Azeotropes are liquid mixtures of two or more substances that retains the same composition in the vapor state as in the liquid state when distilled under a certain pressure.
Ethanol forms with water at normal pressure a binary azeotrope, which contains 95.6 percent ethanol and 4.4 percent water. Therefore pure ethanol cannot be obtained water-free by simple distillation. In former times the dehydration was achieved by a ternary azeotrope of ethanol/water/benzene, which is boiling at 64.85 oC. With the condensation a phase separation develops, whereby the lower phase contains water with little ethanol. Into special rectifying columns the water can be separated and the benzene be led back in a cycle. Because of the poisoness of the benzene this was later replaced by cyclohexane.