Chemical reagents, reactants, or reagents ( English: reactant or reagent) is a substance that causes or consumed in a chemical reaction.

For example, hydrochloric acid is a reactant which reacts with zinc metal to produce hydrogen, or reacts with calcium carbonate to produce carbon dioxide.

The term reagent is also used to refer to the chemical purity sufficient for an analysis or experiment. For example, a reagent water should not contain many impurities such as sodium ions, chloride, or bacteria, and also has a high electrical resistance