What happens to the vapor pressure of a solvent when a solute is added?

When a solute is added to a solvent, the vapor pressure of the solvent decreases. This phenomenon is explained by Raoult's Law, which states that the vapor pressure of a solvent in a solution is proportional to the mole fraction of the solvent. When a non-volatile solute is added, it occupies space on the surface of the liquid and reduces the number of solvent molecules that can escape into the vapor phase. As a result, there are fewer solvent molecules in the vapor above the liquid, leading to a decrease in vapor pressure.

This behavior is consistent across most solutions containing non-volatile solutes, demonstrating a colligative property, which depends on the number of solute particles present rather than their identity. Thus, the addition of a solute effectively diminishes the escape tendency of solvent molecules, lowering the vapor pressure.