Question

Some liquids on mixing form ‘Azeotropes’. What are ‘Azeotropes’?

Answer 1

Hint: An Azeotrope is also called a constant boiling mixture. An Azeotrope has a definite composition. A mixture of water and hydrochloric acid is an example of Azeotrope.

Complete step by step answer:
In the case of solutions showing positive deviation from Raoult’s law, at one of the intermediate compositions, the total vapor pressure is the highest and the boiling point is the lowest. In the case of showing negative deviations from Raoult’s law, for one intermediate composition, the vapor pressure is the lowest and the boiling point is the highest. For liquid pairs, such intermediate compositions, the composition of the liquid and the vapor pressure is the same. So, when a liquid mixture does not change its composition on vaporization, and the liquid obtained by the condensation of the vapor also has the same composition i.e. the mixture distills over as if it were a pure liquid. The composition of such mixtures is fixed. So, these mixtures are called a constant boiling mixture or an azeotrope mixture or simply an azeotrope.
If there is a positive deviation, we get minimum boiling azeotropes and if there is a negative deviation, we get maximum boiling azeotrope.
In the case of a minimum boiling azeotrope, the boiling point of the azeotrope is less than the boiling point of either of the pure components. A mixture of water and ethanol is an example of a minimum boiling azeotrope.
In the case of a maximum boiling azeotrope, the boiling point of the azeotrope is more than the boiling point of either of the pure components. A mixture of water and hydrogen chloride is an example of a maximum boiling azeotrope.

Note: It must be noted that we cannot separate an azeotropic mixture using the process of fractional distillation. If we try to concentrate a dilute solution of alcohol, we can do so only till it becomes 95.4% because after that it becomes an azeotrope.