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 :

Solution

Chemistry ma’am · 4 steps

Judge each statement on its own: vapour pressure first, then which molecules actually freeze.

1. Vapour pressure. A non-volatile solute takes up part of the surface, so fewer solvent molecules can escape into the vapour. The solution's vapour pressure is lower than the pure solvent's. A is true and B is false.
2. What freezes. When a dilute solution is cooled, the crystals that form are pure solvent. The solute stays dissolved in the remaining liquid. So D is true and C is false.
3. Link to ΔTf. The solid that forms is pure solvent, so it has the same vapour pressure as pure solid solvent. The solution's lower vapour pressure meets that solid curve at a lower temperature. That is why the freezing point drops: Δ T_f = K_f m.
4. So. The true statements are A and D, which is option (c).

The idea to keep. In freezing point depression, the solute lowers the vapour pressure and stays in the liquid. Only pure solvent separates out as the solid, so the solution must be cooled further before it freezes.

Answer from the NTA final answer key. Solution by PrepWiser.