Give a brief description of the principles of the following techniques taking an example in each case.

(a) Crystallisation

(b) Distillation

(c) Chromatography

Asked by Abhisek | 11 months ago |  53

##### Solution :-

(a) Crystallisation

Crystallisation is one of the most commonly used techniques for the purification of solid organic compounds.

Principle: It is based on the difference in the solubilites of the compound and the impurities in a given solvent. The impure compound gets dissolved in the solvent in which it is sparingly soluble at room temperature, but appreciably soluble at higher temperature. The solution is concentrated to obtain a nearly saturated solution. On cooling the solution, the pure compound crystallises out and is removed by filtration. For example, pure aspirin is obtained by recrystallising crude aspirin. Approximately 2 – 4 g of crude aspirin is dissolved in about 20 mL of ethyl alcohol. The solution is heated (if necessary) to ensure complete dissolution. The solution is then left undisturbed until some crystals start to separate out. The crystals are then filtered and dried.

(b) Distillation

This method is used to separate volatile liquids from non-volatile impurities or a mixture of those liquids that have a sufficient difference in their boiling points.

Principle:It is based on the fact that liquids having different boiling points vapourise at different temperatures. The vapours are then cooled and the liquids so formed are collected separately. For example, a mixture of chloroform (b.p = 334 K) and aniline (b.p = 457 K) can be separated by the method of distillation. The mixture is taken in a round bottom flask fitted with a condenser. It is then heated. Chloroform, being more volatile, vaporizes first and passes into the condenser. In the condenser, the vapours condense andchloroform trickles down. In the round bottom flask, aniline is left behind.

(c) Chromatography

It is one of the most useful methods for the separation and purification of organic compounds.

Principle:It is based on the difference in movement of individual components of a mixture through the stationary phase under the influence of mobile phase.

For example, a mixture of red and blue ink can be separated by chromatography. A drop of the mixture is placed on the chromatogram. The component of the ink, which is less adsorbed on the chromatogram, moves with the mobile phase while the less adsorbed component remains almost stationary.

Answered by Pragya Singh | 11 months ago

### Related Questions

#### Which of the following carbocation is most stable?

Which of the following carbocation is most stable?

(a) $$(CH_3)_3 C.C^{+}H_2$$

(b) $$(CH_3)_3\; C^{+}$$

(c) $$CH_3 CH_2C^{+}H_2$$

(d) $$CH_3 C^{+}HCH_2CH_3$$

#### Indicate the σ and π bonds in the molecules: CH3NO2

Indicate the σ and π bonds in the molecules: CH3NO2

#### The reaction: CH_3 CH_2 I+KOH_{(aq)}CH_3 CH_2 OH +KI is classified as : (a) electrophilic substitution

The reaction:

$$CH_3 CH_2 I+KOH_{(aq)} \rightarrow CH_3 CH_2 OH +KI$$ is classified as :

(a) electrophilic substitution

(b) nucleophilic substitution

(c) elimination

#### The best and latest technique for isolation, purification and separation of organic compounds is

The best and latest technique for isolation, purification and separation of organic compounds is:

(a) Crystallisation

(b) Distillation

(c) Sublimation

(d) Chromatography

#### In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is obtained

In the Lassaigne’s test for nitrogen in an organic compound, the Prussian blue colour is obtained due to the formation of:

(a) $$Na_4[Fe(CN)_6]$$

(b) $$Fe_4[Fe(CN)_6]_3$$

(c) $$Fe_2[Fe(CN)_6]$$

(d) $$Fe_3[Fe(CN)_6]_4$$