Question

An azeotropic solution of two liquids has boiling point lower than either of them when it:
A.Shows a negative deviation from Raoult’s law
B.Shows no deviation from Raoult’s law
C.Shows positive deviation from Raoult’s law
D.Is saturated

Answer 1

Azeotropic solutions are those solutions which are made up of at least two liquids and exhibit the same concentration at liquid and vapour phase. Raoult’s law states that vapour pressure of an ideal solution is proportional to the mole fraction of solvent. There can be positive and negative deviations from Raoult’s law, which can decide the boiling point.

Complete answer:
The correct answer to this question is option C, the boiling point of an azeotropic solution will be lower than the boiling point of either of them when it will show positive deviation from Raoult’s law.
An azeotropic solution is a solution made up of two or more liquids which exhibit the same concentration at liquid and vapour phase. Thus, what the statement in the question means is that the boiling point of the azeotropic solution is less than the individual boiling point of both the liquids.
Now, Raoult’s law states that the vapour pressure of each volatile component in the mixture is directly proportional to its mole fraction. Only ideal solutions follow Raoult’s law properly, non-ideal solutions show either positive or negative deviation from Raoult’s law.
The deviation from Raoult’s law defines the boiling point of the solution, if the vapour pressure of the solution is higher than the previous one, then the solution is said to exhibit positive deviation. As the vapour pressure increases, the boiling point of the solution decreases.
Similarly, the solutions which show lower vapour pressure are said to exhibit negative deviation. As the vapour pressure decreases, the boiling point increases.
Thus, as the boiling point of the solution is lower than the boiling point of either of them, the solution is said to exhibit positive deviation.

Note:
Another way to find out if the solution shows positive or negative deviation is through the change in enthalpy. In the case of an ideal solution, the change in enthalpy is zero, in case of solutions showing positive deviation, the change in enthalpy is positive, and in case of solutions showing negative deviation, the change in enthalpy is negative. In solutions showing positive deviation, absorption of heat takes place, and in solutions showing negative deviation, evolution of heat takes place.