Raoult's Law Raoult's Law One of the colligative properties of solutions is the vapor pressure depression that takes place when solute is added to a pure solvent. A man by the name of Raoult found that when he added solute to a pure solvent it depressed the vapor pressure of the solvent. The more he added the more depressed the vapor pressure became. This became known as Raoul't Law. The molecular view of Vapor Pressure Depression From a molecular view if we introduce non-volatile solute particles, ions or molecules, into the pure solvent, some of the solute particles will take the position of solvent molecules on the surface of the solution.
Evaporation is a surface phenonmenon.
Molecules or ions can not enter the vapor state when immersed beneath the surface. The submerged particles have other particles surrounding them and forces between the particles are suffcient to prevent the submerged particles from overcoming these forces with the available Kinetic Energy at their disposal. However surface particles have only the particles below them holding them in the liquid state. Therefore, it is the surface molecules that have he best chance of overcoming these inter particle forces and enetering the vapor state. If solute particles take the place of solvent molecules and these solute particles are non-volatile, they succeed in only blocking solvent molecules from vaporizing.