Question

In the depression of freezing point experiment 

A. Vapour pressure of the solution is less than that of pure solvent 

B. Vapour pressure of the solution is more than that of pure solvent 

C. Only solute molecules solidify at the freezing point 

D. Only solvent molecules solidify at the freezing point 

Choose the most appropriate answer from the options given below:

• A. A only
• B. $B$ and $C$ only
• C. A and C only
• D. A and D only

Answer 1

The Correct Option is D

To understand the question about the depression of the freezing point, let's explore the concept. The freezing point of a solution is lower than that of the pure solvent, due to the presence of a solute, affecting the vapour pressure and solidification behaviour.

1. Vapour Pressure and Freezing Point: When a non-volatile solute is added to a solvent, the solution's vapour pressure is lower than that of the pure solvent. This is due to the fact that the solute particles occupy space on the surface and reduce the number of solvent molecules at the surface. Hence, statement A is correct.
2. Solidification Process: At the freezing point, the pure solvent molecules solidify, because freezing is a process where the liquid turns into solid. The solute generally does not crystallize, especially in cases where it is non-volatile or stays in solution. Therefore, statement D is correct.
3. Analysis of Other Options:
   • B - Contrary to the correct process as we've elucidated, vapour pressure of the solution is lower than that of the pure solvent.
   • C - Only solvent molecules solidify, not solute molecules, under typical conditions.

Conclusively, the most appropriate answer is A and D only. The vapour pressure of the solution is indeed less than that of the pure solvent, and at the freezing point, it is predominantly the solvent molecules that solidify.