Hello,

Vapor pressure lowering is a colligative property of solutions. The vapor pressure of a pure solvent is greater than the vapor pressure of a solution accomodating a non volatile liquid. This lowered vapor pressure leads the way to boiling point elevation.

The number of solvent molecules getting away from vapour phase gets reduced and as a result the pressure applied by the vapour phase is also reduced. This is known as relative lowering of vapour pressure.

Thank you.