What Is a Reactant in Chemistry? Definition and Examples

Reactants and Products in Chemistry
In a chemical reaction, reactants are starting materials that are consumed to make products.

In chemistry, a reactant is a starting material in a chemical reaction that is consumed to form products. The activation energy required to initiate a chemical reaction breaks the bonds between reactant atoms. The reactant undergoes a chemical change, forming new bonds that result in products. The term “reactant” first came into use around 1900 to 1920.

Examples of Reactants

Reactants are what you start with in a reaction. They are different from what you get after the reaction occurs. Any chemical change involves reactants and products.

  • The wax of a candle and oxygen in air are reactants in a combustion reaction. The products are carbon dioxide and water vapor.
  • When you burn methane gas, the reactants are methane (CH4) and oxygen in air (O2). The products of the reaction are carbon dioxide (CO2) and water (H2O).
  • When water forms from its elements, the reactants are hydrogen (H2) and oxygen (O2) gas. The product is water (H2O).
  • In photosynthesis, the reactants are carbon dioxide (CO2) and water (H2O). The product is glucose (C6H12O6). Note that sunlight is not considered a reactant. Reactants are matter (atoms, molecules, ions), not energy.

Identifying Reactants and Products in Chemical Equations

Look at the reaction arrow to identify the reactants and products in a chemical equation. In a reaction that only proceeds in the forward direction, the arrow points from left to right. The reactants are to the left of the arrow, while the products are to the right of the arrow. If any chemical species are listed on both sides of the equation (e.g., solvent or spectator ions), they are neither reactants nor products.

In the following reaction, A and B are reactants and C is the product:

A + B → C

However, there doesn’t need to be more than one reactant. In this reaction, A is the reactant, while B and C are products:

A → B + C

The number and type of atoms is the same for the products and the reactants in a balanced chemical equation. For example, the number of hydrogen and oxygen atoms is the same for the reactants (H2 and O2) and product (H2O).

2 H2(g) + O2(g) → 2 H2O(l)

The number of each type of atom is its coefficient multiplied by its subscript (or by 1 if no coefficient or subscript is listed). So, there are 4 atoms of hydrogen on the reactant side (2 x 2) and 2 atoms of oxygen (1 x 2). There are 4 atoms of hydrogen on the product side (2 x 2) and 2 atoms of oxygen (2 x 1). The state of matter (s = solid, l = liquid, g = gas, aq = aqueous or dissolved in water) is stated following each chemical formula.

Many reactions proceed in both directions to reach an equilibrium state. Here again, the reaction arrow identifies the reactants and products, but the arrow points both ways! In this type of reaction, the chemical species on each side of the reaction are both reactants and products.

An example is the Haber process, which forms ammonia from nitrogen and oxygen:

N2(g) + 3H2(g) ⇌ 2NH3(g)

The reaction could as easily be written:

2NH3(g) ⇌ N2(g) + 3H2(g)

The equilibrium constant for the reaction indicates how strongly the arrow points one direction versus the other, but it’s not shown in the equation.

Difference Between Reactant and Reagent

In common usage, the terms “reactant” and “reagent” are used interchangeably. Technically, the two words have different meanings. In analytical chemistry, a reagent is a substance added to cause a chemical reaction or test whether one has occurred. Reagents aren’t necessarily consumed in a reaction.

Similarly, solvents, catalysts, and substrates may be involved in a reaction, but they are considered to be neither reactants nor products.


  • IUPAC (1997) “Reactant”. Compendium of Chemical Terminology (the “Gold Book”). Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook