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When purchasing or preparing chemicals, reagent describes chemical substances of sufficient purity for use in chemical analysis, chemical reactions or physical testing. Purity standards for reagents are set by organizations such as ASTM International. For instance, reagent-quality water must have very low levels of impurities like sodium and chloride ions, silica, and bacteria, as well as a very high electrical resistivity.
In a broader definition a reagent is any substance used in a chemical reaction. It usually implies a chemical that is added in order to bring about the chemical change.
For example hydrochloric acid is the chemical reagent that would cause calcium carbonate to release carbon dioxide. Similarly, but less obvious, hydrochloric acid is the chemical reagent that reacts with zinc to produce hydrogen even though in this case the hydrogen comes from the acid and not the metal.
To classify any of the chemicals involved in a chemical reaction as the "Reagent" is thus largely a matter of convention or perspective.
The term reagent is named after its discoverer, Victor Grignard see Grignard reagent