The substances that enter into a reaction are called reactants. Reactants are the starting materials of a chemical reaction. They are transformed into products through a chemical process. For example, in the combustion of methane, the reactants are methane (CH4) and oxygen (O2). These reactants react to form the products carbon dioxide (CO2) and water (H2O).

Reactants can be elements, compounds, or ions. They can be in any state of matter, but they are most commonly in the solid, liquid, or gas state. Reactants are not limited to a specific number or type. A chemical reaction can involve one reactant or multiple reactants. The number and type of reactants in a reaction are determined by the stoichiometry of the reaction.