Sometimes, when a chemical reaction takes place, it proceeds for a period of time and then seems to stop before all the reactants are consumed. But the reaction does not actually stop. Instead, the reaction reaches a point of chemical equilibrium in which the reverse reaction is converting products into reactants as fast as products are formed in the forward reaction. At equilibrium, with both the forward and reverse reactions taking place at the same rate, the concentration of every species no longer changes.
Every reaction has a point in which equilibrium is established. For many reactions, it occurs at the point when essentially all reactants are converted to products; for practical purposes, scientists say that the reaction has gone to completion. But for other reactions, equilibrium occurs when only part of the reactants are converted into products. In these cases and with enough information, it is possible to calculate the concentration of one or more species at equilibrium. In addition, it is possible to predict how the equilibrium will be affected by a change in temperature or an increase or decrease in concentration of a reactant or product. The state of equilibrium is especially important in solutions, and there are many vital equilibria in the chemistry of the body.