1 Definition

Colligative properties

• are properties of a solution that depend on the number of molecules or ions of a non-volatile solute dissolved in it, and not on the type (identity) of the solute.
• ("Colligative" means "depending on the collection".)

They include four properties:

1. Decrease in the vapour pressure of the liquid.
2. Elevation (rise) of the boiling point.
3. Depression (decrease) of the freezing point.
4. Osmotic pressure (and the phenomenon of osmosis).

• Important: a non-volatile solute is one that does not evaporate appreciably (salt, sugar, urea).
• The more particles in the solution, the bigger the effect.
• One mole of NaCl produces two moles of particles (Na⁺ and Cl⁻) so its colligative effect is about double that of one mole of glucose, which stays as single molecules; one mole of CaCl₂ gives three moles of particles (about three times the effect).