In a depression in freezing point experiment, it is found that:

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

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

(3) Only solute molecules solidify at the freezing point

(4) Only solvent molecules solidify at the freezing point

1, 4

The correct answer is option 3. 1, 4.

The freezing point is the temperature at which the solid and liquid phases of a substance coexist in equilibrium. At this point, the vapor pressure of the solid phase is equal to the vapor pressure of the liquid phase.

When a non-volatile solute is added to a solvent, Raoult's law states that the vapor pressure of the resulting solution decreases. This occurs because the presence of solute particles reduces the number of solvent molecules able to escape into the vapor phase.

At lower temperatures, the vapor pressure of the solution eventually reaches a point where it becomes equal to the vapor pressure of the pure solvent. This decrease in vapor pressure leads to a lowering of the freezing point, a phenomenon known as depression in freezing point.

In the depression in freezing point experiment, the vapor pressure of the solution is lower than that of the solvent. Moreover, as the vapor pressure of the solution reaches equilibrium with the vapor pressure of the pure solvent, only the solvent molecules solidify at the freezing point, while the solute molecules remain in the liquid phase.

Therefore, option (1) Vapour pressure of the solution is less than that of the pure solvent and option (4) only solvent molecules solidify at freezing point, are correct.