Question

what is an azeotrope? why can't a solution with a maximum boiling azeotrope be completely separated by distillation at a single pressure?

Answer 1

Azeotropes are otherwise known as constant boiling liquid solutions. Some miscible liquids when present together in certain composition, tend to boil at the same temperature irrespective of their individual boiling points. The boiling point of an azeotropic mixture will usually be low and at this point ,the composition of the liquid and vapour phases of the mixture would be the same.The azeotropes may be binary mixtures ( two components ) or ternary (three components ).

For instance ,water forms an azeotrope with ethanol where their composition is 4.5:95.5 and the mixture is homogeneous and it boils down at 351.1K as opposed to boiling points of water and ethanol being 373 K , 351.3 K respectively.

As mentioned earlier, azeotrope boils at a constant temperature, at the time of distillation both the components of the mixture come along to the receiver flask. However ,fractional distillation procedure helps in this regard to separate components by adding another component which would disturb the already existing composition .

Azeotropic mixture of water-ethanol is separated by adding a suitable amount of benzene followed by fractional distillation.