Azeotropic or constant boiling mixture

In ethanol water solution the mole composition of each component in liquid phase is different from the mole composition of that component in vapour phase, but at a point (351.15 K) the composition of the liquid and vapour phases become the same. Liquid mixture at this point vaporises without change in composition and the condensate contains same composition. i.e., mixture distills like a pure liquid. At this point, solution or mixture is called as an Azeotropic mixture.

In case of positive deviation we get minimum boiling azeotropes while in case of negative deviation, we get maximum boiling azeotropes.


 

Note: Azeotropic mixture cannot be separated into their constituents by fractional distillation. For example if we try to concentrate a dilute solution of ethanol, only 95.4% ethanol can be obtained after this it becomes a constant boiling mixture (Azeotrope).