Which one of the following responds to Tollen's test? (a) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CHO}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}^{-1}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\)

A qualitative chemical test is designed to identify the presence or absence of a particular substance or type of substance, rather than measuring its quantity. These types of tests rely on chemical reactions that produce a specific and noticeable change when a target compound is present.

One classic example of a qualitative chemical test is Tollen's test. It's a straightforward procedure used by chemists to detect aldehydes, a type of organic compound. Tollen's test invokes a reaction between the aldehyde and a reagent, which in this case, is an ammoniacal silver nitrate solution, known as Tollen's reagent. The presence of an aldehyde is confirmed by the appearance of a silver layer or a black precipitate – a reaction that doesn't occur with ketones or other types of organic compounds.

Aldehydes and ketones are two classes of organic compounds that are structurally similar but have distinct reactivities and are differentiated by the presence of a carbonyl group. The carbonyl group in aldehydes is bonded to at least one hydrogen atom, while in ketones, it's bonded to two carbon atoms.

Tollen's test exploits this difference in structure to differentiate between them. When Tollen's reagent is added to an aldehyde, it gets oxidized to form a carboxylic acid, and in the process, the silver ion in the reagent is reduced to metallic silver. This distinctive reaction produces a visible silver coating, also known as the silver mirror, on the inside of the test tube. Ketones, however, will not be oxidized by Tollen's reagent and will not cause the silver mirror to form, thus providing a clear and simple differentiation between the two classes of compounds.

The silver mirror reaction is a term coined for the characteristic result of Tollen's test when testing an aldehyde. It refers to the formation of a shiny layer of metallic silver on the inside of the glass test tube where the reaction occurs.

The reaction mechanism involves the reduction of silver ions to silver metal. When the aldehyde compound reacts with Tollen's reagent, electrons are transferred from the aldehyde to the silver ions present in the reagent. This reduction of silver ions results in the deposition of silver atoms on the surface of the glass, creating a mirror-like appearance. This striking visual change is the 'positive' result of Tollen's test, indicating the presence of an aldehyde. Notably, only aldehydes and a few other reducing sugars or compounds will produce this reaction, making it an effective tool for chemists to detect the presence of aldehydes in a sample.