Advanced Placement (AP) Chemistry Practice Exam

What does vapor pressure refer to?

1. The pressure exerted by a solid over a liquid

2. The pressure exerted by a vapor over a liquid

3. The pressure of a gas in a closed container

4. The pressure exerted during chemical reactions

The correct answer is: The pressure exerted by a vapor over a liquid

Vapor pressure specifically refers to the pressure exerted by a vapor in equilibrium with its liquid (or solid) form in a closed system. When a liquid evaporates, molecules at the surface enter the gas phase. In a closed container, these vapor molecules exert pressure on the walls of the container. Once the rate of evaporation equals the rate of condensation—the point at which a dynamic equilibrium is established—the pressure exerted by the vapor is referred to as vapor pressure. This concept is vital in understanding phase changes, and it plays a crucial role in various applications, including predicting how substances behave under different temperatures and conditions. The ability of a substance to vaporize and the resulting vapor pressure can significantly affect boiling points and physical properties of substances. Other options do not accurately capture the definition. The pressure exerted by a solid over a liquid or during chemical reactions does not pertain to the equilibrium of a vapor with its liquid phase. Pressure of a gas in a closed container is a more general term that does not specifically involve vaporization or the relationship between a vapor and its liquid state.