Chemistry Class 11 NCERT Solutions: Chapter 12 Organic Chemistry Some Basic Principles and Techniques Part 11

Q: 19. Describe the method, which can be used to separate two compounds with different solubilities in a solvent S.

Answer:

Fractional Crystallisation is the method used for separating two compounds with different solubilities in a solvent S. The process of fractional Crystallisation is carried out in four steps.

(A) Preparation of the solution:

The powdered mixture is taken in a flask and the solvent is added to it slowly and stirred simultaneously. The solvent is added till the solute is just dissolved in the solvent. This saturated solution is then heated.

(B) Filtration of the solution:

The hot saturated solution is then filtered through a filter paper in a China dish.

(C) Fractional Crystallisation:

The solution in the China dish is now allowed to cool. The less soluble compound crystallises first, while the more soluble compound remains in the solution. After separating these crystals from the mother liquor, the latter is concentrated once again. The hot solution is allowed to cool and consequently, the crystals of the more soluble compound are obtained.

(D) Isolation and Drying:

These crystals are separated from the mother liquor by filtration. Finally, the crystals are dried.

Q: 20. What is the difference between distillation, distillation under reduced pressure and steam distillation ?

Answer:

The differences among distillation, distillation under reduced pressure, and steam distillation are given in the following table.

Distillation types and properties
Distillation, under reduced pressure and steam distillation

Distillation

Distillation Under Reduced Pressure

Steam Distillation

1

It is used for the purification of compounds

that are associated with

non3volatile impurities or

those liquids, which do not

decompose on boiling. In

other words, distillation is

used to separate volatile

liquids from non3volatile impurities or a mixture of

those liquids that have

sufficient difference in

boiling points.

This method is used to

purify a liquid that tends

to decompose on boiling.

Under the conditions of

reduced pressure, the

liquid will boil at a low

temperature than its

boiling point and will,

therefore, not decompose.

It is used to purify an

organic compound, which

is steam volatile and

immiscible in water. On

passing steam, the

compound gets heated up

and the steam gets

condensed to water. After

some time, the mixture of water and liquid starts to

boil and passes through

the condenser. This

condensed mixture of

water and liquid is then

separated by using a

separating funnel.

2

Mixture of petrol and

kerosene is separated by

this method.

Glycerol is purified by

this method. It boils with

decomposition at a

temperature of 593 K. At

a reduced pressure, it

boils at 453 K without

decomposition.

A mixture of water and

aniline is separated by

steam distillation.

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