A chemical equation is an expression in which symbols and formulae represent a chemical reaction. The reactants (the starting substances) are shown on the left-hand side of the equation, and the products (the substances formed in the reaction) are shown on the right-hand side. An arrow separates the reactants from the products.

For example, the following equation shows the reaction of hydrogen and oxygen to form water:

$$2H_2 + O_2 → 2H_2O$$

In this equation, the reactants are hydrogen gas ($H_2$) and oxygen gas ($O_2$), and the product is water ($H_2O$). The coefficients in front of the formulae tell us how many molecules of each substance are involved in the reaction. In this case, two molecules of hydrogen react with one molecule of oxygen to form two molecules of water.

Chemical equations can also include other information, such as the temperature and pressure at which the reaction occurs, and the enthalpy change of the reaction.