Why you can never obtain 100% ethanol from distillation of a ethanol-water mixture

Distillation is an imperfect method for separating mixtures of liquids that formazeotropes. An azeotrope, also called a constant boiling mixture, is a mixture of two or more liquids at a specific ratio, whose composition cannot be altered by simple distillation. Every azeotrope has a characteristic boiling point, which may be lower (a positive azeotrope or minimum-boiling mixture) or higher (a negative azeotrope ormaximum-boiling mixture) than the boiling points of the individual liquids that make up the azeotrope.

Ethanol forms a positive azeotrope with water. The boiling point of a mixture of 95.6% ethanol (by weight) with 4.4% water is 78.1 °C, which is lower than the boiling point of pure water (100 °C) or pure ethanol (78.4 °C). Because the azeotropic mixture boils at a lower temperature, it is impossible to use simple distillation to produce ethanol at concentrations higher than 95.6%.