Reactants and reagents are often used interchangeably in chemistry, as both refer to substances involved in chemical reactions. A reactant is specifically a substance that participates directly in the reaction, while a reagent is typically used to detect or produce other substances. The distinction is largely a matter of convention, with "reagent" being the more common term in certain contexts, such as testing kits. For example, hydrochloric acid acts as a reagent to facilitate the reaction of calcium carbonate, which is the reactant. In terminological discussions, "reagent" is generally preferred for contexts like drug testing, though "reactant" can also be acceptable.