Question

By fractional distillation, only $68$% $HN{O}_3$ can be formed. Further increasing concentration is not possible. Give reason.

Answer 1

An azeotrope or a constant boiling mixture is a mixture of two or more liquids whose proportions cannot be altered by simple distillation. This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture.

Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling mixtures.

By fractional distillation, only $68$% ${HNO}_3$ can be formed. Further increasing concentration is not possible because an azeotropic mixture is formed between ${HNO}_3$ and $H_{2}O$.