(a) Crystallisation
Crystallisation is used to purify solid organic compounds.
Principle: The principle on which it works is the difference in the solubility of the compound and impurities in a given solvent. The impure compound is made to dissolve in the solvent at a higher temperature since it is sparingly soluble at lower temperatures. This is continued till we get an almost saturated solution. On cooling and filtering it, we get its’ crystals. For example, by crystallising 2-4 g of crude aspirin in 20 ml of ethyl alcohol, we get pure aspirin. It is heated if needed and left undisturbed until it crystallises. The crystals are then separated and dried.
(b) Distillation
This method is used to separate non-volatile liquids from volatile impurities. It is also used when the components have a considerable difference in their boiling points.
Principle: The principle on which it works is that liquids having different boiling points vaporise at different temperatures. They are then cooled, and the formed liquids are separated.
For example, a mixture of aniline (b.p = 457 K) and chloroform (b.p = 334 K) is taken in a round bottom flask having a condenser. When they are heated, chloroform vaporises first due to its high volatility and is made to pass through a condenser, where it cools down. The aniline is left behind in the round bottom flask.
(c) Chromatography
It is widely used for the separation and purification of organic compounds.
Principle: The principle on which it works is that individual components of a mixture move at different paces through the stationary phase under the influence of the mobile phase.
For example, Chromatography can be used to separate a mixture of blue and red ink. This mixture is placed on a chromatogram where the component which is less absorbed by the chromatogram moves faster up the paper than the other component, which is almost stationary.
