Silver mirror test with Tollens' reagent. When a few drops of an aldehyde solution are added to a clear solution of Tollens' reagent a precipitate start to form. Gradually it darkens and then silver coats the test tube with a mirror surface. The test differentiates between aldose and ketose sugars. Tollen's reagent is made from dilute sodium hydroxide added aqueous silver nitrate and then ammonia is added to the resulting solution.