posted by candy .
During fractional distillation of ethanol/water mixture, why can't pure ethanol be obtained (i.e. distillated contains a little water)?
Ethanol and water can be separated by fractional distillation (pure water distills at some points and pure ethanol at some points) until the mixture in the distilling pot reaches approximately 95% ethanol-5% water. This 95/5 mixture is called an azeotrope or azeotropic mixture. Azeotropes are unique for the mixture and they are part of the non-ideal nature of solutions. At any rate. at the 95/5 point, that is what distills. The only way to prevent the 5% water distilling at the same time as the 95% alcohol, is to add something to the distilling flask that will hold back the water. In such cases, absolute alcohol (pure alcohol without the water) is prepared. It may be interesting to you that if we take absolute alcohol (with no water), pour it into a beaker, allow the beaker to stand in an open setting, the pure alcohol will pick up moisture from the atmosphere until it reaches the 95/5 level. Here is a site that you can read more about azeotropes.