Proof: The Science of Booze

by Adam Rogers

Because ethanol is more volatile than water at every temperature, distilling will eventually pull all the ethanol out of a solution—to a point. At 95.57 percent alcohol, the vapor has the same ethanol concentration as the liquid, and you can’t squeeze any more out. It’s called the azeotropic limit, and it’s the upper bound on how potent any spirit can ever be: 194.4 proof. (“Proof” is an old word when it comes to alcohol content; in the United States it’s just twice the percent alcohol-by-volume. “Eighty proof” is 40 percent ABV. In the United Kingdom it has a slightly different calculus—100 proof is 57.15 percent ABV, a definition that comes from the days when British navy rations included rum for sailors. The sailors would mix gunpowder into the rum, and if it ignited when lit, that was proof the alcohol content was high enough to serve.)