Vapor pressure is defined as:

• A. the tendency of a substance to evaporate.
• B. the mass per unit volume of a substance under specified conditions of pressure and temperature.
• C. a process in which liquids or gasses mix with one another.
• D. a concentration of a gas or liquid that is not too rich to ignite.

Answer: (A)

Vapor pressure describes how readily a liquid tends to escape into the vapor phase. At a given temperature, molecules continuously leave the liquid and, once enough vapor collects, it exerts pressure in equilibrium with the liquid. So, the statement that vapor pressure is the tendency of a substance to evaporate captures the core idea: a substance that evaporates more easily has a higher vapor pressure. The other descriptions refer to density, mixing of phases, or flammable concentration limits, none of which define vapor pressure.