Depending on the concentration of the Tollen's Reagent and the surface of the reaction vessel, the free silver deposition will form a layer of silver atoms on the inside of the reaction vessel which is highly reflective. This is called the silver mirror effect and is a positive indication of the presence of an aldehyde. Other oxidizing agents like Fehlings Solutions and Benedict's solution will also oxidize aldehydes to a salt of the acid. This ion reacts with the Hydroxide present to produce a orange red colored solid precipitate signaling the presence of an aldehyde. However Benedict's solution will also react with reducing sugars which have aldehyde groups or Hydroxyl groups within the molecule to give the same positive indication.
Ketones are resistent to oxidation.
Only Methyl ketones will undergo oxidation in the presence of Potassium Hydroxide and Iodine solution. This is referred to as the Iodoform reagent and results in the production of the salt of an acid and a compound known as Iodoform, CHI 3 which is insoluble in water and precipitates as a yellow crystalline solid. This is indicative of the presence of a methyl ketone or an alcohol with a methyl group on the carbanol carbon like ethanol. Aldehydes will not react with Iodoform Reagent. Menu R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.