Question

In the depression of freezing point it is found that:
A. Vapour pressure of the solution is less than that of the pure solvent.
B. Vapour pressure of the solution is more than that of the pure solvent.
C. Only solute molecules solidify at freezing point.
D. Only solvent molecules solidify at freezing point.

Answer 1

In addition to non-volatile solute to the solvent, the vapour pressure decreases. Then at low temperature, vapour pressure of both solution and pure solvent becomes equal.

Complete step by step answer:
The freezing point is the temperature at which the solid phase and liquid phase of a substance remain in equilibrium. So, at freezing point, vapour pressure of solid equals the vapour phase of liquid.
As the non-volatile solute is added to the solvent, according to Raoult’s law the vapour pressure of the solution decreases. Now, at low temperature, the vapour pressure of the solution becomes equal to the vapour pressure of the pure solvent.So, lowering the vapour pressure lowers the freezing point which is known as depression in freezing point.
So, in the depression in the freezing point the vapour pressure of the solution becomes less than the vapour pressure of the solvent and because vapour pressure of the solution becomes equal to the vapour pressure of the pure solvent so, only solvent molecules solidify.

**Therefore, option (A) Vapour pressure of the solution is less than that of the pure solvent and option (D), only solvent molecules solidify at freezing point, are correct.

Note: **
On adding the non-volatile solute to the solvent the vapour pressure decreases. Non-volatile solute means the solute particle does not tend to become vapour. After the addition of non-volatile solute, the solution has fewer particles that can evaporate compared to the pure solvent, so the vapour pressure decreases.