How does adding a solute to a liquid solvent affect its vapor pressure?

When a solute is added to a liquid solvent, the vapor pressure of the solvent decreases. This phenomenon occurs due to the presence of solute particles, which disrupt the ability of solvent molecules to escape into the vapor phase.

In a pure solvent, there is an equilibrium between the liquid and vapor phases, meaning that some molecules are constantly escaping into the vapor, while others are condensing back into the liquid. However, when a solute is introduced, it takes up space at the liquid's surface and reduces the number of solvent molecules that can transition into the vapor state. As a result, fewer solvent molecules are available to exert vapor pressure, leading to a decrease in the overall vapor pressure.

The decrease in vapor pressure is generally proportional to the concentration of the solute; an increase in the amount of solute leads to a greater disruption of the solvent's ability to vaporize, resulting in a larger drop in vapor pressure. This principle is a fundamental aspect of colligative properties, which are properties that depend on the number of solute particles in a solution, not on their identity.

This understanding aligns with the correct answer, which states that the vapor pressure decreases by an amount proportional to the amount of solute added.