Illustrated Glossary of Organic Chemistry

Tollens' test: A chemical reaction used to test for the presence of an aldehyde or a terminal α-hydroxy ketone. To perform the test Tollens' reagent (a colorless Ag+ solution) is added to the unknown. If an aldehyde is present Ag+ is reduced to Ag0 which precipitates, often as a silver mirror. A terminal α-hydroxy ketone gives a positive Tollens' test because Tollens' reagent oxidizes the α-hydroxy ketone to an aldehyde.

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Before addition of the unknown the
Tollens' reagent solution is colorless.
If the unknown contains an aldehyde or an α-hydroxy
Ag+ is reduced to Ag0 which often forms a mirror.