Introduction: Equilibrium, or a condition where forward and reverse processes occur at equal rates so that no change occurs, requires several very specific conditions. Conditions for equilibrium are as follows: (1) the reaction must occur in a closed system; (2) the rates of the forward and reverse reactions must be equal; (3) as a result of the forward and reverse reactions occurring at equal rates, the concentrations of both the products and reactants tend to be constant. Equilibrium can occur in both chemical and physical changes. Some examples of these types of changes include the following: (a) melting, where solid and liquid are in equilibrium; (b) the Haber process, in which the amounts of ammonia gas, hydrogen gas, and nitrogen gas involved in the process are constant; and (c) evaporation, where gas and liquid are in equilibrium.

Equilibrium can be seen even more simply in our everyday lives. For example, when a gallon tank of water is left closed but is not full, some liquid water droplets tend to build up on the inside of the bottle. This is because equilibrium is occurring in a closed system between the liquid and gas phases of water. A similar idea applies to soda bottles. In a closed soda bottle, some of the carbon dioxide is escaping from the liquid and forming a high-pressure gas. When the bottle is opened, the solubility of the gas particles decreases and the gas particles tend to escape, since pressure is decreasing.