Consider the given plots of vapour pressure (VP) vs temperature(T/K). Which amongst the following options is correct graphical representation showing Δ T_f, depression in the freezing point of a solvent in a solution?

Solution

Chemistry ma’am · 3 steps

Freezing happens where the solid's vapour pressure curve meets the liquid's, so look for those two crossings.

1. Freezing point rule. A liquid freezes at the temperature where its vapour pressure equals the vapour pressure of the solid solvent. So on a VP vs T plot, a freezing point is a crossing with the solid curve. A crossing with a horizontal line belongs to boiling, not freezing.
2. Where each curve sits. A non-volatile solute lowers vapour pressure, so the solution curve must lie below the liquid solvent curve. The steep solid curve then meets the pure liquid first, at T_f°, and meets the lower solution curve at a lower temperature, T_f.

   Option	What the plot shows	Verdict
   (a)	Two curves cut by a horizontal constant-pressure line, with no liquid solvent curve	a boiling-style plot, wrong
   (b)	Solution curve drawn above the liquid solvent curve	solute should lower VP, wrong
   (c)	Solution below liquid solvent, and the solid curve meets the solvent at T_f° and the solution at a lower T_f	correct
   (d)	Solution curve above the liquid solvent curve, read at a horizontal line, with no solid curve	boiling-style and wrong order

3. So. Only (c) shows the solid curve cutting both liquid curves, with the solution's crossing to the left of the solvent's. The gap T_f° - T_f is Δ T_f.

The idea to keep. Boiling point: read where the liquid curve hits the constant external pressure line. Freezing point: read where the liquid curve hits the solid curve. In both cases the solution's curve sits below the pure solvent's.

Answer from the NTA final answer key. Solution by PrepWiser.