Lowering of vapor pressure is best described as which type of change?

• A. Chemical change
• B. Physical change
• C. Phase change
• D. Nuclear change

Answer: (B)

Lowering vapor pressure is a physical change. When a solute is added to a solvent, the chemical identities of the substances don’t change (no new substances are formed in the common case of dissolution), but the observable property—the vapor pressure of the solvent—decreases. This happens because the presence of dissolved particles reduces the fraction of solvent molecules at the surface that can escape into the vapor, described by Raoult’s law (P_solution = X_solvent × P°_solvent). The liquid remains the same phase, and no chemical bonds are formed or broken in the process. A phase change would mean the substance actually changes between solid, liquid, or gas; a chemical change would involve a chemical reaction altering composition; a nuclear change involves changes at the nucleus—none of which are required here.