We have implied that all reactions only go in the direction of the products, but this is not always the case. Sometimes as products are formed, they react with one another or decompose to form the reactants. Thus, the reaction is going in both directions at the same time, and if allowed to continue indefinitely, would result in a constant amount of products and reactants. Reactions that go in both directions are called equilibrium reactions, and when the rate of formation of product is the same as the rate of formation of reactant, they are said to be in equilibrium. In writing an equation, we indicate equilibrium by drawing arrows pointing in opposite directions
As an example of an equilibrium reaction, consider the dissociation of a compound into ions:
Na2CO3 « 2Na+ + CO3-2
Sodium carbonate in solution dissociates into sodium ions and carbonate ions. Some of the ions come back together to form sodium carbonate. Thus, an equilibrium is established.