An azeotrope is a liquid compound whose boiling point is either above or below those of its component parts, meaning that its proportions cannot be altered by simple distillation. Ethanol and water form a so-called positive azeotrope, one whose boiling point is lower than that of either compound, at 95.63 percent of the former and 4.37 percent of the former. Pure 100 percent alcohol, then, is impossible to create through normal distillation: the ethanol that volatilizes and comes through the still is in fact actually the ethanol-water azeotrope. All normal distillation can do is separate the azeotrope from any water above that 4.37 percent. That final portion of water can only be removed by breaking the azeotrope with vacuum distillation or other heroic measures, such as adding benzene or cyclohexane to the liquid to be distilled.
See also ethanol.
American Chemical Society. https://www.acs.org (accessed January 29, 2021).
Rowley, Matthew B. Moonshine! New York: Sterling, 2007.
By: Doug Frost