15 6. Comparing the four colligative properties
1Vapour pressure
Effect of a non-volatile solute
- decreases
Cause
- fewer free surface molecules; attraction to solute
Everyday example
- salt water evaporates more slowly than fresh
2Boiling point
Effect of a non-volatile solute
- rises
Cause
- lower vapour pressure must be raised to atmospheric pressure
Everyday example
- salted pasta water boils slightly hotter
3Freezing point
Effect of a non-volatile solute
- falls
Cause
- solute hinders the formation of the ice lattice
Everyday example
- salt on icy roads; sea water freezing near −1.9 °C
4Osmotic pressure
Effect of a non-volatile solute
- arises between solutions of different concentration
Cause
- solvent crosses a semi-permeable membrane toward the more concentrated solution
Everyday example
- cells, root absorption, salt-preserved food
All four depend on the number of dissolved particles and are essentially independent of the identity of the solute.