Reagent
A substance added to a system to bring about a chemical reaction or to test for the presence of another substance.
Unit
Dimensionless
In depth
Reagents are graded by purity — technical, laboratory, analytical (AR) and primary standard — and the grade required depends on how directly the impurity would affect the result. A reagent consumed in the reaction is distinguished from a catalyst, which is recovered unchanged.
Examples in the real world
Fehling's solution as a reagent for aldehydes, and lithium aluminium hydride as a reducing reagent in synthesis.