What observable change occurs when Tollen's reagent is applied to an aldehyde?

When Tollen's reagent, which contains silver ions in an ammonia solution, is applied to an aldehyde, a distinctive observable change occurs. Aldehydes are known to be reducing agents, meaning they can donate electrons to other substances. When Tollen's reagent encounters an aldehyde, the silver ions (Ag+) are reduced to metallic silver (Ag), while the aldehyde is oxidized to a carboxylic acid.

This reduction process results in the formation of a shiny, reflective layer of silver on the surface of the reaction vessel, often described as a "silver mirror." This is because the metallic silver deposits on the glass create a mirror-like effect, indicating the successful reduction of the silver ions.

The other options do not align with the chemical behavior of Tollen's reagent when reacting with an aldehyde. The green precipitate typically corresponds to a reaction involving transition metal complexes, turning blue might relate to copper (II) ions in another context, and releasing gas generally pertains to specific reactions that do not involve Tollen's reagent with aldehydes.