Definition of reactant


A reactant in chemistry is a substance that starts a chemical reaction. Reactants interact with each other during the reaction to form new substances, called products.


The term "reactant" is universally applicable across all subfields of chemistry, including organic chemistry, physical chemistry, biochemistry, and inorganic chemistry.

Related Terms

  • Product
  • Chemical Reaction
  • Catalyst
  • Reagent
  • Substrate


For example, in the combustion of methane (CH4), methane and oxygen (O2) are the reactants, which react to form carbon dioxide (CO2) and water (H2O) as the products.

Chemical Structures/Formula

In the chemical reaction CH4 + 2O2 -> CO2 + 2H2O, CH4 and O2 are the reactants.

Complexity Level

Basic. The concept of a reactant is a fundamental term in chemistry and is introduced early in chemistry education.

source: Atkins, P., & de Paula, J. (2010). Physical Chemistry. Oxford University Press. McMurry, J., & Fay, R. C. (2015). Chemistry. Pearson.