The freezing point of a solvent is lowered by the present of a solute.

• A. Molarity
• B. Freezing Point Depression
• C. Molality
• D. Raoult's Law

Answer: (B)

Lowering of a solvent’s freezing point when a solute is added is a classic colligative property. The best descriptor for this effect is freezing point depression, which arises because introducing solute particles disrupts the orderly formation of the solid phase, so the liquid must be cooled further to freeze.

Quantitatively, the extent of the lowering is related to the amount of solute via ΔTf = i Kf m, where m is the molality (moles of solute per kilogram of solvent), and i is the van’t Hoff factor that accounts for how many particles the solute produces in solution. For nonelectrolytes, i is about 1. This shows why molality is the relevant measure in this context, rather than molarity, which is based on volume and can vary with temperature.

Raoult’s Law, on the other hand, describes vapor pressure lowering, a different colligative effect.