Reagent

In chemistry, a reagent ( ree-AY-jənt) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. A reactant is a substance or compound that is consumed in a chemical reaction. The terms reactant and reagent are often used interchangeably, but reactant specifies a substance consumed in the course of a chemical reaction; reagent is used in the context of chemical analysis, while reactant is used in the context of reaction itself. Solvents, though involved in the reaction mechanism, are usually not called reactants. Similarly, catalysts are not consumed by the reaction, so they are not reactants. In biochemistry, especially in connection with enzyme-catalyzed reactions, the reactants are commonly called substrates.