What do you call a substance or molecule that participates in a chemical reaction?

In a chemical reaction, a substance or molecule that takes part in the process is referred to as a reactant. Reactants are the starting materials that undergo a transformation during the reaction to form products. This transformation occurs as atoms and molecules rearrange, bond, and interact under specific conditions.

For instance, in the combustion of methane gas ((CH_4)), methane and oxygen are the reactants. During the reaction, they combine to generate carbon dioxide and water, which are the products formed.

Other terms, while related to chemical reactions, refer to different concepts. A product is the substance formed as a result of a chemical reaction, while a solvent is a substance that dissolves a solute to form a solution, and a catalyst is a substance that speeds up a chemical reaction without being consumed itself. Therefore, the term "reactant" specifically describes the molecules that are present at the beginning of a reaction and undergo change.