reagent
n.n. a substance used in a chemical test or experiment to cause a reaction. It helps you see if another substance is present or how it behaves.
n. a substance that is added to another substance to cause a chemical reaction. Often used in qualitative analysis to identify the presence of specific ions or compounds.
The student added a few drops of the reagent to the solution.
In the lab, the technician used a specific reagent to detect the presence of chloride ions in the water sample.
The chemist carefully selected a reagent that would react selectively with the target molecule without interfering with the surrounding organic structure.
Possibly from Latin reagō. Compare with Norwegian Bokmål reagens. The sequence of act → action → agent → agency shows morphologic and semantic parallel, and apparent cognate relation, with react → reaction → reagent → reagency.