In most reactions two or more substances react to form products which themselves react to give back the original substances. Thus A and B may react to form C and D which react to reform A and B.

A reaction which can go in the forward and backward direction simultaneously is called a reversible reaction. Such a reaction is represented by showing a pair of arrows between the reactants and products.

The arrow pointing right indicates the forward reaction, while that pointing left shows the reverse reaction.

In a chemical equilibrium, the forward and reverse reactions do not stop, rather they continue to occur at the same rate, leading to constant concentrations of the reactants and the products. Plots showing how the reaction rates and concentrations change with respect to time are.

Figure 2. A mixture of N₂O₄ and NO₂ moves toward equilibrium.