The vapor pressure of a compound refers to the pressure generated by the vapour of a compound in equilibrium with its condensed stage. The major determinant of a compound vapour pressure is temperature. The vapour pressure of an aqueous solution of sodium chloride will be lower than that of pure water. This is because, dissolution of solute in a solvent always reduces the vapour pressure of the solution as a result of interactions between the molecule of the solute and the solvent. Thus, a pure compound will always have a higher vapour pressure than a solution.