The dilute solutions of non-volatile solutes exhibit certain characteristic properties which do not depend upon the nature of the solute but depend only on the number of particles (molecules or ions) of the solute i.e., on the molar concentration of the solute. These are called colligative properties (colligative, from Latin: co means together, ligare means to bind). Thus, the properties of the solutions which depend only on the number of solute particles but not on the nature of the solute are called colligative properties.

The four important colligative properties are:

1. Relative lowering in vapour pressure
2. Elevation in boiling point
3. Depression in freezing point
4. Osmotic pressure

What are colligative properties useful for?

Determining molar mass

You are correct. Colligative properties are useful for determining the molar mass of unknown compounds. They are not used to determine boiling and melting temperatures because the addition of a solute will change the boiling and melting temperatures of the solution. The change in boiling and melting temperatures is proportional to the number of solute particles in the solution, so it cannot be used to determine the molar mass of the solute.

Equivalent weight can only be determined if the molar mass is known. The van't Hoff factor is a measure of the number of particles that a solute dissociates into in solution. It can be determined by measuring the colligative properties of a solution and comparing them to the colligative properties of a solution of a known van't Hoff factor.

Here is a summary of the colligative properties and their uses: