Colligative PropertiesJEE Main

Concept

Colligative properties depend only on the number of dissolved particles, not their identity. Dissolving a solute lowers the freezing point and raises the boiling point — which is why salt melts ice and antifreeze protects engines.

Key formula

Derivation

A solute lowers the solvent's vapour pressure (Raoult's law), shifting the phase boundaries: the liquid must be cooled further to freeze and heated further to boil.

The shifts scale with molality $m$ and the van't Hoff factor $i$ — the number of particles each formula unit releases (NaCl → 2 ions, so $i = 2$). For water, $K_f = 1.86$ and $K_b = 0.512$ K·kg·mol⁻¹.

Scenarios to explore

• Colligative Properties — Freezing-point depression & boiling-point elevation.

Real-world applications

• De-icing roads and antifreeze in radiators.
• Determining molar masses of unknown solutes.
• Osmosis in cells and desalination.

JEE exam tips

• Ranking effect: more particles (higher $i \times m$) means larger $\Delta T$.
• For weak electrolytes, $i = 1 + (n-1)\alpha$ links to the degree of dissociation.

Common mistakes

• Forgetting the van't Hoff factor for ionic solutes.
• Using molarity instead of molality.
• Sign error — freezing point goes down, boiling point goes up.

Exam traps to avoid

• Equal molality of NaCl and glucose are not equal in effect — NaCl's $i=2$ doubles it.
• Colligative properties ignore the solute's chemical nature entirely.