Although the terms reactant and reagent are often used interchangeably, reagent is often used in a more specialized sense as "a test substance that is added to a system in order to bring about a reaction or to see whether a reaction occurs". Examples of such analytical reagents include Fehling's reagent and Tollens' reagent. In organic chemistry, reagents are compounds or mixtures, usually composed of inorganic or small organic molecules, that are used to effect a transformation on an organic substrate. Examples of organic reagents include the Collins reagent, Fenton's reagent, and Grignard reagent.
In another use of the term, when purchasing or preparing chemicals, "reagent-grade" 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 .