Many reactions are irreversible. But in a reversible reaction, the products can react to produce the original reactants. At equilibrium, the concentrations of reactants and products do not change. Ammonia is widely used in fertilisers and is manufactured using the Haber process.

Sulfuric acid is used in the manufacture of paints, detergents and fertilisers. The contact process demonstrates a reversible reaction used in the production of sulfuric acid.