Vapor pressure is the pressure exerted by a substance's vapor phase when it reaches dynamic equilibrium with its liquid phase at a given temperature — the state where the rate of evaporation (surface molecules with kinetic energy above an escape threshold, per the Maxwell-Boltzmann-like distribution, leaving the liquid) equals the rate of condensation (vapor molecules losing kinetic energy and returning to the liquid). This equilibrium concept belongs to thermodynamics and phase-change theory, and it explains volatility: substances with higher temperature, weaker intermolecular forces, or lower molecular mass require a greater fraction of molecules in the vapor state to reach equilibrium, yielding higher vapor pressure. Boiling is defined as the specific case where vapor pressure equals the surrounding atmospheric (or ambient) pressure, linking vapor pressure to the pressure-dependence of boiling point established in phase-diagram theory.
Vapor pressure is the pressure exerted by a substance's vapor phase when it reaches dynamic equilibrium with its liquid phase at a given temperature — the state where the rate of evaporation (surface…