from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A liquid mixture of two or more substances that retains the same composition in the vapor state as in the liquid state when distilled or partially evaporated under a certain pressure.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A mixture of two or more substances whose liquid and gaseous forms have the same composition (at a certain pressure); the substances cannot be separated by normal distillation.
Ethanol and water at the 96/4 proportions form an azeotrope, which is a mixture of stuff that boils in the same proportions as it is in liquid form.
Things like methylene chloride being rather more weirdly polar as a solvent than you'd expect, or the fact that some amines will stick to solid magnesium sulfate drying agent (but not sodium sulfate), or how you can azeotrope out acetic acid with toluene, or how you want palladium tetrakis to be lemon yellow and not orange
Water and ethanol form an azeotrope - a mixture whose composition can't be changed by distillation - at a ratio of 95.6: 4.4 ethanol: water.
For chemical as well as regulatory reasons, it is unlikely that 100\% ethanol is used; given the formation of an azeotrope at approximately 95 percent, it's hard even to get 100\% ethanol.
You have to work hard to break up the ethanol and water azeotrope, and if you do, you’ve probably wasted your time, because exposure to air will allow the ethanol to absorb enough water to form the azeotrope again.
a Sum of distillation to azeotrope and azeotropic distillation