MCAT General Chemistry Practice Exam 2025 – Comprehensive Test Prep

Adding a nonvolatile solute to a solvent decreases the vapor pressure of the solution. 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 in that solution. When a nonvolatile solute is added, it occupies space at the surface of the solution, which reduces the number of solvent molecules that can escape into the vapor phase.

Since the vapor pressure is dependent on the concentration of solvent molecules that are able to vaporize, the presence of a nonvolatile solute effectively lowers the mole fraction of the solvent. Consequently, this leads to a decrease in the vapor pressure compared to that of the pure solvent. The impact of adding a solute is particularly significant because it alters the physical properties of the solution without the solute contributing to the vapor pressure itself, as it does not evaporate. Thus, the correct answer reflects the fundamental principle of colligative properties in solutions, specifically how solute particles affect the behavior of solvent molecules in terms of vaporization.