Question

An azeotropic mixture of two liquids boils at a lower temperature than either of them when:
a) It is saturated
b) It does not deviate from Raoult’s law
c) It shows negative deviation from Raoult’s law
d) It shows positive deviation from Raoult’s law

Answer 1

Hint: Azeotropic mixtures or azeotropes are the non-ideal solutions of binary mixtures having the same composition in liquid as well as in vapour phase and boils at a constant temperature. As they are non-ideal solutions they do not obey Raoult’s law.

Complete step by step answer:
There are mainly two types of azeotropic mixtures that are known.
Firstly, minimum boiling azeotropes are the binary mixtures where the boiling point of the mixture is less than either of the two components. These non-ideal solutions show a very large positive deviation from Raoult’s law. The reason for this kind of deviation is that the force of attraction between the newly formed A-B molecules is very less than the attraction between the pure solvent molecules (A-A and B-B). Hence the volume of mixing and enthalpy of mixing became more than zero.
For example: a mixture of $94.5\% $ ethyl alcohol and $4.5\% $ water by volume.
Secondly, maximum boiling azeotropes are the binary mixtures where the boiling point of the mixture is more than either of the two components. These non-ideal solutions show a very large negative deviation from Raoult’s law. . The reason for this kind of deviation is that the force of attraction between the newly formed A-B molecules is more than the attraction between the pure solvent molecules (A-A and B-B). Hence the volume of mixing and enthalpy of mixing became less than zero.
For example: a mixture of $68\% $ nitric acid and $32\% $ water by mass.
Thus the correct option is (d).

Note:
As the force of attraction between newly formed A-B molecules is less, the escaping tendency of those molecules increases. Hence the partial pressure for A solvent and B solvent will be more than the partial pressure of pure solvents.