Question

Why is vapor pressure reduced in a solution?

Answer 1

Because there are solvent-solute interactions as well as solvent-solvent interactions.

Explanation:

In a pure solvent (i.e. free from SOLUTE), molecules in the liquid phase must overcome solvent-solvent interactions. In a solution a solvent molecule must overcome these interactions as well SOLUTE/SOLVENT interactions to enter the gas phase and contribute to vapour pressure.

Also the number of solvent molecules on the interface between the vapour and the solution is reduced in a solution (why? because there are solute molecules on or near this interface). The net result is that the vapour pressure of a solution is reduced to an extent proportional to the number (or the concentration) of solute particles.

See this link for an undergraduate treatment.