When salt (non-volatile solute) is added to water kept in a sealed vessel, boiling point of the solution will be elevated or increased. So, vapour pressure of water (solution) will be decreased. Lowering of vapour pressure (∆p=p∘−p) is also proportional to mole fraction of the solute (χB) present in the solution. ∆p∝χB[p∘= vapour pressure of pure water p= vapour pressure of solution]