Two volatile and miscible liquids can be separated by fractional distillation into pure components under the conditions when :

Azeotropes are concoctions of two or more liquids. They might have a lower boiling point in their mixture than either of the individual parts, or vice versa. When the proportion of the liquids cannot be changed by distillation, azeotropes develop. Due to the fact that the composition of the vapour and the liquid combination are identical, they have a constant boiling point like pure compounds. When two volatile and miscible liquids do not combine to produce an azeotropic mixture, they can be separated by fractional distillation into their individual pure components.