Colligative properties (Vapour pressure and Raolt's Law)

Let's take two solutions to explain this.

The first solution is one of a pure liquid.  Let's keep in mind that the vapour pressure is caused by some molecules turning from liquid to gas.

On the other hand, the second solution is one made of water (solvent) and a solute that is not volatile, meaning that it will not turn into a gas, i.e. salt. If more space is taken by the salt molecules, then less water molecules can turn to gas. So vapour pressure is lower.

In other words, the more solute or the more concentration of the solution, the lower the vapour pressure because less molecules will be able to turn into a gas.

This is not only a rule though. Later on, François Marie Raolt discovered that the...

Vapour pressure of a solution (P)= Vapour pressure of the pure solvent (P_O) x Molar fraction of the pure solvent (x_O)