12 PROPERTIES OF SUBSTANCES

12. PROPERTIES OF SUBSTANCES

12 PROPERTIES OF SUBSTANCES

1. Melting point : The temperature at which the state of a

substance changes from solid to liquid is called the melting point

of the substance. Example : The melting point of ice is 0°C while

that of iron is 1535 °C.

2. Boiling point : The temperature at which a liquid boils is called

the boiling point of the substance. Example : The boiling point

of water is 100 °C while that of iron is 2750 °C.

3. Distillation : It is a process in which a liquid is first heated up

to its boiling point and vapours thus formed are cooled to obtain

the liquid again. Example : Drinking water can be obtained from

sea water by distillation.

4. Fractional distillation : If two or more substances are

dissolved in a liquid, fractional distillation can be used for

separating them. The components of crude oil are separated by

the process of fractional distillation. It is easy to separate the

various components of crude oil because of the differences in

their boiling points which is necessary for fractional distillation.

Q. 1 Answer the following questions :

* (1) Define the following :

(1) Melting point (2) Boiling point (3) Distillation.

Ans. (1) Melting point : The temperature at which the state

of a substance changes from solid to liquid is called the melting

point of the substance.

(2) Boiling point : The temperature at which a liquid boils is

called the boiling point of the substance.

(3) Distillation : It is the process in which a liquid is first

heated up to its boiling point and the vapours thus formed are

cooled to obtain the liquid again. This process is called distillation.

Answers to the questions given on page 118 of the textbook

* (1) In which state is the LPG in the cylinder of the

cooking stove ?

Ans. LPG in the cylinder, is stored in the liquid state.

* (2) What is the characteristic of naphthalene balls ?

Ans. Naphthalene balls at room temperature or when

they are heated, the state, of nephthalene balls change from

solid to gaseous state, i.e. sublimation takes place.

* (3) How is drinking water obtained from sea water ?

Ans. The sea water is purified by distillation. Take sea

water in a round bottom flask. Fit the round bottom flask with

a two-holded rubber stopper. Introduce a thermometer

through one hole in the flask. In the other, fit a bent glass tube.

Connect the outer end of the bent tube to a condenser by a

small rubber tube. Fit another small rubber tube to the lower

end of the condenser and put its end into a beaker.

Burner

Sea

water

Round

bottom

flask

Stand

Pure distilled

water

Beaker

Cold

water

Condenser

Water

Rubber

tube

Thermometer

Stand

Fig. 12.1 : Distillation

Set up the apparatus as shown in the figure. Heat the

water. The water in the flask changes to steam at 100 °C. The

steam passes through the condenser, it is cooled by the water

around it and condenses back into water again. This drips

down the tube as pure water and is collected in the beaker.

When all the water in the flask is vaporised, the salt from the

sea water is left behind in the flask.

* (2) Draw a neat labelled diagram showing the process of

distillation.

Ans. For reference, see the diagram to Q. (3) on previous pages.

Answers to the questions given on page 119 of the textbook

* (1) Which method will you use to obtain salt from the

sea water in salt pans– evaporation or distillation ?

Ans. The method used to obtain salt from the sea water

in salt pans is evaporation.

* (2) Which salt is obtained as a result of the chemical

reaction between hydrochloric acid (HCl) and sodium

hydroxide (NaOH) ? Which method will you use to

separate it from the salt solution ?

Ans. The salt obtained from the chemical reaction

between hydrochloric acid and sodium hydroxide is sodium

chloride (NaCl). It is separated from solution by distillation.

(3) Explain the process of separation of components of

crude oil by fractional distillation.

Ans. The crude oil obtained from oil wells contain various

components in the dissolved state. These components of crude oil

are separated by the fractional distillation.

}

naphtha 110 °C to 150 °C

kerosene 190 °C to 250 °C

Diesel 250 °C to 300 °C

Other substances

300 °C to 400 °C

residue 400 °C and higher

}

petrol 40 °C to 70 °C

mineral oil

(heat)

petroleum gas

Fig. 12.2 : Separation of components of crude oil

The crude oil contains components like petrol, naphtha,

kerosene, diesel, etc. These components are separated by fractional

distillation. When crude oil is heated, petrol evoporates. It is

condensed and collected separately. Petrol is highly inflammable

12. 153

and has a low boiling point followed by naphtha, kerosene and

diesel respectively. The components of crude oil are separated at

the proper temperatures. The various components of crude oil are

separated by fractional distillation. It is possible because of

differences in their boiling points.

Answers to the questions given on pages 120 & 121 of the

Textbook

* (1) What method will you use to separate ethanol

from a mixture of ethanol (boiling point 78 °C) and water

(boiling point 100°C)?

Ans. The method used to separate ethanol from a mixture

of ethanol and water is fractional distillation.

* (2) Could we separate water and petrol quickly

without heating ?

Ans. Yes. Take a mixture of water and petrol in a steel

tank or separating funnel. Allow the mixture to stand for some

time. Petrol is immiscible with water, it forms an upper layer

in the mixture. The lower layer of water can be removed from

the tap.

* Q. 2 Answer the following questions :

(1) What will you do to find out the boiling point of water?

Burner

Water

Stand

Stand

Stirrer

Beaker

Thermometer

Fig. 12.3 : Boiling point of water

Ans. Take some water in a

beaker. Introduce a thermometer

into the water in such a way that the

bulb of the thermometer is

completely immersed in water.

Insert a stirrer into the water.

Arrange the appartus as shown in

the figure.

Heat the water with a burner.

Keep on stirring water. After some

time, the temperature starts rising

till it reaches 100°C. At this

temperature, water starts boiling

and is converted into steam. The

temperature remains constant till a

large quantity of water is converted into steam. The temperature so

recorded is the boiling point of water.

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(2) What changes do you see in the appearance of a candle

after you have lit it ?

Ans. When you lit a candle, wax melts as it has low melting

point. The heat of the candle melts the nearby wax and that melted

wax is absorbed by the candle wick. The melted wax burns on the

wick producing carbon dioxide and water. The incomplete burning

of the candle produces carbon monoxide and carbon particles.

Therefore, when a candle burns, it changes into solid, liquid and

gaseous state.

(3) Can a solid and a solvent be separated from their

mixture by distillation ?

Ans. Yes, a solid and a solvent can be separated from their

mixture by distillation.

Q. 3 Give scientifc reasons :

* (1) A condenser has two taps.

Ans. A condenser is used in distillation process to cool the

vapours and condense them to liquid. It consists of two glass tubes.

The inner glass tube is placed inside another glass tube of a bigger

diameter. The outer glass tube has an inlet and an outlet to allow

circulation of cold water around the inner glass tube of the

condenser.

* (2) At dawn, in winter, we see dew drops on the leaves of

trees.

Ans. As the temperature falls, the capacity of air to hold water

vapour becomes less. During a cold night, the temperature of the air

may fall to the dew point, or even below the dew point. If the

temperature falls below the dew point, the excess of water vapour in

the air condenses on the leaves of trees and dew is formed.

Q. 4 Distinguish between Distillation and Fractional

distillation.

Ans. Distillation Fractional distillation

1. In distillation the condenser

is connected to distillation

flask.

2. The liquid and the solute in a

solution can be separated by

the process of distillation.

1. In fractional distillation, there

is a fractionating column

between the distillation flask

and the condenser.

2. If there is a significant

difference between the boiling

points of liquids, they can be

separated from a mixture by

fractional distillation.

Q. 5 Answer the following questions in one sentence

each :

(1) State the melting point of ice.

Ans. The melting point of ice is 0 °C.

(2) State the boiling point of water.

Ans. The boiling point of water is 100 °C.

(3) State the factors due to which the rate of evaporation

increases.

Ans. The rate of evaporation increases due to high

temperature and wind.

(4) Which method is used to obtain drinking water from

sea water ?

Ans. Distillation method is used to obtain drinking water from

sea water.

(5) Where do you use fractional distillation ?

Ans. If two or more substances are dissolved in a liquid,

fractional distillation can be used for separating them.

(6) Name the components of crude oil.

Ans. The components of crude oil are petrol, naphtha,

kerosene, diesel, etc.

(7) Name the component of crude oil whose boiling point

is lowest.

Ans. Petrol has lowest boiling point.

(8) Which method is used to separate the components of

crude oil ?

Ans. The Fractional distillation method is used to separate the

components of crude oil.

Q. 6 State whether the following statements are True or

False . If a statement is false, correct it and rewrite :

(1) If a solid substance is heated, it changes into a liquid.

(2) During condensation, a gas changes into a solid.

(3) The melting point of iron is about 2750 °C.

(4) The rate of evaporation increases with increase in

temperature.

(5) The components of crude oil are separated by distillation.

(6) Petrol is highly inflammable component of crude oil.

(7) The salt and the water in a solution can be separated by

distillation.

(8) The temperature at which the state of a substance changes

from solid to liquid is called the boiling point of the

substance.

(9) Any given substance has a specific melting point and

boiling point.

Ans. (1) True. (2) False. During condensation, a gas changes

into a liquid (3) False. The melting point of iron is about 1535 °C

(4) True. (5) False. The components of crude oil are separated

by the fractional distillation. (6) True. (7) True. (8) False. The

temperature at which the state of a substance changes from solid

to liquid is called the melting point of the substance. (9) True.

Q. 7 Fill in the blanks :

(1) During evaporation, a liquid changes into a ……… .

(2) If two or more substances are dissolved in a liquid, ………

can be used for separating them.

(3) ……… is the most inflammable and has the lowest boiling

point.

(4) The mixture of salt and water is separated by ……… .

(5) The temperature at which the state of a substance changes

from solid to liquid is called the ……… of the substance.

Ans. (1) gas (2) fractional distillation (3) Petrol

(4) distillation (5) melting point.

Q. 8 Choose the correct alternatives and complete the

following statements :

(1) The melting point of iron is ……… .

(a) 1535 °C (b) 1635 °C (c) 1435 °C (d) 1000 °C

(2) The boiling point of iron is ……… .

(a) 2650 °C (b) 2750 °C (c) 2550 °C (d) 2850 °C

(3) In ………, a liquid changes into a gas.

(a) condensation (b) freezing

(c) expansion (d) evaporation

(4) The temperature at which a liquid boils is called the ………

of the substance.

(a) freezing point (b) boiling point

(c) melting point (d) sublimation point

 (5) The rate of ……… increases due to wind and high

temperature.

(a) evaporation (b) condensation

(c) freezing (d) expansion

Ans. (1) 1535°C (2) 2750°C (3) evaporation (4) boiling

point (5) evaporation.

* Q. 9 Match the following :

‘A’ ‘B’

(1) Melting point

(2) Boiling point

(3) Fractional distillation

(4) Distillation

(a) Separating common salt and

water from their mixture

(b) Obtaining diesel from crude oil

(c) Change of a state of a liquid

into a gas

(d) Change of a state from solid to

Liquid

Ans. (1) Melting point – Change of a state from solid to liquid

(2) Boiling point – Change of a state of a liquid into a gas

(3) Fractional distillation – Obtaining diesel from crude oil

(4) Distillation – Separating common salt and water from their

mixture.