Question

Assertion : When $NaCl$ is added to water, depression in the freezing point is observed.
Reason : The lowering of the vapour pressure of a solution causes depression in the freezing point.
(A) Both assertion and reason are correct and the reason is the correct explanation for the assertion
(B) Both assertion and reason are correct but Reason is not the correct explanation for the assertion.
(C) The assertion is correct but the reason is wrong.
(D) Both assertion and reason are incorrect.

Answer 1

We know that colligative properties are properties that depend on the number of the solute particles or in other words the concentration of the solute. Depression in freezing point, lowering of vapour pressure are all colligative properties and when a solute is added into a solvent all of these effects are observed.

Complete answer:
We know that in addition to a non-volatile solute like $NaCl$ to water, $NaCl$ solution is formed in water. We can say that when this happens there is a relatively lesser number of water molecules at the surface of a liquid because of the $NaCl$ molecules that might occupy the surface. This means that the solution now exerts a lower vapour pressure as compared to that of pure water.
We know that a solution will freeze when its vapour pressure equals the vapour pressure of the pure solid solvent. According to Raoult’s law, when a non-volatile solid is added to the solvent its vapour pressure decreases and now it would become equal to that of solid solvent at a lower temperature. Thus, the freezing point of the solvent decreases. From this, we can say that due to the lowering of vapour pressure, depression in the freezing point of water is observed.
Thus the correct answer for this question will be option (A) Both assertion and reason are correct and the reason is the correct explanation for the assertion.

Note:
There are many other colligative properties that we know about. Elevation in boiling point, Depression in freezing point, Relative lowering of vapour pressure are some of these. All of these work on the same principle that when a non-volatile solvent is added to a solvent it will reduce its vapour pressure causing these effects.