Which term refers to the pressure exerted by vapor in equilibrium with a liquid?

• A. Osmotic pressure
• B. Atmospheric pressure
• C. Vapor pressure
• D. Partial pressure

Answer: (C)

Vapor pressure is the pressure exerted by vapor in equilibrium with a liquid. At a given temperature, molecules continually escape from the liquid into the vapor phase and, as soon as equilibrium is reached, the rate of evaporation equals the rate of condensation. The resulting steady pressure in the space above the liquid is the vapor pressure. This pressure rises with temperature because more molecules have enough energy to escape into the vapor.

This concept is distinct from osmotic pressure (driven by solute concentration across a membrane), atmospheric pressure (the total external pressure from the surroundings, not just the vapor), and partial pressure (the portion of the total pressure contributed by a specific gas in a mixture).