Reversible chemical reactions follow a similar pattern. The reactants are initially the only molecules around. They react to form products. The amount of reactants dwindles and the forward reaction slows down. The product amounts increase at the same time the reactants are disappearing. They products "decompose" to form reactants. The rate for this reverse reaction increases as the amount of product grows. Ultimately there comes a time when the forward reaction rate and the reverse reaction rates are equal. The mixture is at equilibrium.