Why Freezing Point Depression Is a Colligative Property

Freezing point depression is colligative because it depends only on how many solute particles are dissolved, not on what they are. A solute dilutes the solvent, which makes the liquid more stable relative to the pure solid, so the solution must be cooled further to freeze.

The formula

ΔT_f = i · K_f · m

• i: van't Hoff factor, the number of particles per formula unit.
• K_f: solvent constant.
• m: molality, mol solute per kg solvent.

The freezing point falls by ΔT_f from the pure solvent value.

Particle count, not identity

• 1 m glucose: i = 1.
• 1 m NaCl: i near 2.
• 1 m CaCl₂: i near 3, so the largest drop.

Why molality

Molality does not change with temperature, unlike molarity, so it is used for freezing point and boiling point problems.

Common traps

• Forgetting i for ionic compounds.
• Using molarity instead of molality.
• Raising instead of lowering. Boiling point goes up; freezing point goes down.
• Thinking the solute freezes out. In ideal cases pure solvent freezes first.