Question

Freezing point of solution is less than that of pure solvent, which of the following statements are correct ?
(A) Vapour pressure of solution is less than that of pure solvent
(B) Vapour pressure of solution is greater than that of pure solvent
(C) Only solvent molecules will freeze
(D) Only solute molecules will freeze

• A. A & B only
• B. A & C only
• C. C & D only
• D. A, B & D only

Answer 1

Answer: (B)

A & C only

Answer 2

The freezing point denotes the temperature where a substance's solid and liquid phases coexist harmoniously. Consequently, at this freezing point, the vapor pressure of the solid matches that of the liquid's vapor phase.

Upon introducing a non-volatile solute to the solvent, Raoult's law dictates that the solution's vapor pressure declines. Consequently, at lower temperatures, the solution's vapor pressure aligns with that of the pure solvent. As a result of this reduction in vapor pressure, the freezing point decreases, a phenomenon termed "depression in freezing point."

During this depression in the freezing point, the solution's vapor pressure falls below that of the solvent. Subsequently, since the solution's vapor pressure coincides with the vapor pressure of the pure solvent, only the solvent molecules undergo solidification.

The Correct Option is (B): $A \;\& \;C \;only$