Chemical equilibrium hinges on the fact that most chemical reactions are reversible. As the forward reaction proceeds, products accumulate; these products can reform reactants.

Equilibrium occurs when the reversible reactions [local] are occuring at the same rate, i.e., the forward reaction forms products at the same rate that the reverse rate reforms reactants.When reactions are kept under closed conditions, and the reaction is reversible, chemical equilibrium can occur.

The state of chemical equilibrium can be represented mathematically by the mass action expression. [local]