The silver mirror test is used as a test for aldehydes. Tollens' reagent is ammoniacal silver nitrate (silver nitrate in ammonia solution) and is used as a weak oxidising agent to oxidise aldehydes to carboxylic acids. Ketones cannot be further oxidised so Tollens' reagent can be used to distinguish between aldehydes and ketones.Aqueous sodium hydroxide is added to aqueous silver nitrate until a brown precipitate of silver oxide Ag2O is formed. Dilute aqueous ammonia is added until the precipitate just dissolves and forms a colourless solution.