Question

In the depression of freezing point experiment, it is found that the:
This question has multiple correct options
$A.$ 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 solidity at the freezing point
$D.$ Only solvent molecules solidity at the freezing point

Answer 1

Hint: At the freezing point, the solute molecules do not solidify and the solution's vapor pressure is no greater than that of pure solvent.

Complete step-by-step answer:
A substance's freezing point is defined as the temperature at which the solid vapor pressure is in equilibrium with the substance's liquid form. It can also be said that the substance's solid and liquid forms have the same concentration of vapour.
Depression at the freezing point defines the mechanism in which applying a solute to a solvent reduces the solvent's freezing point. For example, salt in water, alcohol in water or the mixture in a finely ground substance of two solids such as impurities where the added compound is the solvent, and the original solid is solvent. The resulting solution or non-solid mixture has a lower freezing point than the pure solvent. This effect is what allows seawater to stay liquid at temperatures below ${0^\circ }C$, the freezing point of pure water which is a combination of salt and other things in water. By the above principle we can infer that the solution's vapor pressure is less than that of pure solvent, only solvent molecules at the freezing point solidifies.
When the solvent is pure then the whole exposed surface will be only solvent molecules present on the surface or else, we can say the area that is exposed on the outer side is solvent molecules. When it will be in solution form along with solvent molecules there will be solute molecules because of which the vapour pressure decreases.
Hence, we can say that option A and D is the correct answer and as per this in the depression of freezing point experiment, it is found that the vapour pressure of the solution is less than that of pure solvent and only solvent molecules solidity at the freezing point.

Note: Vapor pressure is the pressure exerted by a vapor in the thermodynamic equilibrium with its condensed phases in a closed system at a given temperature. We must remember the properties of solute and solvent for answering these types of questions.