A compound is heated with zinc dust and ammonium chloride and the reaction mixture is filtered into Tollen's reagent. Formation of silver mirror indicates the presence of following group :

For the purpose of detecting aldehydes, Tollens' reagent is an alkaline solution of ammoniacal silver nitrate. In the presence of hydroxide ions, silver ions precipitate out of solution as a brown substance called silver(I) oxide, or ${\mathrm{Ag}}_2\mathrm{O}\left(\mathrm{s}\right)$. The diamminesilver(I) ion,${\left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}$+, is created when this precipitate dissolves in aqueous ammonia. With Tollens' reagent, ketones do not react.

$\begin{array}{l}2{\mathrm{Ag}}^{+}\left(\mathrm{aq}\right)+2\mathrm{OH}-\left(\mathrm{aq}\right){\mathrm{Ag}}_2\mathrm{O}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\\ {\mathrm{Ag}}_2\mathrm{O}\left(\mathrm{s}\right)+4{\mathrm{NH}}_3\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)2{\left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}\left(\mathrm{aq}\right)+2\mathrm{OH}-\left(\mathrm{aq}\right)\end{array}$