A reversible chemical reaction is one in which the conversion of reactants into products and the conversion of products back into reactants occur simultaneously. A double arrow between the reactants and products signifies its reversible nature.

When the rate of formation of products, or the rate of the forward reaction, equals the rate of formation of reactants, or the rate of the reverse reaction, chemical equilibrium is achieved.

Chemical equilibrium is in fact a dynamic process. From the outside, the whole system appears static—however, at the molecular level, it is highly active.

Consider the thermal decomposition of phosphorus pentachloride.

When heated in a closed container, gaseous phosphorus pentachloride decomposes into a mixture of phosphorus trichloride and chlorine gas in a reversible reaction. As the reaction proceeds, the concentration of phosphorus pentachloride decreases while phosphorus trichloride and chlorine gas increase.

As a result, the rate of the forward reaction decreases and that of the reverse reaction increases.

Eventually, the rates of the forward and reverse reactions become equal, and the system achieves chemical equilibrium. In this state, the relative concentrations of phosphorus pentachloride, phosphorus trichloride, and chlorine no longer change.

Such an equilibrium state is also called dynamic equilibrium because both the forward and reverse reactions continue to occur but at the same rates. Since the reactants and products form at the same rates as they are consumed, their concentrations remain constant, though not necessarily equal.

A chemical equilibrium is homogeneous if all the reactants and products in a given reversible reaction are in the same phase — either gaseous or aqueous. In a heterogeneous equilibrium, reactants and products exist in different phases.