There are four colligative properties:

1. Relative lowering of vapour pressure

2. Elevation in boling point.

3. Depression in freezing point.

4. Osmotic pressure

Depression in freezing point:

Freezing point is the temperature at which vapour pressure of solution and liquid phase becomes equal.

Reason:

On adding non-volatile solute, vapour pressure decreases so Freezing Point also decreases depression in Freezing Point.

Liquid solvent in solution is in equilibrium with solid solute. So, at Freezing Point only solvent solidify, so that amount of solute remains unchanged in the solution and concentration of solution increases.

Raoult’s Law :( Relative lowering of vapour pressure)

When non-volatile solute (B) is added to pure solvent (A).

On adding non-volatile solute vapour, pressure decreases and according to Raoult’s Law, mole fraction of non-volatile solute becomes equal to relative lowering in vapour pressure.

When a Solute is added in the solvent, the solute particles the surface of liquid which is exposed to environment reduces and in turn lowers the number of Solvent molecules Going into the vapours and then IT is responsible for Low vapour pressure at a given Temperature. Similarly, the Freezing point is also Reduced, suppose water Freezes at 0C the if we add salt to it it may freeze at below zero.