The silver mirror test

This is used as a test for aldehydes (see also Fehling's solution and Benedict's solution). A clean test tube of Tollen's reagent is warmed in a water bath and a couple of drops of the suspected aldehyde are added. Tollen's reagent must be freshly prepared from silver nitrate solution. A drop of sodium hydroxide solution precipitates the coffee-coloured silver(I) oxide. This is then just dissolved in dilute ammonia solution. The solution must be used, and disposed of with plenty of water, promptly as explosive material is formed.

With a positive test, after a short time, a mirror of silver appears on the inner surface of the test tube. This is a sensitive test for aldehydes. The chemical principle is the ready oxidation of the aldehyde to a carboxylic acid, accompanied by the reduction of the Ag⁺ ion to metallic silver. The following video shows a positive test between Tollen's reagent and propanal. It has been speeded up four times.

The result of a positive silver mirror test

Video - a positive silver mirror test