reagent
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.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.