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Dynamic equilibrium is a state in which the rates of the forward and reverse reactions in a system are equal. This results in a constant concentration of reactants and products, even though individual molecules may be changing. #Step 2: Define Vapor Pressure#

Vapor pressure is the pressure exerted by a vapor in equilibrium with its condensed phase (liquid or solid). It depends on the temperature and the chemical nature of the substance. In the case of a liquid, it means the pressure exerted by the molecules that have escaped from the surface of the liquid to form a vapor above the liquid. #Step 3: Explain Dynamic Equilibrium in terms of Vapor Pressure#

In a closed system containing a liquid, some molecules at the surface of the liquid will have enough energy to overcome the intermolecular forces holding them in the liquid phase, and they will escape into the vapor phase. This process is called evaporation. As the vapor concentration increases, the likelihood of vapor molecules colliding with the surface of the liquid and re-entering the liquid phase (called condensation) also increases. As the system evolves, the rates of evaporation and condensation will eventually become equal, and a state of dynamic equilibrium is reached. At dynamic equilibrium, the vapor pressure remains constant, and the number of molecules evaporating from the liquid equals the number condensing back into it. This constant vapor pressure, which depends on the temperature and the chemical nature of the substance, represents the balance between the forces driving evaporation and those driving condensation.