Tollens' reagent is a chemical reagent most commonly used to determine whether a known carbonyl-containing compound is an aldehyde or a ketone. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous diamminesilver(I) complex is present. It was named after its discoverer, Bernhard Tollens.
A positive test with Tollens' reagent results in elemental silver precipitating out of solution, occasionally onto the inner surface of the reaction vessel, producing a characteristic and memorable "silver mirror" on the inner vessel surface.
Read more about Tollens' Reagent: Laboratory Preparation, Analytical Uses, In Silver Mirroring, Safety