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What is meant by a pure substance?

A substance which consists of a single type of particle and it cannot be separated into other kinds of matter by any physical process is called a pure substance.


List the points of differences between homogenous and heterogeneous mixtures?



Homogenous mixture

Heterogeneous mixture


It has a uniform composition throughout its mass.

It has no uniform composition .


It has no visible boundary or boundaries of separation between its constituents.

It has visible boundary or boundaries of separation.



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. 

Mass of sodium chloride  = 36 g
Mass of the solution                = 100 g
Mass of the solution                = 100 + 36 = 136 g
Concentration of the solution    = 100 = 26.47 %.


How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25ºC), which are miscible with each other?

We can separate a mixture containing kerosene and petrol by simple distillation because kerosene and petrol do not decompose on heating and their boiling points are sufficiently apart. The apparatus used for simple distillation is given below.

Take the given mixture in a distillation flask. Heat the mixture slowly keeping a close watch at the thermometer. At a certain point, temperature becomes constant. Petrol having a lower boiling point vaporizes, condenses in the condenser and is collected in the receiving flask. Stop heating when the temperature starts rising further . Kerosene is left behind in the distillation flask.


Name the technique to separate
(i) butter from curd,
(ii) salt from sea-water,
(iii) camphor from salt.


i) butter from curd - Centrifugation
ii) salt from sea-water - Crystallisation
iii) camphor from salt - Sublimation


What are the types of mixtures that are separated by the technique of crystallisation?

Crystallisation is used to purify salt that we get from sea water and to obtain pure substances from their impure samples.


 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


• cutting of trees is a physical change.

• melting of butter in a pan is a physical change.

• rusting of almirah is a chemical change.

• boiling of water to form steam is a physical change.

• passing of electric current through water and the water breaking down into hydrogen and oxygen gases is a chemical change.

• dissolving common salt in water is a physical change.

• making a fruit salad with raw fruits is a physical change.

• burning of paper and wood is a chemical change.



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.

(c) Small pieces of metal in the engine oil of a car.

(d) Extraction of different pigments  from the 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.


(a)  evaporation and crystallisation.

(b) sublimation.

(c) centrifugation.

(d) chromatography.

(e) centrifugation.

(f) separating funnel.

(g) hand-picking.

(h) magnetic separation.

(i) winnowing.

(j) centrifugation.


Write the steps you would use for making tea. Use the words solution, solvent,  solute, dissolve, soluble, insoluble, filtrate  and residue. 

Take the solvent, water in a kettle. When the solvent boils, add the solute which is the milk. Milk and water form a solution. Then pour some tea leaves over sieve. Pour slowly hot solution of milk over tea leaves. Colour of the tea leaves gets into solution as filtrate. The remaining tea leaves being insoluble remain as residue. Add requisite sugar which dissolves and the tea is ready.


(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.

(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

(d) What is the effect of change of temperature on the
solubility of a salt? 


a. At 313 K, 62 grams of potassium are added in 100 gram of water to form a saturated solution. So 50 grams of water require 31 grams of potassium nitrate.

b. As the solution cools, Potassium chloride gets crystallized or precipitated because at 353 K, solubility of potassium chloride is 54 g per 100 g of water whereas at room temperature it is 35 g. So excess of potassium chloride dissolved, gets precipitated.

c. At 20o C or 293 k ammonium chloride has the highest solubility.

d. As the temperature increases, solubility increases.


Explain the following giving examples.

(a) saturated solution

(b) pure substance

(c) colloid

(d) suspension

a. Saturated

A solution in which no more of the solid can be dissolved at a given temperature is called a saturated solution. Suppose 40 g of a solute is the maximum amount that can be dissolved in 100 g of water at 298 K. Then 140 g of solution so obtained is the saturated solution at 298K.

b. Pure substance
The substances which always have the same colour, taste or texture at a given temperature and pressure are pure substances. A pure substance has a characteristic melting point or boiling point at a given pressure. For example, pure water is always colourless, odourless and tasteless and boils at 373 K at normal atmospheric pressure.

c. Colloid

The particles of a colloid are uniformly spread throughout the solution. A colloid has the following characteristics:

a. It is a heterogeneous mixture.

b. The size of particles of a colloid lie between 1-100 nm and are not visible to the naked eye.

c. The particles of a colloid can scatter a beam of light.

d. The particles of a colloid cannot be scattered from the mixture by filtration.

e. The particles of a colloid do not settle if left undisturbed.

Examples are smoke, milk, fog and cloud.


d. Suspension
A suspension is a heterogeneous mixture and the particles of a suspension are greater than 100 nm and are visible to the naked eye. The particles of suspension can scatter a beam of light.
The examples of suspension are fine clay particles in water.



Classify each of the following as a homogeneous or heterogeneous mixture. soda water, wood, air, soil, vinegar, filtered tea. 

Soda water, air, vinegar, filtered tea are homogenous mixture. Wood and soil are heterogeneous mixture.


How would you confirm that a colourless liquid given to you is pure water?

Water boils at 373 K at 1 atmospheric pressure. If the given colourless liquid boils at 373 K then we can confirm that the given liquid is water.


Which of the following materials fall in the category of a "pure substance"?

(a) Ice

(b) Milk

(c) Iron

(d) Hydrochloric acid

(e) Calcium oxide

(f)  Mercury

(g) Brick

(h) Wood

(i)  Air

Ice, Iron, Hydrochloric acid, calcium oxide and mercury are pure substances.


Identify the solutions among the following mixtures.

(a) Soil

(b) Sea water

(c) Air

(d) Coal

(e) Soda water.

Sea water and soda water


Which of the following will show "Tyndall effect"?

(a) Salt solution

(b) Milk

(c) Copper sulphate solution

(d) Starch solution.

Tyndall effect is shown by colloids and so milk and starch show tyndall effect.


Classify the following into elements, compounds and mixtures.

(a)  Sodium

(b)  Soil

(c)  Sugar solution

(d)  Silver

(e)  Calcium carbonate

(f)   Tin

(g)  Silicon

(h)  Coal

(i)   Air

(j)   Soap

(k)  Methane

(l)   Carbon dioxide

(m) Blood

(a)  Element

(b) mixture

(c) mixture

(d) element

(e) compound

(f)  element

(g) element

(h) mixture

(i) mixture

(j) mixture

(k) compound

(l)  compound

(m)  mixture


Which of the following are chemical changes?

(a) Growth of a plant

(b) Rusting of iron

(c) Mixing of iron filings and sand

(d) Cooking of food

(e) Digestion of food

(f) Freezing of water

(g) Burning of a candle

Rusting of iron, cooking of food, digestion of food, burning of candle undergo chemical changes.

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