The colligative properties are important for explaining a vareity of natural phenomena and for controlling processes in practice. The effects of the collgiative properties are freezing point depression, boiling point elevation and osmotic pressure. Although most books do not discuss in this way, the vapor pressure lowering is the key feature that unifies these three effects. Since the lowering of the vapor pressure upon addition of a non-volarile solute is a property of the solvent (and not the solute) the colligative ffects are properties of the solvent that dependon the addition of solute.
Vapor pressure lowering
One could make the case that the essence of the colligative properties is the lowering of the vapor pressure of a solvent when a non-volatile solute is added. In reality this too is a colligative property since it occurs irrespective of the identity of the solute. The vapor pressure lowering can be seen as a consequence of Raoult's law, P1 = x1P*1. The vapor pressure is lowered proportional to the mole fraction of the solute. The effect of this lowering is easiest to see for boiling. If water is boiling at 1 atm of pressure and one adds 50 grams of salt to the water it will immediately cease boiling. The reason can be understaood from Raoult's law. The vapore pressure has been reduced by the addition of a non-volatile solute. Therefore, we will need to raise the temperature in order to reestablish equilibrium and start the water boiling again. The boiling point has increased.
