Silver mirror test. Test tube being taken from a beaker of warm water after the silver mirror test has been completed. The silver mirror test is used to distinguish aldehydes from ketones. Tollen's reagent (ammoniacal silver nitrate) and ethanol are added to a solution. If the solution contains aldehydes, the ethanol reduces the silver ions to metallic silver, which deposits on the inside of the reaction vessel. The test tube was placed in a beaker of warm water to speed up the reaction.