The mass action law describes the ratio of the activities (or simplified, the concentrations) of the substances involved in a chemical reaction in the state of equilibrium. This ratio is constant in the state of equilibrium.

The state of chemical equilibrium can be thought of as a situation in which the rates of the outward and backward reactions are equal in each case, without the chemical reaction having come to a “standstill”.

The law of mass action applies to all reversible reactions that are in equilibrium. The derived equilibrium constant K has a fixed value for the reaction under consideration, but it can be influenced by external influences such as temperature or (for gas reactions) pressure.

With the possibility of influencing the position of a chemical equilibrium and calculating the concentration of one reaction partner (e.g. a reaction product) from those of the others, the law of mass action assumes fundamental importance in the quantitative consideration of chemical reactions and in the optimization of yields in industrial processes.