At the freezing point of a solution of a non-volatile solute, which of the following are in equilibrium?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Liquid solvent, solid solvent
The freezing point of a liquid is the temperature at which its solid and liquid phases are in dynamic equilibrium.
When a solution of a non-volatile solute is cooled, the solid that separates out is pure solid solvent (ice from an aqueous solution); the solute stays dissolved in the liquid.
So at the freezing point of the solution, the solvent in the liquid solution is in equilibrium with the solid solvent.
Why the other options are incorrect
Option (2) Liquid solvent, solid solute — Incorrect. The solute does not freeze out; the solid that forms is the solvent.
Option (3) Liquid solute, solid solute — Incorrect. The solute is not present as a separate liquid or solid phase; it stays dissolved in the solution.
Option (4) Liquid solute, solid solvent — Incorrect. The solid solvent is right, but the liquid it is in equilibrium with is the solvent of the solution, not a liquid solute.
NCERT (2026-27 edition):
1.6.3 Depression of Freezing Point – “… We know that at the freezing point of a substance, the solid phase is in dynamic equilibrium with the liquid phase. … A solution will freeze when its vapour pressure equals the vapour pressure of the pure solid solvent … Thus, the freezing point of the solvent decreases.”