Fractional distillation. The vapours of lower boiling fraction reach the top of the column first followed by vapours of higher boiling fractions. Distillation under reduced pressure: This method is used to purify liquids having very high boiling points and those, which decompose at or below their boiling points. Such liquids are made to boil at a temperature lower than their normal boiling points by reducing the pressure on their surface.
A liquid boils at a temperature at which its vapour pressure is equal to the external pressure. The pressure is reduced with the help of a water pump or vacuum pump (Fig. . ).
Glycerol can be separated from spent-lye in soap industry by using this technique. Fig. . Distillation under reduced pressure.
A liquid boils at a temperature below its vapour pressure by reducing the pressure. Steam Distillation: This technique is applied to separate substances which are steam volatile and are immiscible with water. In steam distillation, steam from a steam generator is passed through a heated flask containing the liquid to be distilled. The mixture of steam and the volatile organic compound is condensed and collected.
The compound is later separated from water using a separating funnel. In steam distillation, the liquid boils when the sum of vapour pressures due to the organic liquid ( p ) and that due to water ( p ) becomes equal to the atmospheric pressure ( p ), i.e. p = p + p . Since p is lower than p , the organic liquid vaporises at lower temperature than its boiling point.
Thus, if one of the substances in the mixture is water and the other, a water insoluble substance, then the mixture will boil close to but below, 373K. A mixture of water and the substance is obtained which can be separated by using a separating funnel. Aniline is separated by this technique from aniline – water mixture (Fig. .
, Page ). . . Differential Extraction When an organic compound is present in an aqueous medium, it is separated by shaking it with an organic solvent in which it