250
241

200

150

100
Compound ‘A’
50

0
10 20 30 40 50 60 70 80
Temperature (°C)

Fig. 5.6, page 77 — solubility curves of compounds ‘A’ and ‘B’ in water.

Compound ‘B’ — at every temperature on the graph its curve lies far above that of compound
‘A’.

Page 13 of 80

as e
Class 9 Science Chapter 5 Exploring Mixtures and their Separation
a g l AglaSem · NCERT Solutions

co m
s e m.
400
m l a
m .co agCompound ‘B’
l a se ≈360
g
Solubility (g per 100 g water)

a 300 287

co m
241
em . ag
200
g l as
a
100
. com
m a s em
Compound ‘A’

. co ag l ≈66 at 80 °C
e m
g l as 0
a 10 20 30 40 50 60 70 80
Temperature (°C)
a s
comredrawn from Fig. 5.6. Read a temperature on the
Solubility curves of compounds ‘A’ and ‘B’ in .water, agl
m
toethe curve, then across to the y-axis.
x-axis, go up s
a
agl

co m
At 20 °C: B ≈ 205 g per 100 g water, A ≈ 37 g per 100 g water
m .
m as e
.co
At 60 °C: B = 287 g per 100 g water, A ≈ 56 g per 100 g water
a g l
s m 80 °C: B ≈ 360 g per 100 g water, A ≈ 66 g per 100 g water
eAt
gl a
a
se m
com
How to read the graph: Pick a temperature on the x-axis, go straight up to a curve,
g l a
m . a
e
as at that temperature. Since B’s curve is above A’s
then straight across to the y-axis — that reading is the maximum mass of solute

a g l
which 100 g of water can hold
everywhere, B always dissolves more in the same mass of water at the same
temperature.
co m
m .
m ase
.co a g l
a s emQ2
gl
(i) The solubility of compound ‘A’ in water at 20 °C is ______________ (less than/more
a than/similar to) its solubility at 60 °C.
c
m .
m a s e
. co agl

e m
Less than.
g l as
a

co m
m .
m as e
.co


a g l Page 14 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Solubility of A at 20 °C ≈ 37 g per 100 g of water

Solubility of A at 60 °C ≈ 56 g per 100 g of water

Increase ≈ 56 g − 37 g = 19 g per 100 g of water

Why it happens: Dissolving a solid pulls its particles apart from the crystal, and that
costs energy. Heating supplies the energy and also makes the water molecules move
faster, so they attack the crystal surface more often. Both effects raise the solubility
of a solid solute as the temperature rises.

Q3 (ii) The solubility of compound ‘B’ at 20 °C is ____________ (less than/more than/similar
to) its solubility at 60 °C.

Less than.

Solubility of B at 20 °C ≈ 205 g per 100 g of water

Solubility of B at 60 °C = 287 g per 100 g of water

Increase ≈ 287 g − 205 g = 82 g per 100 g of water

Tip: B’s curve is not only higher than A’s, it is also much steeper. A steep solubility
curve is exactly what makes a compound easy to purify by crystallization, because a
small drop in temperature throws out a large mass of solid.

Q4 (iii) The solubility of _____________ increases more than that of ______________ with an
increase in the temperature.

The solubility of compound ‘B’ increases more than that of compound ‘A’ with an increase in
the temperature.

Page 15 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

From 20 °C to 60 °C:

Compound B: 287 g − 205 g ≈ 82 g per 100 g of water

Compound A: 56 g − 37 g ≈ 19 g per 100 g of water
Compound B gains about 82 ÷ 19 ≈ 4 times as much

Why it happens: How steeply solubility rises is a property of the particular solute —
it depends on how much energy is absorbed when that solid dissolves. Compound B
absorbs much more heat on dissolving than compound A, so warming helps B far
more. On the graph this shows up directly as the steeper slope of B’s curve.

In-text Questions — Page 77
5.2.3 Solubility of substances

Q1 What do you think will happen if you make a saturated solution at a higher
temperature and cool it slowly?

Part of the dissolved solute will come out of the solution as pure solid crystals. This is the
process of crystallization.

Saturated solution of compound B at 60 °C: 287 g in 100 g of water

Solubility of B at 40 °C (from Fig. 5.6): 241 g per 100 g of water

Mass that must separate out = 287 g − 241 g = 46 g

Why it happens: A saturated solution holds the maximum the solvent can carry at
that temperature. When the temperature falls, that maximum falls with it, so the
solution now holds more solute than it is allowed to. The extra solute has nowhere to
go but out, and it leaves the solution as solid. If the cooling is slow, the particles
have time to arrange themselves in a regular geometric pattern and you get large,
well-shaped, shiny crystals; if it is rapid, many tiny crystals form at once and each
stays small.

Page 16 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Did you know? Impurities usually stay dissolved because they are present in small
amounts and are nowhere near their own saturation level. That is why crystallization
gives a pure solid and is used to purify compounds.

Activity 5.3: Let us prepare — Page 78
5.3.1 Crystallization

ACTIVITY

Q1 Leave it for some time. What do you observe?

The water evaporates from the drop of saturated copper sulfate solution on the glass plate, and
small blue crystals of copper sulfate are left behind, spread over the plate.

Why it happens: Here the solution is not being cooled — the solvent is being
removed. As water molecules escape from the surface, the amount of water falls
while the amount of copper sulfate stays the same, so the solution passes its
saturation point and the excess solute separates as solid. The crystals are blue
because copper sulfate crystallises with water in its structure, as hydrated copper
sulfate.

Q2 Did you get crystals? If yes, is this a good way to experiment? Explain.

Yes, crystals do form — but this is not a good method if you want good crystals or a pure
product.

The crystals are tiny and badly shaped. Evaporation from a thin film is fast, so many
crystals start growing at the same moment and none of them gets time or material to grow
large.
The product is impure. Everything dissolved in the drop — including the impurities — is left
behind on the plate when the water goes. Nothing is removed.
No control. The rate depends on the room temperature, the breeze and the humidity, so the
result cannot be repeated reliably.

The proper method is the one in steps 1 – 6: filter the hot saturated solution to take out
insoluble impurities, cover it, and let it cool slowly and undisturbed.

Page 17 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why slow cooling is better: When cooling is slow, the solution stays only slightly
supersaturated at any moment. Few crystal nuclei form, and each of them has time
to collect particles from the solution and grow into a large, shiny, well-shaped
crystal. The soluble impurities remain in the liquid left over, which is poured off —
that is what makes crystallization a purification technique, not just a drying
technique.

Think as a Scientist — Page 79
5.3.1 Crystallization

THINK AS A SCIENTIST

Q1 If a hot, saturated solution of copper sulfate is cooled rapidly in ice-cold water,
smaller and less well-formed crystals will form than if it is cooled slowly at room
temperature. How would you design and perform an experiment to test this
hypothesis?

Design a controlled experiment in which the rate of cooling is the only thing you change.
Hypothesis being tested: faster cooling of a saturated copper sulfate solution gives smaller
and less well-formed crystals.
Variables

TYPE IN THIS EXPERIMENT

Independent variable (what Rate of cooling — ice-cold water bath versus still air at room temperature
you change)

Dependent variable (what Size and shape of the crystals formed
you measure)

Controlled variables (kept Same stock solution, equal volumes, same starting temperature, same type of
the same) beaker, both left undisturbed for the same time

Procedure

1. Prepare a hot saturated solution of copper sulfate as in Activity 5.3 and filter it while hot to
remove insoluble impurities.
2. Divide the hot filtrate into two equal parts in two identical beakers, and note the starting
temperature of both with a thermometer.
3. Stand beaker 1 in a bath of ice-cold water. Leave beaker 2 on the bench at room
temperature. Cover both with watch glasses and do not disturb them.

Page 18 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

4. Record the temperature of each beaker every two minutes, so that you have evidence of the
co m
two different cooling rates.
se m.
o m g l a
.cdry them on a watch glass.
5. After the same fixed time, filter out the crystals from each beaker, rinse them with a little cold
m a
se them: measure the longest edge of about ten crystals from each beaker with a
water and
l a
ag
6. Compare
scale or a magnifying glass, and note how sharp and regular the faces look.

o m
. c ag
Expected result: beaker 1 gives many small crystals with rough, poorly formed faces; beaker 2

s e m flat faces. That supports the hypothesis.
gives fewer but larger, shiny crystals with clear
a
agl
Why it happens: Crystals grow in two competing steps — new nuclei appear, and
existing nuclei grow. Rapid cooling makes the solution strongly supersaturated all at
co m
once, so a very large number of nuclei appear together and share the same limited
em.
m l as
.co g
solute; each therefore stays small. Slow cooling keeps the supersaturation gentle, so

em a
s
few nuclei form and each has time to add particles in an orderly way, producing
a
gl large well-shaped crystals.
a
m a s
.co
Tip: A single trial is not proof. Repeat the whole comparison at least three times; if
em agl
a s
the same difference appears every time, the evidence is far stronger. Safety first:
agl it under adult supervision and never with bare
copper sulfate is toxic — handle
hands.

co m
m .
m as e
.co a g l
em and Ponder — Page 79
as Crystallization
Pause
a g l
5.3.1

PAUSE AND PONDER
se m
com g l a
m . a
ase
agl
Q4 Refer to the solubility curves given in Activity 5.2. If equal masses of hot, saturated
solutions of compounds ‘A’ and ‘B’ are cooled from 80 °C to 60 °C, which solution is
likely to deposit more solid?

co m
m .
m of compound ‘B’ will deposit far more solid. glas e
o

. c a
s e m
The solution

a
agl
Step 1 — read the solubilities off Fig. 5.6 (in g per 100 g of water):

.c
s e m
com a
agl
Compound B: at 80 °C ≈ 360 g at 60 °C = 287 g

Compound A: at 80 °C ≈ 66 gem
.
a s
agl
at 60 °C ≈ 56 g

Step 2 — solid deposited per 100 g of water

co m
m .
m ase
.co


a g l Page 19 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Compound B: 360 g − 287 g ≈ 73 g

Compound A: 66 g − 56 g ≈ 10 g

Step 3 — the question says equal masses of solution, so scale to 100 g of solution

B: 100 g water + 360 g solute = 460 g of solution

deposit = (73 g ÷ 460 g) × 100 g ≈ 15.9 g per 100 g of solution

A: 100 g water + 66 g solute = 166 g of solution

deposit = (10 g ÷ 166 g) × 100 g ≈ 6.0 g per 100 g of solution

Either way of counting gives the same verdict: B deposits more — about 2.6 times as much for
the same mass of solution.

Why it happens: What decides the yield of crystals is not how much solute a
compound dissolves, but how steeply its solubility falls as the solution cools. B’s
curve drops sharply between 80 °C and 60 °C, so a large excess is thrown out. A’s
curve is almost flat, so hardly anything separates. This is why compounds with steep
solubility curves, such as potassium nitrate, are purified by crystallization, while
nearly-flat ones such as sodium chloride are obtained by evaporating the solvent
instead.

Tip: Values read off a graph are approximate. Quote them as “about 360 g”, not
360.0 g — only 287 g and 241 g are printed exactly on Fig. 5.6.

Q5 Will there be any change in the size of common salt crystals if the rate of
evaporation is increased or decreased? Explain.

Yes. Slow evaporation gives large crystals; fast evaporation gives many small ones.

Page 20 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

RATE OF WHAT HAPPENS IN THE SOLUTION CRYSTALS
EVAPORATION OBTAINED

Slow (shallow pan, mild sun, The solution creeps past saturation gently, so only Few, large, well-
still air) a few nuclei form and each keeps growing shaped crystals

Fast (strong heating, dry The solution shoots far past saturation, so a huge Many tiny, poorly
wind, thin film) number of nuclei appear together formed crystals

Why it happens: The total mass of salt obtained is fixed by how much was dissolved
— evaporation rate cannot change that. What it changes is how that fixed mass is
shared out. Fast evaporation creates a large number of growth centres, so each
gets only a small share; slow evaporation creates few, so each grows big. The
particles also need time to line up in the regular geometric pattern of a crystal, and
rapid evaporation does not give them that time.

Did you know? Indian salt makers used exactly this idea long before the science was
written down: karkatch salt was made by the slow evaporation of seawater in shallow
pans, while panga salt was made by boiling concentrated brine — and the two
methods gave crystals of clearly different sizes.

Activity 5.4: Let us describe a process — Page 79

Page 21 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

5.3.1 Crystallization

ACTIVITY

Q1 Observe Fig. 5.9, it shows how salt crystals are obtained from seawater. Can you
describe the process in your own words?

Seawater Saturated solution Salt crystals

Fig. 5.9, page 79 — steps followed in the salt manufacturing process.

Seawater → saturated solution → salt crystals. It is crystallization driven by the evaporation
of the solvent, carried out on a huge scale in shallow salt pans along the coast.

1. Seawater is let into shallow pans. Seawater is a dilute solution — roughly 3.5 g of dissolved
salts in every 100 g of water — so it is nowhere near saturated to begin with. The pans are
made wide and shallow so that a very large surface is exposed to the sun and wind.
2. The water evaporates and the brine becomes a saturated solution. Water molecules
leave from the surface, but the salt cannot; so the mass of solvent falls while the mass of
solute stays the same and the concentration climbs steadily. Once the water can hold no
more salt at that temperature, the brine is saturated.
3. Further evaporation deposits salt crystals. Every further gram of water that escapes
leaves behind salt it can no longer hold, and the excess separates out on the floor of the pan
as cubic crystals of common salt. These are raked up, washed and dried.

Why evaporation rather than cooling: Cooling only helps for a solute whose
solubility changes sharply with temperature. Common salt dissolves to about 36 g
per 100 g of water at 10 °C and only about 37 g at 80 °C (Table 5.4) — practically a
flat curve. Cooling seawater would therefore give almost no crystals. Removing the
solvent is the only route that works, and the sun does it for free.

Did you know? Along India’s coasts this was an old craft: karkatch salt was made by
evaporating seawater in pans, and panga salt by boiling concentrated sea brine —
the two methods yielding crystals of different sizes.

Page 22 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Ready to Go Beyond — Page 79
5.3.1 Crystallization

READY TO GO BEYOND

Q1 Have you visited a place where large crystal deposits can be observed in nature?

Natural crystals grow wherever a mineral-rich solution is left undisturbed for a very long time —
in caves, in mines and inside the Earth’s crust.
Sample answer: On a school trip to Sohra (Cherrapunji) in Meghalaya we went into the
Mawsmai Cave (Fig. 5.10a). Water dripping through the limestone roof carries dissolved
calcium compounds. As each drop hangs at the roof and slowly loses water and carbon dioxide,
the solution becomes saturated and a little solid is deposited. Over thousands of years these
deposits build up into stalactites hanging down and stalagmites rising from the floor. Quartz
(Fig. 5.10b), with its clear six-sided pointed crystals, is another beautiful example found in mines
and rock cavities.

Why nature makes better crystals than we do: The size of a crystal depends on
how slowly it grows. In the laboratory we cool a solution over an hour; in a cave the
solution is fed drop by drop over centuries. Extremely slow growth means very few
nuclei and unlimited time for each to add particles in a perfectly regular geometric
pattern — which is why natural crystals can be enormous and beautifully formed.

Try this: If you cannot visit a cave, look at rock salt (sendha namak), mishri, alum or
quartz from a local shop under a magnifying glass. Sketch the shape you see — each
compound has its own characteristic crystal shape.

In-text Questions — Page 80
5.3.2 Distillation

Q1 However, we may also want to recover the solvent. What should we do then?

Use distillation instead of plain evaporation.

Page 23 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

co m
em.
Evaporation: solution → solute recovered, solvent lost to the air
m l as
.co
Distillation: solution → solute left in the flask and solvent condensed and collected
m a g
l a se
g
aWhy it works: Both methods do the same first step — the solvent is boiled off as

co m
vapour. The difference is what happens next. In evaporation the vapour simply

em .
escapes into the room. In distillation the vapour is led into a condenser, where ag
g l as
circulating cold water takes away its latent heat of vaporisation, so it turns back into
a
a liquid and drips into a receiving flask. Nothing is thrown away, so you end up with

m
both components in pure form.
co
e m.
m l as
.co
Did you know? This is how a camp still gives you drinking water from salty or
a g
a s em water: the water is boiled off and condensed, while the salt and dirt stay
gl behind in the boiling vessel.
muddy
a
m a s
m .co agl
l a se
g
How can we separate the two miscible liquids?
a
Q2

o m
By distillation, provided their boiling points differ by at least about 25 °C.m.c

o m l a se
.c a g
se m
g l a
a
se m
com g l a
. a
Thermometer
e m
asWater outlet
agl Water condenser
Stand

co m
m .
as e
comflask
Distillation
.
vapour cools → distillate
a g l
m
ase
Water inlet

agl Mixture of acetone vapour rises
c
acetone and
m .
water
m a s e
Burner em
.co agl
as
Conical flask

a g l

com
m .
m as e
.co


a g l Page 24 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Distillation set-up. The lower-boiling liquid vaporises first, the condenser turns the vapour back into
liquid, and the distillate collects in the conical flask.

Boiling point of acetone = 56 °C

Boiling point of water = 100 °C

Difference = 100 °C − 56 °C = 44 °C, which is more than 25 °C → distillation will work

Heat the mixture in a distillation flask. Acetone, the lower-boiling liquid, boils first at about 56
°C; the thermometer bulb at the mouth of the side arm holds steady near 56 °C while it distils
over. Its vapour passes through the water condenser, is cooled and collected in the conical flask
as pure acetone. Water, needing 100 °C, stays behind in the distillation flask.

Why the 25 °C rule: No liquid waits politely for its turn — even below its boiling
point some of it evaporates. If the two boiling points are close, the vapour rising
from the flask is a mixture of both liquids and the distillate is impure. A gap of about
25 °C or more means that when the first liquid is boiling, the second contributes only
a small amount of vapour, so the distillate is essentially pure. For smaller gaps,
fractional distillation is used instead.

Q3 Is it possible to separate the mixture of two miscible liquids by evaporation and
obtain both the liquids?

No. Evaporation can give you back only one of the two — and even that one is not obtained
pure.

The liquid that evaporates escapes into the air and is lost; there is no arrangement to catch
it. So one component is gone for good.
Both liquids evaporate at the same time, because evaporation goes on at all temperatures.
The liquid left behind therefore still contains some of the other liquid — it is not pure either.

Evaporation is useful only when the solute is a non-volatile solid, such as salt in water, and you
do not care about recovering the solvent.

Why distillation succeeds where evaporation fails: Distillation adds two things —
a controlled temperature that makes the lower-boiling liquid vaporise first, and a
condenser that catches that vapour and turns it back into liquid. Because the vapour
is captured instead of released, both components are recovered.

Page 25 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Activity 5.5: Let us investigate — Page 82
5.3.3 Paper Chromatography

ACTIVITY

Q1 Observe the paper as the water rises through the paper. What do you notice?

The water creeps steadily upwards through the paper, well above the level in the jar, and when
the wet front reaches the ink spot it begins to drag the ink up with it. The single black spot
stretches into a streak, and colours start to appear at the edges.

Why water climbs the paper: Chromatographic paper (and filter paper) is a mesh
of fine cellulose fibres with countless narrow channels between them. Water
molecules are attracted to the cellulose (adhesion) and to one another (cohesion), so
they are pulled into these narrow channels and rise against gravity. This is capillary
action — the same reason a cotton wick draws oil up in a lamp, and a towel soaks
up water.

Tip: The line must be drawn with a pencil, not a pen. Pencil is graphite, which is
insoluble and does not travel with the solvent, so it cannot spoil the result.

Q2 As the water rises, the ink starts to separate into different colour spots. What can
you infer from this?

That the “black” ink is not a single substance at all — it is a mixture of several coloured dyes,
and each dye travels up the paper at its own speed.

Because separate coloured spots appear, at least that many different components are
present.
The number of spots tells you how many components the ink contains.
The spot that has travelled farthest is the component least held by the paper and most
attracted to the water.

Page 26 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why the components separate: Every dye is caught between two competing pulls
— the paper (the stationary phase) holds it back, while the rising water (the mobile
phase) carries it forward. A dye that dissolves readily in water and clings only weakly
to the cellulose is swept far up the strip. A dye that clings strongly to the paper and
dissolves poorly is left near the start. Since each dye has its own balance of these
two attractions, each ends up at a different height, and what began as one spot
becomes a row of separate spots.

Try this: Repeat with green food colour, or with the green extract of spinach leaves,
or with the colours of flower petals. For some of these, water will not do the job and
you will need alcohol, or a mixture of solvents, as the mobile phase.

In-text Questions — Page 82
5.3.3 Paper Chromatography

Q1 Will water work as a solvent in every case?

No. Water works only when the components of the mixture actually dissolve in water. In many
cases a different solvent, or a mixture of solvents, must be used.

MIXTURE TO BE SUITABLE MOBILE REASON
SEPARATED PHASE

Black or coloured sketch-pen Water, or 2 % m/v salt The dyes are water-soluble
ink solution

Green pigments of spinach Alcohol, or a solvent Chlorophyll is insoluble in water but
leaves mixture dissolves in alcohol

Coloured pigments of flower Alcohol or a water–alcohol Depends on which pigment the petal contains
petals mixture

Page 27 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why the choice of solvent matters: A component can only move up the paper if
the mobile phase can dissolve it and carry it. If the solvent cannot dissolve a
component at all, that component simply stays on the starting line and nothing is
separated. The solvent must also not dissolve every component equally well, or all of
them would race up together and stay as one spot. A good solvent is one that
separates the components by treating each of them a little differently.

Pause and Ponder — Page 82
5.3.3 Paper Chromatography

PAUSE AND PONDER

Q6 State whether the following statements are True or False. Also, correct the False
statements. (i) Salt can be separated from a salt solution by evaporation or
distillation. (ii) Distillation can be used for separation of two liquids even when
these have the same boiling point. (iii) In paper chromatography, the solvent level
should be above the sample spot at the beginning of the experiment. (iv)
Evaporation and crystallization are the same processes.

STATEMENT TRUE / CORRECTED STATEMENT, WHERE NEEDED
FALSE

(i) Salt can be separated from a salt True —
solution by evaporation or distillation.

(ii) Distillation can be used for separation False Distillation can separate two miscible liquids only
of two liquids even when these have the when their boiling points differ by at least about 25
same boiling point. °C.

(iii) In paper chromatography, the solvent False The solvent level should be below the sample spot
level should be above the sample spot at at the beginning of the experiment.
the beginning of the experiment.

(iv) Evaporation and crystallization are False They are different: evaporation removes the solvent
the same processes. to leave the solute behind, while crystallization
deposits a pure solid as regular crystals from a
saturated solution.

(i) Why it is true. Salt is a non-volatile solid: it does not vaporise at the temperatures involved.
So if the water is driven off — either by evaporating it into the air or by boiling it off and
condensing it in a distillation set-up — the salt is left behind in the vessel in both cases.

Page 28 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Distillation does more: it also gives you back the pure water.
c o m
.
mIf two liquids
s e
(ii) Why it is false. Distillation depends on one liquid vaporising before the other.
a same
.
boil at the same
c om
temperature they vaporise together, and the distillate has lthe
a g
s
composition
e m as the mixture — nothing is separated.
a
a l it is false. If the solvent level starts above the spot, the ink is simply washed off into
(iii)gWhy
the solvent in the jar and lost. The whole method depends on the solvent rising through the

co m
ag
paper and reaching the spot from below, so the level must start below the spot.

m .
(iv) Why it is false.
as e
a g l
POINT EVAPORATION CRYSTALLIZATION

co m
m.
What is The solvent is driven off The solvent stays; the excess solute
removed
m
comes out
as e
. c o a g l
What m
a s e drives it Heating or exposure to air Slow cooling of a hot saturated solution

aglPurity of the All dissolved impurities are left behind Impurities remain in the liquid; the

s
product with the solute crystals are pure
m a
m .co agl
ase
Form of the A crust or fine powder Well-shaped crystals with regular geometry

agl
solid

co m
m .
m Let us separate — Page 83 as e
.co
Activity 5.6:
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 29 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

5.4.1 Separation of two immiscible liquids

ACTIVITY

Q1 Let it stand undisturbed. What do you observe?

Laboratory stand

Separating funnel

Mustard oil

Water

Stopcock

Conical flask

Glass stopper

Fig. 5.16, page 83 — redrawn sketch of the separating-funnel set-up used in Activity 5.6.

The mixture, cloudy at first from being poured, soon clears into two sharply separated layers
— a yellow layer of mustard oil (5 mL) floating on a colourless layer of water (20 mL), with a flat,
clearly visible boundary between them.

Why it happens: Mustard oil and water are immiscible: their molecules attract their
own kind far more strongly than they attract the other kind, so they do not spread
through each other. Standing still lets gravity sort the two liquids purely by density —
the denser liquid sinks and the lighter one floats — and once sorted, there is nothing
to mix them again.

Page 30 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Q2 The yellow-coloured mustard oil forms the upper layer and water forms the lower
layer. Can you explain why?

Because mustard oil is less dense than water, so an equal volume of it weighs less and it floats.

Density of water = 1.00 g cm⁻³

Density of mustard oil ≈ 0.91 g cm⁻³

Since 0.91 g cm⁻³ < 1.00 g cm⁻³, the oil rises to the top

Why it happens: Take 1 cm³ of each liquid. The water cube has a mass of 1.00 g and
the oil cube only about 0.91 g. Gravity pulls harder on the heavier water, so the water
sinks to the bottom and pushes the lighter oil up. Put another way, the buoyant
force on the oil from the surrounding water is greater than the oil’s own weight, so
the oil rises until it sits on top. This density difference is precisely what the
separating funnel exploits: open the stopcock and the lower liquid runs out first.

Tip: Discard the small middle portion where the two liquids meet. Collecting it would
contaminate both, and losing a millilitre matters far less than losing purity.

What if … — Page 83
5.4.1 Separation of two immiscible liquids

WHAT IF …

Q1 two immiscible liquids of the same density are mixed in a separating funnel, how
will the layers form?

They will not form two clean layers one above the other, and a separating funnel will fail to
separate them.

Page 31 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why it happens: A separating funnel works only because gravity acts more strongly
on the denser liquid, pushing the lighter one up. If the two densities are equal, there
is no net upward or downward push on either liquid — neither has any reason to
float or to sink. The droplets of one liquid stay scattered through the other, so the
mixture stays cloudy for a very long time and, if it does eventually settle, the
boundary is blurred and its position is unpredictable. The liquids still do not mix
(they remain immiscible and form separate droplets), but they no longer stack.

What could be done instead:

Centrifugation — spinning the tube produces a force many times stronger than gravity, and
even a very small density difference is then enough to drive one liquid outwards.
Change one density — for example, dissolve a salt in the aqueous layer to make it denser,
so the two separate normally.
Distillation, if the two liquids have sufficiently different boiling points, since that method
does not depend on density at all.

In-text Questions — Page 83
5.4.1 Separation of two immiscible liquids

Q1 Can you think of any heterogeneous mixtures with a gas as one of the components?

Yes. Gases mix freely with one another, so a mixture of gases is almost always homogeneous —
but if the other component is a solid or a liquid spread through the gas, the mixture is
heterogeneous.

MIXTURE WHAT IS DISPERSED IN THE GAS TYPE

Smoke Fine solid carbon and ash particles in air Heterogeneous

Fog, mist, clouds Tiny liquid water droplets in air Heterogeneous

Dust in the air Solid dust particles in air Heterogeneous

Air freshener spray, an aerosol Liquid droplets in air Heterogeneous

Hydrogen and oxygen (rocket fuel) Gas mixed with gas Homogeneous

Page 32 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why most gas mixtures are homogeneous: Gas particles are very far apart and
move freely in every direction at high speed, so within moments they spread evenly
through the whole container and no part differs from any other. But a solid or a
liquid particle floating in air is a separate phase with a visible boundary — you can
point to where the smoke is and where it is not — so such mixtures are
heterogeneous.

Activity 5.7: Let us explore — Page 84
5.4.2 Sublimation

ACTIVITY

Q1 Observe the inner wall of the funnel carefully. Do you notice any solid deposits?

Yes — a layer of white solid camphor collects on the cool inner wall of the inverted funnel,
while the sand stays behind in the china dish.

Camphor (solid) —heat→ camphor vapour —cooling on the funnel wall→ camphor (solid)

Solid → vapour directly = sublimation

Vapour → solid directly = deposition

Why it happens: On gentle heating (kept below its melting point) camphor passes
straight from the solid state to the vapour state without ever becoming a liquid — its
particles gain enough energy to escape from the solid surface directly. The vapour
rises and meets the funnel wall, which is much cooler because it is exposed to the
room air. There the vapour loses energy and settles straight back into the solid state.
Sand does not sublime at these temperatures, so it is left in the dish. The mixture is
separated because one component sublimes and the other does not.

Tip: The cotton plug in the funnel nozzle is not decoration — it stops the camphor
vapour from escaping so that all of it condenses inside the funnel. Naphthalene,
ammonium chloride and dry ice (solid carbon dioxide) also sublime and can be used
the same way.

Page 33 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

co m
m.
Pause and Ponder — Page 84
m ase
5.4.2 Sublimation
.co a g l
se m
l a
PAUSE AND PONDER

a g
Why do immiscible liquids form two separate layers in a separating funnel?
m
Q7

. co ag
em
g l as
a
Because they do not dissolve in each other, and they have different densities.

co m
Denser liquid → sinks → lower layer
se m.
o m g l a
m .c liquid → floats → upper layer
Less dense
a
l a se
a g Example: water (1.00 g cm⁻³) below, mustard oil (≈ 0.91 g cm⁻³) above

m a s
em
Why it happens — two separate reasons working together:
.co agl
a s
aglso instead of spreading between the water molecules
1. They stay apart. Water molecules attract one another strongly. Oil molecules
cannot join in that attraction,
they gather with their own kind. The two liquids therefore remain as two separate
phases with a boundary between them.
. com
m a s
2. Gravity stacks them. Once they are separate, gravity sorts them em by density. For
o volume the denser liquid is heavier, so it sinks and
the.csame
a glpushes the lighter one
as em
l
up, and the arrangement with the denser liquid at the bottom is the stable one. That
a g is why the layers always form in the same order and stay that way.

se m
com g l a
m . a
e
Tip: This is the whole basis of the separating funnel. Open the stopcock and the

g l as it as the boundary reaches the tap, discard the small
lower layer runs out first; close
a the upper layer separately.
mixed portion, then collect

co m
m .
mIs sublimation different from evaporation? Justify. glas e
. c o a
m
Q8

l a se
ag ANSWER
.c
s e m
m a
agl
Yes, they are different. Sublimation is solid → vapour directly; evaporation is liquid → vapour
. co
m
from the surface. The key difference is the state you start from and whether a liquid stage

as e
l
appears at all.
a g

com
m .
m ase
.co


a g l Page 34 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

POINT OF SUBLIMATION EVAPORATION
COMPARISON

Change of state Solid → vapour, with no liquid stage Liquid → vapour

Starting material A solid A liquid

Where it occurs At the surface of the solid At the surface of the liquid

Which substances Only a few — camphor, naphthalene, Every liquid, at all temperatures
ammonium chloride, dry ice

Reverse process Deposition (vapour → solid) Condensation (vapour → liquid)

Used to separate A sublimable solid from a non-sublimable A non-volatile solute from its solvent
one (camphor from sand) (salt from salt solution)

Why only some solids sublime: In most solids the particles are held so tightly that
they must first loosen into a liquid before they can escape as vapour. In camphor
and naphthalene the forces between the particles are weak enough that, at ordinary
pressure, a particle at the surface can break away straight into the vapour state — so
the solid never passes through a liquid stage at all.

In-text Questions — Page 84
5.4.2 Sublimation

Q1 Can you think of any other mixtures where sublimation can be used to separate the
components?

Yes — any mixture in which exactly one component sublimes on gentle heating.

Page 35 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

MIXTURE COMPONENT THAT COMPONENT LEFT
SUBLIMES BEHIND

Naphthalene and sand Naphthalene Sand

Ammonium chloride and common Ammonium chloride Common salt
salt

Camphor and common salt Camphor Common salt

Iodine and sand Iodine Sand

Dry ice mixed with an ordinary solid Solid carbon dioxide The other solid

The condition for the method to work: sublimation separates a solid–solid mixture
only when one component vaporises directly below its melting point while the other
stays unchanged at that temperature. If both sublime, or if neither does, the method
is useless and you must use some other difference — solubility, magnetism or
density.

Q2 Is it possible to dissolve one metal in another?

Not at room temperature — but yes, if the metals are melted first. The homogeneous solid
formed when such a molten mixture solidifies is called an alloy.

ALLOY APPROXIMATE COMPOSITION WHY IT IS MADE

Brass About 80 % copper and 20 % zinc Harder than copper, easy to
shape, does not corrode readily

Bronze About 80 % copper and 20 % tin Strong and hard-wearing; used
for statues and bells

Stainless Iron with carbon 0.03 – 0.8 %, chromium 16 – 18 %, Strong and rust-resistant; used for
steel nickel 10.0 – 14.0 %, molybdenum 2.0 – 3.0 % utensils and instruments

Page 36 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why melting is needed: In a solid metal the atoms are locked into a fixed lattice
and cannot move past one another, so one metal cannot spread into another.
Melting frees the atoms to move, and the two kinds of atoms then mix right down to
the atomic level. On cooling, the mixture solidifies with the atoms still mixed, so the
alloy is a homogeneous mixture and looks like a single metal. Because the mixing
is atomic and not just physical, no physical method can separate an alloy back
into its metals.

In-text Questions — Page 85
5.4.3 Suspensions

Q1 How can we separate mud from water?

Start with the simplest method and add another only if the water is still not clear.

1. Sedimentation and decantation. Let the muddy water stand. The heavier mud particles
settle at the bottom; pour off the clearer water carefully without disturbing the sediment.
2. Filtration. Pass it through filter paper or a clean cotton cloth. Particles larger than the pores
are held back as residue.
3. Coagulation, then filtration. Add powdered alum (fitkari). It makes the fine particles clump
into larger flocs that settle by gravity and can then be removed by decantation or filtration.
4. Centrifugation. Spin a sample at high speed; the centrifugal force drives even very fine
particles to the bottom of the tube.

Why one method is often not enough: Muddy water contains particles over a very
wide range of sizes. The big ones settle in minutes and are caught by filter paper.
The finest clay particles are so light that molecular jostling keeps them suspended,
and they are small enough to pass straight through the pores of the paper — which
is why the water often stays cloudy even after filtering. Those particles must first be
made bigger (coagulation) or be pushed down by a force stronger than gravity
(centrifugation).

Page 37 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Q2 If the muddy water is still not clear even after keeping for some time, how can it be
cleaned?

By centrifugation and/or coagulation — the two techniques the chapter introduces for exactly
this situation.
Coagulation. Add a pinch of powdered alum, a white crystalline substance used in water
purification. Alum makes the fine suspended particles stick together in clumps. These larger,
heavier clumps settle rapidly under gravity — the process called sedimentation — and the clear
water above can then be decanted or filtered off.
Centrifugation. Take the cloudy water in a tube and spin it at high speed. The tubes swing out
to the horizontal, and the centrifugal force acting outwards on each particle is many times
greater than its weight, so even the finest particles are driven to the bottom of the tube while
clear liquid remains on top.

Why these work when filtration does not: Filtration is a sieve, and it can only
remove particles bigger than its pores. Coagulation attacks the problem from the
other side — instead of making the sieve finer, it makes the particles bigger until
they settle on their own. Centrifugation changes the force instead of the particle: it
replaces gravity with a much stronger outward force, so particles that would take
days to settle sink in minutes.

Did you know? A sewage treatment plant uses this very sequence — sedimentation,
then coagulation, then filtration — before the treated water is released or reused for
flushing and watering plants.

Threads of Curiosity — Page 85
5.4.3 A. Centrifugation

THREADS OF CURIOSITY

Q1 The spinning game is a folk dance called phugadi in Marathi and kikli in Punjabi.
What is this called in your local language?

The same two-person spinning game is played all over India and has a different name in each
language.

Page 38 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

co m
m.
LANGUAGE / REGION NAME OF THE GAME OR DANCE

m as e
.co
Marathi (Maharashtra) Phugadi (फु गडी)
a g l
se m
g l a
a
Punjabi (Punjab) Kikli (ਿਕੱਕਲੀ)

Konkani (Goa) Fugdi

co m
m . ag
se
Gujarati (Gujarat) Ghero / Fugdi

l a
agcall it ______. We hold each other’s crossed hands and spin,
Sample answer: “In my language we
and the faster we spin the harder we feel ourselves being pulled outwards.” Ask your

co m
m.
grandparents — they will almost certainly know a local name for it.

o m l a se
.c in the game: When you spin, your body keeps tryinga g to move in a
m
The science

l a se line, but your partner’s grip keeps pulling you into a circle. From where you
straight
a g stand it feels as if something is throwing you outwards. That outward effect on a

s
body in circular motion is the centrifugal force, and it is exactly the force a
m a
.co
centrifuge uses to drive the heavier particles of a mixture to the bottom of a
m agl
spinning tube.
l a se
a g

co m
m .
e
Bridging Science and Society — Page 86
m l as
.co a g
The Paperfuge

a s em SCIENCE AND SOCIETY
gl
BRIDGING

a
se m
m a
Find out more about such simple technological inventions that improve access to
medical care in remote places..co l
Q1

a g
a s em
ANSWER agl
m
The paperfuge is one of a whole family of frugal medical devices: they use everyday materials

. co
m
and no electricity, cost very little, and can be carried to a village that has neither a laboratory

m as e
l
nor a reliable power supply.

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

co m
m .
m ase
.co


a g l Page 39 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

DEVICE HOW IT WORKS WHAT IT MAKES
POSSIBLE

Paperfuge A cardboard disc on twisted strings, like a button Detecting malaria and
whirligig; spinning it at very high speed separates anaemia without electricity
blood by centrifugation

Foldscope A microscope folded from a paper sheet with a tiny Seeing microbes and blood
glass lens cells in the field, at very low
cost

Paper-strip A drop of blood or urine travels along treated paper Instant tests for pregnancy,
diagnostic tests by capillary action and changes colour malaria, diabetes and
infections

Solar-powered An insulated box cooled by a small solar panel Vaccines kept cold on the way
vaccine carrier to villages with no power

Jaipur foot A rubber and wood artificial limb made by hand from Affordable mobility for
local materials people who have lost a leg

Why the paperfuge works: A laboratory centrifuge is only doing one thing —
spinning a tube fast so that the outward force on each blood component is far larger
than its weight, driving the heavier red cells outwards and leaving the lighter plasma
near the axis. Nothing in that idea requires a motor. A disc whipped by twisted
strings can reach speeds of many thousands of revolutions per minute, which is
enough to do the same separation in a few minutes. The physics is unchanged; only
the cost has been removed.

Try this: Make the button-and-string whirligig of Activity 5.8 and watch how a disc
can be driven to a very high speed simply by pulling the strings in and out.

Activity 5.8: Let us make a model — Page 86

Page 40 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

5.4.3 A. Centrifugation

ACTIVITY

Q1 Make your own centrifuge with a cardboard disc and thick thread. You will be able
to see how the heavier particles move outwards. It is a fun and hands-on way to
understand the science behind separation. Which mixture would you like to
separate using this mini centrifuge?

Muddy water is the best mixture to try, because you can see the result with your own eyes and
it needs nothing that is not already in the classroom.
How to do it

1. Push a thick thread through two holes near the centre of a stiff cardboard disc and tie the
ends to make a loop, as in the paperfuge of Fig. 5.20.
2. Tape a small, tightly capped plastic tube of muddy water flat on the disc, close to its rim.
Tape a second, identical tube on the opposite side so that the disc stays balanced.
3. Hold the loop at both ends, wind the disc up by swinging it, then pull the strings apart and
let them come together, over and over. The disc spins one way, then the other, at high speed.
4. Spin for two or three minutes, then stop and hold the tube upright.

What you will see: a layer of mud packed at the far end of the tube — the end that was
pointing outwards — with much clearer water above it. The same tube left standing on the table
would take far longer to become that clear.

Why it happens: A particle going round in a circle is constantly being pulled towards
the centre by the tube walls. From the point of view of the mixture inside the
spinning tube, this feels like a strong outward push — the centrifugal force. Its size
grows with the mass of the particle and with the square of the spinning speed, so at
a few thousand revolutions per minute it is hundreds of times bigger than the
particle’s weight. The heavier mud particles are therefore driven to the outer end far
faster than gravity alone could ever manage, while the lighter water stays nearer the
axis.

Other mixtures worth trying: chalk powder in water, curd separating into thick
curd and whey, or the pulp in unfiltered fruit juice. Safety first: the tubes must be
sealed and taped firmly, the disc must be balanced, and everyone should stand clear
while it is spinning.

In-text Questions — Page 86

Page 41 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

5.4.3 B. Coagulation; 5.4.4 Colloids

Q1 Can you think of any other coagulation processes used in everyday life?

Yes — coagulation is at work in the kitchen, at the water works, and even in your own body.

EVERYDAY PROCESS COAGULANT USED WHAT CLUMPS TOGETHER

Making paneer or chhena Lemon juice or vinegar (an acid) Milk proteins clump and separate from
from milk the whey

Setting curd from milk Lactic acid made by the bacteria Milk proteins form a soft solid network
in the starter

Purifying drinking water at a Alum (fitkari) Fine clay and silt particles form
water works settleable flocs

Sewage treatment Alum or other coagulants Suspended solids settle out before
filtration

Blood clotting over a cut The body’s own clotting proteins Blood cells are trapped in a fibrous
mesh, sealing the wound

Setting an egg while cooking Heat Egg proteins clump into a solid white

Why a coagulant is needed: The fine particles in muddy water all carry the same
kind of electric charge, so they repel one another and stay separated and
suspended. A coagulant neutralises that charge. Once the repulsion is gone,
particles that collide stick together instead of bouncing apart, and they grow into
flocs heavy enough for gravity to pull down. In milk the same idea works through
acid: adding lemon juice changes the surface of the protein particles so they stop
repelling and clump into paneer.

Q2 If the components of blood can be separated by centrifugation and the blood
coagulates, is it a suspension? However, we cannot see blood cells with the naked
eye. Is it a solution?

Neither. Blood is a colloid — it sits exactly between the two.

Page 42 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

EVIDENCE WHAT IT SEEMS WHAT IT REALLY MEANS
TO SAY

Separates on centrifuging; coagulates Looks like a Its particles are big enough to be driven
suspension out by a strong force

Cells cannot be seen with the naked eye; Looks like a solution Its particles are far too small for the eye
blood looks uniform to resolve

Does not settle out on standing Not a suspension Molecular jostling keeps the particles
dispersed

Verdict In between Particle size 1 – 1000 nm → a colloid

Why the two clues do not clash: Particle size settles the question. Blood cells are
much larger than the ions of a salt solution (< 1 nm), so blood is not a true solution;
but they are smaller than the particles of a suspension (> 1000 nm), so gravity alone
cannot pull them down and they never settle out on standing. They do give in to the
far stronger force inside a centrifuge — which is precisely why blood banks use
centrifugation to split donated blood into plasma, platelets, white cells and red cells.

Tip: Milk, ice cream and tomato sauce are colloids for exactly the same reason —
uniform to the eye, but the particles are still big enough to scatter light and show
the Tyndall effect.

Bridging Science and Society — Page 87
Donate Blood

BRIDGING SCIENCE AND SOCIETY

Q1 Do you know your blood group? Find out!

Your blood group is decided by which marker proteins sit on the surface of your red blood cells.
In the common ABO and Rh system there are eight groups: A+, A−, B+, B−, AB+, AB−, O+ and
O−.
How to find out: a blood group test needs only a drop or two of blood. It is done free at almost
every blood donation camp, and at any hospital or pathology laboratory. Some school health
check-ups also record it. Write it down and keep it in your school diary — in an emergency it
saves valuable minutes.

Page 43 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

co m
m.
Why it matters, and how this chapter connects to it: If a patient is given blood of

m l a se
the wrong group, the body treats the incoming cells as foreign and clumps them
o
c
together, .blocking g its group
blood vessels. So every donated unit is testedaand
m
se before use. It is then separated by centrifugation into plasma, platelets,
l a
g cells and red cells (Fig. 5.22) — the same technique you met for muddy water
identified
awhite
— because one donation can then treat several different patients, each getting only
co m
.
the component they need. The components are stored in blood banks and supplied
em ag
on demand.
g l as
a
Did you know? The body replaces donated blood naturally within a few weeks,
co m
se
which is why regular donation is safe for a healthy adult. In Chapter 3, Tissues in
m.
o m l a
g that service on to
.c gases and hormones — a donation quite literally passes
Action, you learnt that blood connects the different parts of the body by transporting
m a
se
nutrients,

g l a
a
someone else.

m a s
In-text Questions — Page 87 sem
.co agl
a
5.4.4 Colloids agl

com
Let us try to understand what colloids are. How are they different from solutions
m .
e
Q1

m l as
.co
and suspensions?
a g
a s em
agl

m
A colloid is a mixture in which particles of one substance, sized between 1 nm and 1000 nm,

a se
com l
stay evenly dispersed through another substance without dissolving and without settling.
. a g
m
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 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

PROPERTY SOLUTION COLLOID SUSPENSION

Nature Homogeneous Heterogeneous (looks Heterogeneous
homogeneous)

Particle size Less than 1 1 – 1000 nm More than 1000 nm
nm

Particles visible? No No Yes, to the naked
eye

Separated by No No Yes
filtration?

Settles on standing? No No Yes

Tyndall effect Not shown Shown Shown

The components of a colloid have their own names: the solute-like dispersed particles form the
dispersed phase, and the substance they are spread through is the dispersion medium.

Why the colloid is the interesting middle case: Its particles are small enough that
random molecular bombardment keeps them permanently suspended, so like a
solution it never settles and passes through filter paper. But they are large enough
to scatter light, so unlike a solution it shows the Tyndall effect and is really a two-
phase, heterogeneous mixture. Size alone explains every entry in the table.

Q2 Can you think of some other substances that could be colloids?

Colloids are everywhere in daily life — in the kitchen, in the bathroom shelf, in the sky and in
your own body.

Page 45 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

COLLOID DISPERSED PHASE DISPERSION MEDIUM

Milk Liquid fat droplets Water (an oil-in-water emulsion)

Butter Water droplets Fat (a water-in-oil emulsion)

Blood Blood cells and proteins Plasma

Ice cream, tomato sauce Fat and solid particles Water

Cloud, fog, mist Water droplets Air

Smoke Solid carbon particles Air

Cold cream, body lotion, vanishing cream Water or oil droplets Oil or water

Try this: Shake a few drops of cooking oil into water containing a few drops of soap
solution. You get a milky mixture that does not separate for a long time — an
emulsion. The soap acts as an emulsifying agent, holding the oil droplets apart. In
milk and butter, proteins do the same job.

In-text Questions — Page 88
5.5 Tyndall Effect

Q1 Have you ever observed light scattering by particles in your surroundings? Think of
some more examples!

Yes — wherever a beam of light passes through air or a liquid that carries suspended particles,
its path becomes visible. That is the Tyndall effect.

A shaft of sunlight entering a dark room through a small hole or a gap in the curtain, picked
out by the dust in the air.
Sunlight streaming through the gaps between the leaves of a dense tree, or through mist in
an early-morning forest.
The beams of floodlights in a sports stadium (Fig. 5.24), and the cone of light from a cinema
projector.
Car headlights on a foggy night; the beam is visible far ahead because water droplets scatter
the light.
A laser or a torch shone through milk diluted with water, or through a beaker of dilute soap
solution.

Page 46 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

The blue of the sky and the red of the setting sun — sunlight scattered by the molecules and
fine particles of the atmosphere.

Why the beam becomes visible: You see an object only when light from it enters
your eye. A beam travelling across your line of sight sends you nothing — unless
something turns part of it sideways. Suspended particles do exactly that: each one
scatters a little of the beam in all directions, and the fraction that reaches your eye
traces out the path of the beam. Clean air and true solutions have nothing big
enough to scatter light, so in them the beam remains invisible.

Pause and Ponder — Page 88
5.5 Tyndall Effect

PAUSE AND PONDER

Q9 Clouds are made up of tiny water droplets or ice crystals floating in the air. Based
on what you know about solutions, suspensions and colloids, what type of mixture
do you think clouds are and why?

A cloud is a colloid — an aerosol, with tiny liquid water droplets or ice crystals as the dispersed
phase and air as the dispersion medium.

TEST WHAT A CLOUD DOES CONCLUSION

Are the droplets dissolved in No — they stay as separate droplets of Not a solution
the air? liquid water

Do they settle out? No — a cloud floats for hours or days Not a suspension

Does it scatter light? Yes — that is why clouds look white and Shows the Tyndall effect
cast shadows

Do you see separate No — the cloud looks like one smooth mass Particles are of colloidal size → a
droplets? colloid

Page 47 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Why the droplets do not fall: Cloud droplets are only a few micrometres across, so
their weight is minute while the drag of the air on them is relatively large; rising air
currents easily hold them up. They fall as rain only when many droplets coalesce into
a drop big enough for its weight to beat the updraught. That is also why a cloud is
white — the droplets scatter all the colours of sunlight almost equally, and all
colours together look white.

Q10 Why do cities with a lot of smoke and dust in the air often look hazy?

Because the smoke and dust particles suspended in the city air scatter sunlight in all
directions — the Tyndall effect on the scale of a whole city.

Why it produces haze: Light coming from a distant building travels to your eye in a
straight line and carries the image of that building. On the way it meets millions of
smoke and dust particles, and each of them turns some of that light aside. Two
things follow. First, the light from the building is weakened, so the building looks
faint. Second, sunlight that was never headed towards you is scattered into your line
of sight, adding a uniform grey-white glow over the whole scene. The image loses
contrast and everything looks washed out and blurred — that is haze.

Did you know? Particles of colloidal size — from vehicle exhaust, construction dust
and the burning of fuel — are the worst offenders. They are small enough to stay
suspended in the air for days, and small enough to be breathed deep into the lungs,
which is why the same haze that hides the skyline is also a health warning.

Activity 5.9 — Page 88

Page 48 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

5.5 Tyndall Effect
co m
em.
m as
ACTIVITY

.co a g l
a s em Table 5.1 and review what you have learnt about solutions, suspensions
gl
Complete
a
Q1
and colloids.

co m
em . ag
S. PROPERTY
g l as SOLUTION SUSPENSION COLLOID
NO. a
1. Nature
co m
(homogeneous/heterogeneous)
em.
m l as
.c2.o a g
sem
Particle size

a
agl 3. Visibility

m a s
agl
4. Separation by filtration

m .co
5. Settling
l a se
6. Tyndall effect a g

co m
m
Table 5.1, page 88 — Properties of different types of mixtures (as printed, with the cells
.
as e
com
left blank for you to fill).

. a g l
m
ase
agl ANSWER
se m
Table 5.1: Properties of different types of mixtures
com g l a
m . a
ase
agl

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

co m
m .
m as e
.co


a g l Page 49 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

S. PROPERTY SOLUTION SUSPENSION COLLOID
NO.

1. Nature Homogeneous Heterogeneous Heterogeneous, though
(homogeneous / it looks homogeneous
heterogeneous)

2. Particle size Less than 1 nm More than 1000 nm 1 – 1000 nm

3. Visibility Particles not visible; Particles visible to Particles not visible;
the mixture is the naked eye the mixture looks
transparent uniform

4. Separation by Cannot be separated Can be separated — Cannot be separated
filtration leaves a residue

5. Settling Does not settle Settles on standing Does not settle
(sedimentation)

6. Tyndall effect Not shown Shown Shown

Examples: solution — salt or sugar in water, vinegar, brass; suspension — chalk powder in
water, muddy water, sand in water; colloid — milk, blood, smoke, ice cream, cloud.

The single idea behind all six rows: particle size. Below 1 nm the particles are so
small that they neither settle, nor get caught by filter paper, nor scatter light —
everything about a solution follows. Above 1000 nm they are heavy enough to settle,
big enough to be trapped by filter paper and visible to the eye. In between, a colloid
inherits half of each list: it behaves like a solution when you filter it or let it stand,
and like a suspension when you shine a beam of light through it.

In-text Questions — Page 89
Separation in nature and in daily life

Q1 Imagine trying to separate all the ingredients in a lemonade once they have been
mixed. Can you do it?

Not completely — and certainly not by any one method. Lemonade (shikanji) is water with
dissolved sugar, salt, lemon juice with its acids and pigments, and often bits of pulp and ice. You
can get some components back, but not every one of them in pure form.

Page 50 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

COMPONENT METHOD THAT WOULD HOW WELL
WORK

Pulp and seeds Filtration Easily removed — they are a suspension

Water Distillation Recovered pure

Sugar and salt together Evaporation or crystallization Recovered, but mixed with each other

Coloured pigments of Paper chromatography Only in traces, not in usable amounts
lemon

Sugar separated from salt Repeated fractional crystallization Very difficult — both are soluble in
water

Why mixing is easier than unmixing: Every separation technique works by
exploiting a difference — in particle size, in solubility, in boiling point, in density, in
how fast a substance travels up paper. Where two components share the property a
method depends on, that method cannot tell them apart. Sugar and salt are both
white, both soluble in water and both non-volatile, so filtering, distilling or
evaporating treats them identically. The more components a mixture has, the more
separate differences you must find, and the longer the chain of techniques becomes.

Did you know? Nature and industry face the same difficulty on a far larger scale —
removing plastic from the oceans, recovering lithium from used mobile-phone
batteries, or treating sewage before it is released. Your own kidneys do it every
minute, separating waste from blood while keeping the useful substances in.

Revise, Reflect, Refine — Pages 90 – 93
End-of-chapter questions

REVISE, REFLECT, REFINE

Q1 Which of the following mixtures are correctly classified as homogeneous (Hm) and
heterogeneous (Ht)? Choose the correct option. (i) Air — Hm, Milk — Ht, Sugar
solution — Hm, Smoke — Hm (ii) Brass — Ht, Fog — Ht, Vinegar — Ht, Muddy water
— Hm (iii) Copper sulfate solution — Hm, Salt solution — Hm, Milk — Hm, Bronze —
Hm (iv) Muddy water — Ht, Milk — Ht, Blood — Ht, Brass — Hm

The correct option is (iv) Muddy water — Ht, Milk — Ht, Blood — Ht, Brass — Hm.

Page 51 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

OPTION THE MISTAKE IN IT

(i) Smoke is called Hm. Smoke is solid carbon particles dispersed in air — a heterogeneous colloid.

(ii) Three mistakes: brass is an alloy and therefore Hm; vinegar is acetic acid dissolved in water and
therefore Hm; muddy water is a suspension and therefore Ht.

(iii) Milk is called Hm. Milk is a colloid — fat droplets dispersed in water — and so is heterogeneous.

(iv) All four are right: muddy water (suspension) Ht, milk (colloid) Ht, blood (colloid) Ht, brass (alloy)
Hm.

Why milk and blood count as heterogeneous: They look uniform to the eye, but
they are two-phase mixtures — droplets or cells of one substance dispersed through
another, with a real boundary around every particle. Only a mixture that is uniform
right down to the level of individual particles, such as a salt solution or a molten-and-
cooled alloy, is homogeneous. The Tyndall effect is the practical test: milk and blood
scatter a beam of light, a sugar solution does not.

Q2 Choose the correct options, and explain the reason for the correct and incorrect
options. Which among the following mixtures show the Tyndall Effect? A mixture of:
(a) air and dust particles (b) copper sulfate and water (c) starch and water (d)
acetone and water — (i) a and b (ii) b and d (iii) a and c (iv) c and d

The correct option is (iii) a and c.

MIXTURE TYPE PARTICLE SIZE TYNDALL EFFECT?

(a) Air and dust particles Suspension / aerosol > 1000 nm Yes — shows it

(b) Copper sulfate and water True solution < 1 nm No

(c) Starch and water Colloid 1 – 1000 nm Yes — shows it

(d) Acetone and water Solution of two miscible liquids < 1 nm No

Why (a) and (c) show it: both contain particles large enough to turn part of a light beam
sideways, so the path of the beam becomes visible from the side.
Why (b) and (d) do not: in both, the substances are dissolved to the level of individual ions or
molecules, far smaller than the wavelength of light (about 400 – 700 nm). Such particles barely
disturb the light wave, so nothing is scattered and the beam passes through unseen. That

Page 52 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

copper sulfate solution is blue makes no difference — colour comes from absorption, scattering
is a different phenomenon altogether.

The rule to remember: a mixture shows the Tyndall effect only if its dispersed
particles are at least about as large as the wavelength of light. That is true for every
colloid and every suspension, and false for every true solution.

Q3 A mixture can be categorised as a solution, a suspension, or a colloid, each
possessing distinct properties. Utilise the words or phrases provided in the box to
fill in the Table 5.2. Words and phrases may be used more than once. [Large-sized
particles; Particles remain evenly distributed; Small-sized particles (less than 1 nm
diameter); Moderate-sized particles (1 – 1000 nm); Settles down when left
undisturbed (more than 1000 nm in diameter); Does not settle down; Scatters light;
Separates by filtration; Transparent; Salt solution; Milk; Sand in water; Smoke;
Heterogeneous mixture; Cannot be separated by filtration; Mud; Butter; Brass.]

SOLUTION SUSPENSION COLLOID

Properties Properties Properties
______________ ______________ ______________
______________ ______________ ______________

Examples Examples Examples
______________ ______________ ______________
______________ ______________ ______________

Table 5.2, page 90 — as printed, with blanks for the properties and examples of each
type of mixture.

Table 5.2 completed

Page 53 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

co m
m.
SOLUTION SUSPENSION COLLOID

as e
com1 nm diameter);
.than
Small-sized particles (less Large-sized particles; Settles
a g l
Moderate-sized particles (1 –
Properties

a s em Transparent; Particles down when left undisturbed 1000 nm); Particles remain

ag l remain evenly
(more than 1000 nm in
diameter); Separates by
evenly distributed; Does not
settle down; Cannot be
distributed; Does not filtration; Scatters light; separated by filtration;

m
.co ag
settle down; Cannot be Heterogeneous mixture Scatters light; Heterogeneous

sem
separated by filtration mixture

a
Examples Salt solution; Brass
agl Sand in water; Mud Milk; Smoke; Butter

co m
m.
How to place each phrase: work from particle size. Small particles cannot settle,

o m l a se
cannot be trapped by filter paper and cannot scatter light — that gives the whole
.c column, and salt solution and brass are the two examplesa g in the box that
m
se uniform at particle level. Large particles do all three, giving the Suspension
Solution

g l a
are
a column with sand in water and mud. The Colloid column then takes one property

m a s
agl
from each side: it does not settle and cannot be filtered (like a solution), but it

m .co
scatters light and is heterogeneous (like a suspension). Milk, smoke and butter all fit

l a se
g
that description.
a
Tip: “Scatters light” and “Heterogeneous mixture” are used twice each — the
co m
m .
question warns you that some phrases repeat, and these are the ones.
m as e
.co a g l
se m
g l a
a Solve the following problems: (i) A cake recipe uses dry ingredients, namely 75 g of
m
Q4

m
sugar for 420 g of all-purpose flour and 5 g of sodium hydrogencarbonate. Express
o l a se
the concentration of each component
c
.copper in the mixture using an appropriate method.
a g
se m
a
(ii) A brass alloy contains 70% by mass. Calculate the quantities of copper
and zinc present in 120 l
ag of brass.
g

. c om
m a s em by mass percentage
(i) All three ingredients are solids, so the appropriate method is the mass
o
(%cm/m).
. a gl
a s em
agl c
m .
m a s e
e m . co agl
g l as
a

co m
m .
m ase
.co


a g l Page 54 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Total mass of the mixture = 75 g + 420 g + 5 g = 500 g

% m/m of sugar = (75 g ÷ 500 g) × 100 = 15 % m/m

% m/m of flour = (420 g ÷ 500 g) × 100 = 84 % m/m

% m/m of sodium hydrogencarbonate = (5 g ÷ 500 g) × 100 = 1 % m/m

Check: 15 % + 84 % + 1 % = 100 % ✓

(ii) “70 % copper by mass” means 70 g of copper in every 100 g of brass.

Mass of copper = (70 ÷ 100) × 120 g = 84 g

Mass of zinc = 120 g − 84 g = 36 g

Check: % m/m of zinc = (36 g ÷ 120 g) × 100 = 30 %, and 70 % + 30 % = 100 % ✓

Why % m/m and not % m/v here: mass by volume percentage needs a volume of
solution, and powders and solid alloys have no well-defined solution volume — a jar
of flour also holds air between the grains, so its volume is not a reliable measure of
how much flour there is. Mass does not suffer from that problem, so a solid mixture
is always described by mass. The chapter uses the same convention for milk powder
and spice mixtures.

Tip: The percentages of all the components of a mixture must add up to 100. Use
that as a quick check on every concentration calculation.

Q5 The label on a cooking oil pack says one litre (910 g). If this oil is mixed with water,
will it form a separate layer? If so, which substance will be on top? How will you
separate the two layers? Also, draw the diagram of the apparatus used.

Yes, it forms a separate layer, and the oil floats on top. The two are separated with a
separating funnel.
Step 1 — find the density of the oil

Page 55 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Density = mass ÷ volume

Volume of oil = 1 L = 1000 mL = 1000 cm³, mass = 910 g

Density of oil = 910 g ÷ 1000 cm³ = 0.91 g cm⁻³
Density of water = 1.00 g cm⁻³

Step 2 — compare. 0.91 g cm⁻³ < 1.00 g cm⁻³, so the oil is lighter than an equal volume of water.
Oil and water are also immiscible. Hence they form two layers, with the oil as the upper layer
and water below.

Glass stopper

Cooking oil, density
0.91 g cm⁻³ (upper layer)

Stand Water, density
1.00 g cm⁻³ (lower layer)

Stopcock

Conical flask —
water drained first

Separating funnel. The less dense oil floats as the upper layer; opening the stopcock lets the denser
water run out first.

Step 3 — how to separate them

1. Pour the mixture into a separating funnel mounted on a laboratory stand and close it with
the glass stopper.
2. Let it stand undisturbed until two clear layers form with a sharp boundary.
3. Remove the stopper and open the stopcock slowly. The lower layer (water) runs out into a
conical flask.
4. Close the stopcock the moment the boundary reaches the tap. Collect the small mixed
portion separately and discard it.

Page 56 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

5. Open the stopcock again and collect the upper layer (oil) in a fresh, clean container.

Why this works: A separating funnel exploits exactly two facts — that the liquids do
not mix, and that they have different densities. Gravity does the sorting on its own,
arranging the denser liquid at the bottom; the tap at the very bottom then lets you
draw off that liquid alone. No heating, no chemicals and no filter paper are needed.

Q6 Assertion (A): Solutions do not exhibit the Tyndall effect. Reason (R): The particles in
solutions are larger than 100 nm, so they cannot scatter light. Choose the correct
option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and
R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A
is false, but R is true.

The correct option is (iii) A is true, but R is false.
Assertion — true. A true solution does not show the Tyndall effect. Shine a laser through a salt
or copper sulfate solution and the path of the beam cannot be seen from the side.
Reason — false, on two counts.

Stated in R: particle size of a solution > 100 nm ✗

Correct value: particle size of a solution < 1 nm ✓

The size is wrong. Solute particles in a solution are smaller than 1 nm, not larger than 100
nm.
The logic is wrong too. Being larger would make a particle scatter light more, not less. It is
precisely because the particles are so small that they fail to scatter.

The physics behind it: Visible light has wavelengths of about 400 – 700 nm. A
particle much smaller than that wavelength hardly disturbs the passing light wave,
so almost nothing is scattered sideways and the beam stays invisible. Once particles
reach roughly the size of the wavelength — as in a colloid (1 – 1000 nm) or a
suspension (> 1000 nm) — they scatter strongly and the beam lights up.

Tip: In assertion–reason questions, always check the reason on its own before
asking whether it explains the assertion. Here the assertion is a correct fact but the
reason contains a factual error, which forces option (iii).

Page 57 of 80

Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

Q7 How would you separate the mixtures given in Table 5.3? Mention the reason for
choosing your method. If a mixture cannot be separated, explain why.

MIXTURE METHOD OF REASON FOR
SEPARATION SELECTION

Mud from muddy water

Plasma from other components in
the blood sample

Naphthalene and sand

Chalk powder and common salt

Common salt and water

Oil from water

Pigments of the flower

Table 5.3, page 91 — as printed, with the two right-hand columns blank.

Table 5.3 completed

Page 58 of 80

as e
a g l
Class 9 Science Chapter 5 Exploring Mixtures and their Separation AglaSem · NCERT Solutions

co m
m.
MIXTURE METHOD OF REASON FOR SELECTION

as e
com
SEPARATION

. a g l
wateras
em
Mud from muddy Sedimentation and decantation, Mud is an insoluble suspension with particles

agl
then filtration; add alum larger than 1000 nm, so it settles and is held back
(coagulation) or centrifuge if it by filter paper. Very fine particles need to be
stays cloudy clumped by a coagulant or thrown down by
m
.co ag
centrifugation.

a sem
agl
Plasma from other Centrifugation Blood is a colloid; its cells do not settle under
components in the gravity and pass through filter paper. Spinning
blood sample produces a much stronger outward force, and the

co m
denser cells collect at the bottom leaving the
lighter plasma on top.
se m.
coandm Naphthalene changesg l a
sandem
.
Naphthalene Sublimation
a directly from solid to
l as
vapour below its melting point and is recovered by

a g deposition on a cool surface; sand does not
sublime and is left behind.

om Salt is soluble in water and chalk is not. Filtration a s
Chalk powder and Add water, filter, then

em
. c agl
a s takes out the chalk as residue; evaporating the

agl
common salt evaporate or crystallise the
filtrate filtrate returns the salt.

om
Common salt and Evaporation (to recover the Salt is a non-volatile solid dissolved in the water,
water salt) or distillation (to recover
Distillation also condensesm . c
so it stays behind when the water is driven off.

m
both)
a s e and collects the water.

m . co agl and have different
ase
Oil from water Separating funnel The two are immiscible

agl
densities, so they form two layers and the lower
one can be run off through the stopcock.

se m
com g l a
.
Pigments of the Paper chromatography The pigments differ in how strongly they are held by
flower
e m the paper and how readily they dissolve in the solvent, a
a s
agl
so they travel different distances up the strip.

The common thread: every method here picks out one difference between the
co m
m .
e
components — particle size (filtration, centrifugation), the ability to sublime
m l as
.co g
(sublimation), solubility (dissolve-and-filter), volatility (evaporation, distillation),

em a
s
density (separating funnel) or rate of movement on paper (chromatography).
l a
ag
Choosing a technique is really a matter of asking: in what single property do these
.c
m
two components differ most?

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

co m
m .
m as e
.co


a g l Page 59 of 80