Assertion (A): A solution of a non-volatile solute in a solvent freezes at a lower temperature than the pure solvent.
Reason (R): At its freezing point, a solution has the same vapour pressure as the pure solid solvent, and a non-volatile solute lowers the vapour pressure of the solvent.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
The freezing point of a substance is the temperature at which its liquid and solid phases have the same vapour pressure, so the two phases are in equilibrium.
Assertion (A): A solution of a non-volatile solute in a solvent freezes at a lower temperature than the pure solvent. — True. This is the depression of freezing point, ΔTf = Tf0 − Tf, a colligative property.
Reason (R): At its freezing point, a solution has the same vapour pressure as the pure solid solvent, and a non-volatile solute lowers the vapour pressure of the solvent. — True. Both parts are the conditions given for freezing of a solution and for the effect of a non-volatile solute.
Why R explains A?
Since the solute lowers the vapour pressure, the solution’s vapour pressure meets that of the solid solvent only when the temperature is brought down further.
So the solution starts to freeze at a lower temperature than the pure solvent, which is A.
NCERT (2026-27 edition):
1.6.3 Depression of Freezing Point – “The lowering of vapour pressure of a solution causes a lowering of the freezing point compared to that of the pure solvent (Fig. 1.8). … 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 lower temperature. Thus, the freezing point of the solvent decreases.”