Question

Give a brief description of the principles of the following techniques taking an example in each case.(a) Crystallisation (b) Distillation (c) Chromatography.

Answer 1

(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 and chloroform 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.

Answer 2

(a) Crystallization:
Principle: Crystallization is like making sugar crystals in a jar. When you dissolve sugar in hot water and let it cool, the sugar forms crystals because it can't stay dissolved in the cold water anymore. This is how we separate things that dissolve differently in a liquid.
Example: If you mix sugar and salt in hot water and then cool it, the sugar will form crystals first because it doesn't dissolve as well in cold water. We can then filter out the sugar crystals from the salt.

(b) Distillation:
Principle: Distillation is like boiling water to make steam. When you heat a mixture of liquids, the one that boils first turns into steam, and then it cools down and becomes liquid again. This way, we can separate liquids that have different boiling points.
Example: If you heat a mixture of water and alcohol, the alcohol will turn into steam first because it boils at a lower temperature than water. When the steam cools down, it turns back into liquid alcohol, and we can collect it separately from the water.

(c) Chromatography:
Principle: Chromatography is like watching colors spread on a paper towel when you spill watercolor paint. When you put a drop of ink on a special paper and let it soak in water, the ink spreads out into different colors because some parts of the ink stick to the paper more than others.
Example: If you put a drop of marker ink on a piece of paper and let it soak in water, the different colors in the ink will spread out. Some colors will move faster and farther than others because they stick to the paper differently. This way, we can see all the colors that make up the ink.