Azeotropic Distillation

Some liquid mixtures resist separation by fractional distillation altogether, however carefully it is carried out -- these are azeotropes, mixtures that boil at one single, CONSTANT temperature and distil over behaving, for all practical purposes, as if they were one pure component (ethanol and water, mixed in the specific ratio 95.87:4.13 by mass, is the textbook example: no amount of further fractional distillation can concentrate ethanol beyond that ratio). Azeotropic distillation gets around this limitation by deliberately introducing a THIRD substance into the mixture -- benzene, carbon tetrachloride, ether, glycerol or glycol are the ones named -- which acts as a dehydrating agent: it selectively depresses the partial vapour pressure of ONE of the original two components, which correspondingly RAISES that component's effective boiling point, so that the OTHER original component is now free to distil over on its own. Specifically: low-boiling additives like benzene and CCl4 depress the partial vapour pressure of the alcohol component more strongly than that of the water component, while high-boiling additives like glycerol and glyc …