Why in dilute soln, there is elevation in B.P and depression in F.P

When non-volatile solid solute is added to a solvent some of the liquid solvent molecules are replaced by solute molecules at the surface.The lesser the number of solvent molecules at the surface that fewer molecules evaporate, and thus the vapor pressure of the solution will be lower than the pure solvent.The change in vapour pressure with added solute is called Raoults law. Other colligative properties are elevation of boiling point, depression of freezing point and Osmotic pressure. These depend only on the number of solute molecules in a fixed volume of solvent, not their nature.