Question

When a substance is dissolved in a solvent, the vapour pressure of the solvent decreases. This brings
(A) An increase in the boiling point of the solution
(B) A decrease in boiling point of a solution
(C) An increase in freezing point of the solvent
(D) None

Answer 1

Hint: The vapour pressure of the solvent decreases with the addition of a substance to the solvent. Vapour pressure means the tendency of the molecules to escape from the surface of a liquid. As the substance is added to the solvent, it will occupy some of the space on the surface, and less area will be available for solvent molecules. As a result, less solvent is available for evaporation.

Complete Step by Step Answer:
The vapour pressure of a solvent is inversely proportional to its boiling point. This is the reason that when a substance is dissolved in a solvent, the vapour pressure of the solvent decreases. This leads to an increase in the boiling point of the solution. Also, there is a decrease in the freezing point of the solution.
Therefore, the correct option is Option (A): An increase in the boiling point of the solution.

Note: Raoult’s law states that a solvent’s partial vapour pressure in a solution is equal to the vapour pressure of the pure solvent multiplied by its mole fraction in the solution. In a graph of vapour pressure versus mole fraction, when the solute is added to the solvent, the partial pressure of the solvent decreases and that of the solute increases. The total pressure of the solution will be the sum of the partial pressures of solute and solvent.