Fig. 2.1: Some consumable items

Have you ever noticed the word ‘pure’

written on the packs of these consumables?

For a common person pure means having no

adulteration. But, for a scientist all these things

are actually mixtures of different substances

and hence not pure. For example, milk is

actually a mixture of water, fat, proteins, etc.

When a scientist says that something is pure,

it means that all the constituent particles of

that substance are the same in their chemical

nature. A pure substance consists of a single

type of particle. In other words, a substance is

a pure single form of matter.

As we look around, we can see that most

of the matter around us exists as mixtures of

two or more pure components, for example,

sea water, minerals, soil, etc., are all mixtures.

2.1 What is a Mixture?

Mixtures are constituted by more than one

kind of pure form of matter. We know that

dissolved sodium chloride can be separated

from water by the physical process of

evaporation. However, sodium chloride is itself

a pure substance and cannot be separated by

physical process into its chemical constituents.

Similarly, sugar is a substance which contains

only one kind of pure matter and its

composition is the same throughout.

Soft drink and soil are not single pure

substances. Whatever the source of a

pure substance may be, it will always have

the same characteristic properties.

Therefore, we can say that a mixture

contains more than one pure substance.

2.1.1 TYPES

OF MIXTURES

Depending upon the nature of the components

that form a mixture, we can have different

types of mixtures.

Activity ______________ 2.1

• Let us divide the class into groups A,

B, C and D.

• Group A takes a beaker containing

50 mL of water and one spatula full

of copper sulphate powder. Group B

takes 50 mL of water and two spatula

full of copper sulphate powder in a

beaker.

• Groups C and D can take different

amounts of copper sulphate and

potassium permanganate or common

salt (sodium chloride) and mix the

given components to form a mixture.

• Report the observations on the

uniformity in colour and texture.

• Groups A and B have obtained a

mixture which has a uniform

composition throughout. Such

mixtures are called homogeneous

mixtures or solutions. Some other

examples of such mixtures are: (i) salt

dissolved in water and (ii) sugar

dissolved in water. Compare the

How do we judge whether milk, ghee, butter,

salt, spices, mineral water or juice that we buy

from the market are pure?

M M

ATTERATTER

ATTER

A A

ROUNDROUND

ROUND

U U

P P

UREURE

URE

hapter

Rationalised 2023-24

More to know

colour of the solutions of the two

groups. Though both the groups have

obtained copper sulphate solution but

the intensity of colour of the solutions

is different. This shows that a

homogeneous mixture can have a

variable composition.

• Groups C and D have obtained

mixtures, which contain physically

distinct parts and have non-uniform

compositions. Such mixtures are called

heterogeneous mixtures. Mixtures of

sodium chloride and iron filings, salt

and sulphur, and oil and water are

examples of heterogeneous mixtures.

Activity ______________ 2.2

• Let us again divide the class into four

groups— A, B, C and D.

• Distribute the following samples to

each group:

− Few crystals of copper sulphate

to group A.

− One spatula full of copper

sulphate to group B.

− Chalk powder or wheat flour to

group C.

− Few drops of milk or ink to

group D.

• Each group should add the given

sample in water and stir properly

using a glass rod. Are the particles

in the mixture visible?

• Direct a beam of light from a torch

through the beaker containing the

mixture and observe from the front. Was

the path of the beam of light visible?

• Leave the mixtures undisturbed for

a few minutes (and set up the

filtration apparatus in the meantime).

Is the mixture stable or do the

particles begin to settle after some

time?

• Filter the mixture. Is there any

residue on the filter paper?

• Discuss the results and form an

opinion.

Groups A and B have got a solution.

Group C has got a suspension.

Group D has got a colloidal solution.

Now, we shall learn about solutions,

suspensions and colloidal solutions in the

following sections.

uestions

1. What is meant by a substance?

2. List the points of differences

between homogeneous and

heterogeneous mixtures.

2.2 What is a Solution?

A solution is a homogeneous mixture of two

or more substances. You come across various

types of solutions in your daily life. Lemonade,

soda water, etc., are all examples of solutions.

Usually we think of a solution as a liquid that

contains either a solid, liquid or a gas dissolved

in it. But, we can also have solid solutions

(alloys) and gaseous solutions (air). In a

solution there is homogeneity at the particle

level. For example, lemonade tastes the same

throughout. This shows that particles of sugar

or salt are evenly distributed in

the solution.

Fig. 2.2: Filtration

Alloys: Alloys are mixtures of two or

more metals or ametal and a non-metal

and cannot be separated into their

components by physical methods. But

still, an alloy is considered as a mixture

because it shows the properties of its

constituents and can have variable

composition. For example, brass is a

mixture of approximately 30% zinc and

70% copper.

IS MATTER AROUND US

PURE?

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SCIENCE16

A solution has a solvent and a solute as its

components. The component of the solution

that dissolves the other component in it

(usually the component present in larger

amount) is called the solvent. The component

of the solution that is dissolved in the solvent

(usually present in lesser quantity) is called

the solute.

Examples:

(i) A solution of sugar in water is a solid

in liquid solution. In this solution,

sugar is the solute and water is

the solvent.

(ii) A solution of iodine in alcohol known

as ‘tincture of iodine’, has iodine (solid)

as the solute and alcohol (liquid) as the

solvent.

(iii) Aerated drinks like soda water, etc., are

gas in liquid solutions. These contain

carbon dioxide (gas) as solute and water

(liquid) as solvent.

(iv) Air is a mixture of gas in gas. Air is a

homogeneous mixture of a number of

gases. Its two main constituents are:

oxygen (21%) and nitrogen (78%). The

other gases are present in very

small quantities.

Properties of a Solution

• A solution is a homogeneous mixture.

• The particles of a solution are smaller

than 1 nm (10

-9

metre) in diameter. So,

they cannot be seen by naked eyes.

• Because of very small particle size, they

do not scatter a beam of light passing

through the solution. So, the path of

light is not visible in a solution.

• The solute particles cannot be separated

from the mixture by the process of

filtration. The solute particles do not

settle down when left undisturbed, that

is, a solution is stable.

2.2.1 CONCENTRATION OF A SOLUTION

In activity 2.2, we observed that groups A and

B obtained different shades of solutions. So,

we understand that in a solution the relative

proportion of the solute and solvent can be

varied. Depending upon the amount of solute

present in a solution, it can be called dilute,

concentrated or saturated solution. Dilute

and concentrated are comparative terms. In

activity 2.2, the solution obtained by group

A is dilute as compared to that obtained by

group B.

Activity ______________ 2.3

• Take approximately 50 mL of water

each in two separate beakers.

• Add salt in one beaker and sugar or

barium chloride in the second beaker

with continuous stirring.

• When no more solute can be dissolved,

heat the contents of the beaker to raise

the temperature by about 5

C.°

• Start adding the solute again.

Is the amount of salt and sugar or barium

chloride, that can be dissolved in water at a

given temperature, the same?

At any particular temperature, a solution

that has dissolved as much solute as it is

capable of dissolving, is said to be a saturated

solution. In other words, when no more solute

can be dissolved in a solution at a given

temperature, it is called a saturated solution.

The amount of the solute present in the

saturated solution at this temperature is called

its solubility.

If the amount of solute contained in a

solution is less than the saturation level, it is

called an unsaturated solution.

What would happen if you were to take a

saturated solution at a certain temperature

and cool it slowly.

We can infer fr

om the above activity that

different substances in a given solvent have

different solubilities at the same temperature.

The concentration of a solution is the amount

(mass or volume) of solute present in a given

amount (mass or volume) of solution.

There are various ways of expressing the

concentration of a solution, but here we will

learn only three methods.

(i) Mass by mass percentage of a solution

Mass of solute

×100

Mass of solution

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IS MATTER AROUND US PURE? 17

(ii) Mass by volume percentage of a solution

Mass of solute

×100

Volume of solution

(iii) Volume by volume percentage of a

solution

Volume of solute

×100

Volume of solution

Example 2.1 A solution contains 40 g of

common salt in 320 g of water. Calculate

the concentration in terms of mass by

mass percentage of the solution.

Solution:

Mass of solute (salt) = 40 g

Mass of solvent (water) = 320 g

We know,

Mass of solution = Mass of solute +

Mass of solvent

= 40 g + 320 g

= 360 g

Mass percentage of solution

Mass of solute

× 100

Mass of solution

= ×100 =11.1%

360

2.2.2 WHAT IS A SUSPENSION?

Non-homogeneous systems, like those

obtained by group C in activity 2.2, in which

solids are dispersed in liquids, are called

suspensions. A suspension is a heterogeneous

mixture in which the solute particles do not

dissolve but remain suspended throughout

the bulk of the medium. Particles of a

suspension are visible to the naked eye.

Properties of a Suspension

• Suspension is a heterogeneous

mixture.

• The particles of a suspension can be seen

by the naked eye.

• The particles of a suspension scatter a

beam of light passing through it and

make its path visible.

• The solute particles settle down when a

suspension is left undisturbed, that is,

a suspension is unstable. They can be

separated from the mixture by the

process of filtration. When the particles

settle down, the suspension breaks and

it does not scatter light any more.

2.2.3 WHAT IS

A COLLOIDAL SOLUTION?

The mixture obtained by group D in activity

2.2 is called a colloid or a colloidal solution.

The particles of a colloid are uniformly spread

throughout the solution. Due to the relatively

smaller size of particles, as compared to that of

a suspension, the mixture appears to be

homogeneous. But actually, a colloidal solution

is a heterogeneous mixture, for example, milk.

Because of the small size of colloidal

particles, we cannot see them with naked eyes.

But, these particles can easily scatter a beam

of visible light as observed in activity 2.2. This

scattering of a beam of light is called the

Tyndall effect after the name of the scientist

who discovered this effect.

Tyndall effect can also be observed when a

fine beam of light enters a room through a small

hole. This happens due to the scattering of light

by the particles of dust and smoke in the air.

Fig. 2.3: (a) Solution of copper sulphate does not

show Tyndall effect, (b) mixture of water

and milk shows Tyndall effect.

(a) (b)

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SCIENCE18

• Colloids are big enough to scatter a

beam of light passing through it and

make its path visible.

• They do not settle down when left

undisturbed, that is, a colloid is quite

stable.

• They cannot be separated from the

mixture by the process of filtration. But,

a special technique of separation known

as centrifugation (perform activity 2.5),

can be used to separate the colloidal

particles.

The components of a colloidal solution are

the dispersed phase and the dispersion

medium. The solute-like component or the

dispersed particles in a colloid form the

dispersed phase, and the component in which

the dispersed phase is suspended is known

as the dispersing medium. Colloids are

classified according to the state (solid, liquid

or gas) of the dispersing medium and the

dispersed phase. A few common examples are

given in Table 2.1. From this table you can

see that they are very common everyday life.

Tyndall effect can be observed when

sunlight passes through the canopy of a dense

forest. In the forest, mist contains tiny droplets

of water, which act as particles of colloid

dispersed in air.

Fig. 2.4: The Tyndall effect

uestions

1. Differentiate between homogen- eous and heterogeneous mixtures with examples.

2. How are sol, solution and suspension different from each other?

3. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water

at 293 K. Find its concentration at this temperature.

Properties of a Colloid

• A colloid is a heterogeneous mixture.

• The size of particles of a colloid is too

small to be individually seen with

naked eyes.

Table 2.1: Common examples of colloids

Dispersed Dispersing Type Example

phase Medium

Liquid Gas Aerosol Fog, clouds, mist

Solid Gas Aerosol Smoke, automobile exhaust

Gas Liquid Foam Shaving cream

Liquid Liquid Emulsion Milk, face cream

Solid Liquid Sol Milk of magnesia, mud

Gas Solid Foam Foam, rubber, sponge, pumice

Liquid Solid Gel Jelly, cheese, butter

Solid Solid Solid Sol Coloured gemstone, milky glass

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IS MATTER AROUND US PURE? 19

2.3 Physical and Chemical

Changes

In the previous chapter, we have learnt about

a few physical properties of matter. The

properties that can be observed and specified

like colour, hardness, rigidity, fluidity,

density, melting point, boiling point etc. are

the physical properites.

The interconversion of states is a physical

change because these changes occur without

a change in composition and no change in the

chemical nature of the substance. Although

ice, water and water vapour all look different

and display different physical properties, they

are chemically the same.

Both water and cooking oil are liquid but

their chemical characteristics are different.

They differ in odour and inflammability. We

know that oil burns in air whereas water

extinguishes fire. It is this chemical property

of oil that makes it different from water.

Burning is a chemical change. During this

process one substance reacts with another to

undergo a change in chemical composition.

Chemical change brings change in the

chemical properties of matter and we get new

substances. A chemical change is also called

a chemical reaction.

During burning of a candle, both physical

and chemical changes take place. Can you

distinguish these?

uestions

1. Classify the following as chemical

or physical changes:

• cutting of trees,

• melting of butter in a pan,

• rusting of almirah,

• boiling of water to form steam,

• passing of electric current,

through water and the water

breaking down into hydrogen

and oxygen gases,

• dissolving common salt in

water,

• making a fruit salad with raw

fruits, and

• burning of paper and wood.

2. Try segregating the things

around you as pure substances

or mixtures.

2.4 What are the Types of Pure

Substances?

On the basis of their chemical composition,

substances can be classified either as elements

or compounds.

2.4.1 ELEMENTS

Robert Boyle was the first scientist to use the

term element in 1661. Antoine Laurent

Lavoisier (1743–94), a French chemist, was the

first to establish an experimentally useful

definition of an element. He defined an element

as a basic form of matter that cannot be broken

down into simpler substances by chemical

reactions.

Elements can be normally divided into

metals, non-metals and metalloids.

Metals usually show some or all of the

following properties:

• They have a lustre (shine).

• They have silvery-grey or golden-yellow

colour.

• They conduct heat and electricity.

• They are ductile (can be drawn into

wires).

• They are malleable (can be hammered

into thin sheets).

• They are sonorous (make a ringing

sound when hit).

Examples of metals are gold, silver,

copper, iron, sodium, potassium etc. Mercury

is the only metal that is liquid at room

temperature.

Non-metals usually show some or all of the

following properties:

• They display a variety of colours.

• They are poor conductors of heat and

electricity.

• They are not lustrous, sonorous

or malleable.

Examples of non-metals are hydrogen,

oxygen, iodine, carbon (coal, coke), bromine,

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SCIENCE20

chlorine etc. Some elements have intermediate

properties between those of metals and non-

metals, they are called metalloids; examples

are boron, silicon, germanium, etc.

More to know

2.4.2 COMPOUNDS

A compound is a substance composed of two

or more elements, chemically combined with

one another in a fixed proportion.

What do we get when two or more elements

are combined?

Activity ______________ 2.4

• Divide the class into two groups. Give

5 g of iron filings and 3 g of sulphur

powder in a china dish to both

the groups.

Group I

• Mix and crush iron filings and

sulphur powder.

Group II

• Mix and crush iron filings and

sulphur powder. Heat this mixture

strongly till red hot. Remove from

flame and let the mixture cool.

Groups I and II

• Check for magnetism in the material

obtained. Bring a magnet near the

material and check if the material is

attracted towards the magnet.

• Compare the texture and colour of the

material obtained by the groups.

• Add carbon disulphide to one part of

the material obtained. Stir well

and filter.

• Add dilute sulphuric acid or dilute

hydrochloric acid to the other part of

the material obtained.(Note: teacher

supervision is necessary for

this activity).

• Perform all the above steps with both

the elements (iron and sulphur)

separately.

Now answer

• Did the material obtained by the two

groups look the same?

• Which group has obtained a material

with magnetic properties?

• Can we separate the components of the

material obtained?

• On adding dilute sulphuric acid or

dilute hydrochloric acid, did both the

groups obtain a gas? Did the gas in

both the cases smell the same

or different?

The gas obtained by Group I is hydrogen,

it is colourless, odourless and combustible– it

is not advised to do the combustion test for

hydrogen in the class. The gas obtained by

Group II is hydrogen sulphide. It is a colourless

gas with the smell of rotten eggs.

You must have observed that the products

obtained by both the groups show different

properties, though the starting materials were

the same. Group I has carried out the activity

involving a physical change whereas in case

of Group II, a chemical change (a chemical

reaction) has taken place.

• The material obtained by group I is a

mixture of the two substances. The

substances given are the elements—iron

and sulphur.

• The properties of the mixture are the

same as that of its constituents.

• The number of elements

known at present are more

than 100. Ninety-two elements

are naturally occurring and the

rest are man-made.

• Majority of the elements are

solid.

• Eleven elements are in gaseous

state at room temperature.

• Two elements are liquid at

room temperature—mercury

and bromine.

• Elements, gallium and cesium

become liquid at a temperature

slightly above room

temperature (303 K).

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IS MATTER AROUND US PURE? 21

• The material obtained by group II is a

compound.

• On heating the two elements strongly we

get a compound, which has totally

different properties compared to the

combining elements.

• The composition of a compound is the

same throughout. We can also observe

that the texture and the colour of the

compound are the same throughout.

Thus, we can summarise the physical

and chemical nature of matter in the

following graphical organiser:

Table 2.2: Mixtures and Compounds

Mixtures Compounds

1. Elements or compounds just mix 1. Elements react to form new compounds.

together to form a mixture and no

new compound is formed.

2. A mixture has a variable composition. 2. The composition of each new substance

is always fixed.

3, A mixture shows the properties of the 3. The new substance has totally different

constituent substances. properties.

4. The constituents can be seperated 4. The constituents can be separated only

fairly easily by physical methods. by chemical or electrochemical

reactions.

What

you have

learnt

• A mixture contains more than one substance (element and/

or compound) mixed in any proportion.

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SCIENCE22

Exercises

1. Which separation techniques will you apply for the separation

of the following?

(a) Sodium chloride from its solution in water

(b) Ammonium chloride from a mixture containing sodium

chloride and ammonium chloride

(d) Different pigments from an extract of flower petals

(e) Butter from curd

(f) Oil from water

(g) Tea leaves from tea

(h) Iron pins from sand

(i) Wheat grains from husk

(j) Fine mud particles suspended in water

• Mixtures can be separated into pure substances using

appropriate separation techniques.

• A solution is a homogeneous mixture of two or more

substances. The major component of a solution is called the

solvent, and the minor, the solute.

• The concentration of a solution is the amount of solute present

per unit volume or per unit mass of the solution.

• Materials that are insoluble in a solvent and have particles

that are visible to naked eyes, form a suspension.

A suspension is a heterogeneous mixture.

• Colloids are heterogeneous mixtures in which the particle

size is too small to be seen with the naked eye, but is big

enough to scatter light. Colloids are useful in industry and

daily life. The particles are called the dispersed phase and

the medium in which they are distributed is called the

dispersion medium.

• Pure substances can be elements or compounds. An element

is a form of matter that cannot be broken down by chemical

reactions into simpler substances. A compound is a substance

composed of two or more different types of elements,

chemically combined in a fixed proportion.

• Properties of a compound are different from its constituent

elements, whereas a mixture shows the properties of its

constituting elements or compounds.

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IS MATTER AROUND US PURE? 23

(a) What mass of potassium nitrate would be needed to

produce a saturated solution of potassium nitrate in

50 grams of water at 313 K?

(b) Pragya makes a saturated solution of potassium chloride

in water at 353 K and leaves the solution to cool at

room temperature. What would she observe as the

solution cools? Explain.

the highest solubility at this temperature?

(d) What is the effect of change of temperature on the

solubility of a salt?

4. Explain the following giving examples.

(a) Saturated solution

(b) Pure substance

(d) Suspension

5. Classify each of the following as a homogeneous or

heterogeneous mixture.

soda water, wood, air, soil, vinegar, filtered tea.

6. How would you confirm that a colourless liquid given to you

is pure water?

2. Write the steps you would use for making tea. Use the words

solution, solvent, solute, dissolve, soluble, insoluble, filtrate

and residue.

3. Pragya tested the solubility of three different substances at

different temperatures and collected the data as given below

(results are given in the following table, as grams of substance

dissolved in 100 grams of water to form a saturated solution).

Substance Dissolved

Temperature in K

283 293

313 333 353

Solubility

Potassium nitrate 21 32 62 106

167

Sodium chloride 36 36 36 37 37

Potassium chloride 35 35 40 46 54

Ammonium chloride 24 37 41 55 66

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SCIENCE24

7. Which of the following materials fall in the category of a

“pure substance”?

(a) Ice

(b) Milk

(d) Hydrochloric acid

(e) Calcium oxide

(f) Mercury

(g) Brick

(h) Wood

(i) Air

8. Identify the solutions among the following mixtures.

(a) Soil

(b) Sea water

(d) Coal

(e) Soda water

9. Which of the following will show “Tyndall effect”?

(a) Salt solution

(b) Milk

(d) Starch solution

10. Classify the following into elements, compounds and mixtures.

(a) Sodium

(b) Soil

(d) Silver

(e) Calcium carbonate

(f) Tin

(g) Silicon

(h) Coal

(i) Air

(j) Soap

(k) Methane

(l) Carbon dioxide

(m) Blood

11. Which of the following are chemical changes?

(a) Growth of a plant

(b) Rusting of iron

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IS MATTER AROUND US PURE? 25

(d) Cooking of food

(e) Digestion of food

(f) Freezing of water

(g) Burning of a candle

Group Activity

Take an earthen pot (mutka), some pebbles and sand. Design a

small-scale filtration plant that you could use to clean

muddy water.

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