Living things carry out many chemical reactions in order to stay alive. A chemical reaction occurs when chemical bonds are made or broken between atoms.
The molecules that take part in a chemical reaction are known as reactants and the final molecules that form as a result of the chemical reaction are known as products. Chemical reactions are reversible so that reactants in the forward reaction become products in the opposite reaction.
When the amount of reactants and products do not change, one can describe this state as chemical equilibrium.
An example of a reversible reaction is when two molecules of hydrogen and one molecule of oxygen combine to form two molecules of water. In this reaction, the reactants are hydrogen and oxygen and the product is water. In the reverse reaction, water can break down into hydrogen and oxygen. In the reverse reaction, water is now the reactant and the products are hydrogen and oxygen.
If these reactions are occurring at the same rate, chemical equilibrium will occur. At equilibrium, the relative concentrations of both the reactants and products do not change.