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 is directly proportional to the mole fraction of the solvent in a solution. When a non-volatile solute is added, it occupies some of the surface area at which the solvent molecules may evaporate, effectively reducing the number of solvent molecules that can escape into the vapor phase. As a result, there are fewer solvent molecules capable of contributing to the vapor pressure, leading to a decrease in that pressure. This is a key concept in colligative properties, which describe how the properties of solutions change when solutes are added. In summary, the addition of a solute lowers the solvent's vapor pressure due to reduced mole fraction and increased intermolecular interactions within the solution.