At equilibrium, the concentration of each gas phase component is the same as the total liquid phase component. In an ideal gas, molecules move around freely and exchange energy (in the form of heat)

When a sealed bottle containing water is half full, both liquid and gaseous phases of water molecules coexist within the container. Initially, water molecules in the liquid phase gain enough energy to overcome the attractive forces between them, such as hydrogen bonding, and escape into the gas phase through the process of evaporation. Similarly, water molecules in the gas phase may lose energy and return to the liquid phase through the process of condensation. As both evaporation and condensation continue within the sealed bottle, a dynamic balance is eventually established. This balance is referred to as equilibrium, specifically vapor-liquid equilibrium, wherein the rate of evaporation equals the rate of condensation. In other words, the number of water molecules leaving the liquid phase matches the number of water molecules returning to it from the gas phase. At equilibrium, the pressure exerted by water molecules in the gas phase, which corresponds to the partial pressure of water vapor, is called the equilibrium vapor pressure. It depends on the temperature of the system and the nature of the liquid. The equilibrium vapor pressure increases with increasing temperature, as more energy is available for water molecules to evaporate from the liquid phase at higher temperatures. It is worth noting that at equilibrium, although the rates of evaporation and condensation are equal, individual water molecules continuously pass between the liquid and gas phases. This dynamic interaction is similar to the way a dynamic equilibrium forms in chemical reactions, where forward and reverse reactions occur simultaneously at equivalent rates, resulting in no net change in the concentrations of reactants and products. In summary, the equilibrium attained in a sealed bottle half full of water represents a dynamic balance between the evaporation and condensation of water molecules. At this point, the number of liquid and gas phase water molecules remains constant over time, the rate of evaporation equals the rate of condensation, and the system achieves a steady state characterized by the equilibrium vapor pressure.