Question

Why the vapour pressure of a saline solution is lower than that of pure water

Answer 1

Because of “impurities”. Now these impurities can be anything. Ions, colloids, salts, etc. As long as it actually reduces the mole fraction of the pure solvent in the solution. The phenomenon might be caused by any of possible actions of the solutes such as intermolecular interactions with water molecules that could keep if from vaporizing or taking up some of the energy of the system at constant temperature which would result to a lower kinetic energy for water molecules in order for them to vaporize.

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