In the depression of freezing point experiment, it is found that:

(A) The vapour pressure of the solution is less than that of the pure solvent.

(B) The vapour pressure of solution is more than that of the pure solvent.

(C) Only the solute molecules solidify at the freezing point.

(D) Only the solvent molecules solidify at the freezing point.

Choose the correct answer from the options given below:

(A) and (D) Only

The correct answer is option 2. (A) and (D) Only.

Let us look at each of the statement carefully:

(A) Vapour pressure of the solution is less than that of the pure solvent. This is true. When a solute is added to a solvent, it disrupts the solvent molecules at the surface, making it harder for them to escape into the vapor phase. This lowers the vapour pressure of the solution compared to the pure solvent.

(B) Vapour pressure of solution is more than that of the pure solvent. This is incorrect. As explained above, adding a solute decreases the vapour pressure.

(C) Only the solute molecules solidify at the freezing point. This is incorrect. At the freezing point, only the solvent molecules solidify.

(D) Only the solvent molecules solidify at the freezing point. This is true. Even though the presence of solute particles disrupts the arrangement of solvent molecules, at the freezing point, only the solvent molecules have enough energy to solidify and form crystals.

Therefore, the experiment demonstrates that adding a solute to a solvent leads to a decrease in the freezing point (depression) and a decrease in the vapour pressure of the solution.