The silver mirror reaction, also known as Tollens' test, is a classic qualitative test in organic chemistry. When an aldehyde is mixed with Tollens' reagent and heated gently, silver ions are reduced to metallic silver, which deposits on the glass surface forming a characteristic mirror-like coating.
Tollens' reagent is prepared in two steps. First, sodium hydroxide is added to silver nitrate solution, forming a brown precipitate of silver oxide. Then, dilute ammonia solution is added dropwise until the precipitate completely dissolves, forming the colorless diamminesilver complex ion. This complex is the active oxidizing agent in the silver mirror test.
The silver mirror reaction is a redox process. The aldehyde acts as a reducing agent and is oxidized to a carboxylate ion, losing electrons in the process. Simultaneously, the silver complex ion acts as an oxidizing agent and is reduced to metallic silver, gaining the electrons from the aldehyde. This electron transfer is the driving force behind the formation of the silver mirror.