Science, Tech, Math › Science Reactant Definition and Examples Chemistry Glossary Definition of Reactant Share Flipboard Email Print Comstock/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated August 10, 2019 Reactants are the starting materials in a chemical reaction. Reactants undergo a chemical change in which chemical bonds are broken and new ones formed to make products. Formulating Chemistry Equations In a chemical equation, reactants are listed on the left side of the arrow, while products are on the right side. If a chemical reaction has an arrow that points both left and right, then substances on both sides of the arrow are reactants as well as products (the reaction proceeds in both directions simultaneously). In a balanced chemical equation, the number of atoms of each element is the same for the reactants and products. The term "reactant" first came into use around 1900-1920. The term "reagent" is sometimes used interchangeably Examples of Reactants A general reaction may be given by the equation: A + B → C In this example, A and B are the reactants and C is the product. There do not have to be multiple reactants in a reaction, however. In a decomposition reaction, such as: C → A + B C is the reactant, while A and B are the products. You can tell the reactants because they are at the tail of the arrow, which points toward the products. H2 (hydrogen gas) and O2 (oxygen gas) are reactants in the reaction that forms liquid water: 2 H2(g) + O2(g) → 2 H2O(l). Notice mass is conserved in this equation. There are four atoms of hydrogen in both the reactant and product side of the equation and two atoms of oxygen. The state of matter (s = solid, l = liquid, g = gas, aq = aqueous) are stated following each chemical formula.