When a solid substance is dissolved in a solvent, some physical properties of solution differ from physical properties of pure solvent.
For example Vapor pressure
If a solution of a non-volatile solute is made in a solvent, then the vapor pressure of solution is due to the vapor of that solvent only.
At a certain temperature, the Vapor Pressure of solution is less than pure solvent's Vapor Pressure.
Evaporation is a surface process so depends on surface area.
Molecules of both solute and solvent are present on the solution surface.
Therefore, the surface area surrounded by solvent molecules is reduced and the number of leaving solvent molecules is reduced.
Due to this the Vapor Pressure of solution decreases.
The decrease in vapor pressure of a solution depends on the amount of non-volatile solute present in solution. It does not depend on its nature.
For example, on dissolving 1.0 mol of glucose in 1 kg of water, the vapor pressure of water decreases as much as 1.0 mol of urea is dissolved in 1kg of water.
In such solutions, Raoult's law is expressed as follows.
Raoult's law – The partial vapor pressure of each volatile component of a solution at a given temperature is proportional to its mole fraction.