JEE Main 2025 Jan 24 Shift 1, Chemistry Q17: Colligative Properties

Two ideas fix the picture:

1. Adding a non-volatile solute lowers the vapour pressure of the liquid at any given temperature, so the curve for the solution lies below the curve for the pure liquid solvent.

2. Freezing point is defined where the vapour pressure of the liquid phase equals that of the solid phase (the frozen-solvent curve). Because the solution's liquid curve is depressed, it meets the solid-solvent curve at a lower temperature.

Therefore the correct sketch must show:

(i) the liquid-solvent curve above the solution curve, and

(ii) the freezing point of the solution $T_f$ lying to the left of the pure solvent's freezing point $T_f^{\circ }$, with $\Delta T_f=T_f^{\circ }-T_f$.

Option (3) shows the liquid-solvent curve above the solution curve and the frozen-solution curve meeting them at the lower $T_f$. $\therefore$ Option (3).