What is indicated by the term "vapor pressure"?

The term "vapor pressure" refers specifically to the pressure exerted by a vapor when it is in thermodynamic equilibrium with its liquid or solid form at a given temperature. At this equilibrium, the rate of evaporation of the liquid into vapor equals the rate of condensation of vapor back into the liquid. This means that the molecules in the vapor phase are in a dynamic balance with those in the liquid or solid state, contributing a measurable pressure that reflects the tendency of the liquid or solid to evaporate. The higher the temperature, the greater the vapor pressure because more molecules have enough energy to enter the vapor phase.

Understanding vapor pressure is significant in the context of various applications, such as distillation, refrigeration, and understanding phase changes in substances. This concept is fundamental for engineers working with thermodynamic systems, specifically in phases of matter and heat transfer processes.