Why does vapor pressure decrease when a solute is added?
Vapor pressure is a measurement of the likelihood of the molecules of a pure solvent to change from the liquid to the vapor phase.
Compare two solvents, water and alcohol. Of these two solvents, alcohol has a greater tendency to have its molecules at the liquid surface escape into the gas phase. We call this evaporation. As a result, the vapor pressure for alcohol is greater than that of water for a given temperature.
If you add a nonvolatile solute to solvent such as water, you decrease the tendency for water molecules to evaporate into the gas phase. In essence, the solute particles obstruct or reduce the likelihood of evaporation.
As a result, fewer molecules change from the liquid to the gas phase, thus reducing the vapor pressure.