A colligative property is a property of a solution that depends only on the number of solute particles (molecules or ions) present in the solvent, and not on the type or nature of the solute. This means it is determined by how many particles are dissolved, regardless of whether they are polar, nonpolar, large, or small.
The key idea is that when solute particles are added to a solvent, they disturb the physical properties of the liquid in predictable ways.
The main colligative properties are:
- Relative lowering of vapor pressure – The vapor pressure of a solution is lower than that of the pure solvent.
- Elevation of boiling point – The solution boils at a higher temperature than the pure solvent.
- Depression of freezing point – The solution freezes at a lower temperature than the pure solvent.
- Osmotic pressure – The pressure required to stop the flow of solvent through a semipermeable membrane into the solution.
For example, adding salt to water lowers its freezing point, which is why salt is used to melt ice on roads.