Tollens' reagent (\([\text{Ag(NH}_3)_2]^+\)) is a mild oxidising agent that distinguishes aldehydes from ketones. Aldehydes are oxidised to carboxylates and silver is reduced to metallic silver (the silver mirror). Ethanal (\(CH_3CHO\)) is an aldehyde → gives positive result. Propanone and benzophenone are ketones → do not react (ketones cannot be further oxidised under mild conditions).