Why is it not possible to obtain pure ethanol by fractional distillation ? What general name is given to binary mixture which show deviation from Raoult's law and whose omponents cannot be separted by fractional distillation. How many types of such mixture are there?

Explanation

A mixture of ethanol and water is genrally obtained when we prepare it commercially with the help of alcoholic fermentation. The two liquids are completely miscible with each other. Their boiling points are 351 K and 373 K respectivly. Since the two liquids differ in theit boiling points by more than `20^(@)`, they should be easily separated by fractional distillationprocess. Howerver, during the operation, a stage is reached when we get a liquid mixture containing 95 peracent ethanol and 5 percent water. This constitults and azeotropic mixture also called constant boiling mixture. Both alcohol and water start boiling together at a temperature whcih is close to the biling point tempreature of alcohol. The azetropic mixture shows positive deviation from Raoult's Law. In fact there are two types of azeotropit mixtures.
Azotropic mixture which shows positive deviation from Raoult's Law (Minimum boiling azeotrope)
Azeotropic mixture which shows which shows nagitive deviation from Raoult's Law ( Maximium boiling azeotrope)
For more ditails, consult section 7.