Tollen's test is used to determine if a reducing sugar is present in a sample, using a reagent called Tollen's reagent. The reagent is a solution containing silver ions (Ag+). When the reagent reacts with the aldehyde group of a reducing sugar, the Ag+ ions are reduced to the elemental form Ag, forming a silver mirror. The chemical reaction of the key functional groups can be viewed in Figure 1.
Figure 1 - Simplified reaction of a reducing sugar with Tollen’s reagent.