Page 19 - Chemistry - XI

Q8. What is meant by seeding?
Ans. It is an addition of a crystal of the same substance in a hot saturated solution while it is kept for
cooling. It helps in speeding up the process of crystallisation.

Q9. What is meant by water of crystallisation?
Ans. The number of molecules of H O associated with crystalline substance.
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Q10. What happens when copper sulphate crystals are heated?
Ans. They will become white amorphous solid.

Q11. What is the solvent used in crystallisation of benzoic acid?
Ans. Benzene

Q12. Which are more pure—small crystals or big crystals?
Ans. Small crystals are more pure than big crystals because impurities get trapped in big crystals.

EXPERIMENT - 8

Objective

Determination of melting point of a solid organic substance.

Principle/Theory
On heating, the kinetic energy of solid molecules increases and when it reaches more than the intermolecular
forces of attraction, the solid melts. In a liquid phase, the molecules get more opportunity to move freely.
Melting point of a substance is the temperature at which the solid changes into a liquid at atmospheric
pressure, i.e. vapour pressure of solid and liquid becomes equal. It is characteristic of any pure substance.

Materials Required
Beaker (250 mL), thermometer (110°C), glass capillary tube, rubber band, laboratory stand with clamp
and ring, Thiele’s tube, distilled water, and naphthalene

Procedure

1. Take a capillary tube of 5 to 8 cm length
and seal one of its ends by heating. Thermometer

2. Take naphthalene and powder it. Now, fi ll Thermometer
the powdered naphthalene in the capillary
tube up to 1 cm length by tapping.
Capillary Tube
3. Tie the capillary tube with the help of a

rubber band to the thermometer as shown Capillary Solid
in the diagram.
4. Take a Thiele’s tube half fi lled with water Water

and dip the thermometer along with the
capillary tube in the water such that both Substance
of them completely dip in the water.
Fig. 8.1

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