Many chemical processes are reversible. Unlike most combustion reactions, reversible processes do not proceed all the way to the products. In reversible chemical systems, both the forward and reverse processes occur at the same time, so starting substances and products exist in the same mixture. When two opposite processes are in balance, the result is an equilibrium mixture. To the eye, the equilibrium mixture may appear unchanging, but this is not the case. Changes are continuously happening on the atomic scale in systems at equilibrium.