Silver is a precious metal, which means that it is a rare metal of high economic price. However, it is not too expensive in comparation with gold, palladium or platinum that are also metals of this group. Silver is chemically pretty nonreactive, so it belongs to the group of noble metals. It is also a very shiny metal. Out of all these reasons, silver is a perfect metal for making mirrors in process called silvering.
In 1835 German chemist Justus von Liebig postulated a process of silvering glass surfaces. This process consists of spraying glass surface with a solution of silver salt and sugar, after which silver mirror is gradually formed.
This experiment shows exactly the silvering process described by Liebig. When glucose is added into a flask containing diamminesilver(I) hydroxide solution, a reaction takes place and colloidal silver is produced. Colloidal silver adheres to the walls of the glass flask and after several seconds a shiny silver coating is formed. If the flask is rinsed with a tap water and tightly closed, silvered flask can be kept for a long period of time with no significant changes.
There is a simple chemistry behind the silvering process. Namely, glucose has an aldehyde (CHO) group that easily undergoes oxidation even with weak oxidizing agents, such as diamminesilver(I). Diamminesilver(I) can be prepared by dissolving silver(I) oxide in concentrated ammonia:
The reaction of glucose oxidation to yield gluconic acid can be shown with the following equation:
During glucose oxidation, silver(I) is reduced to elemental silver that strongly adheres to the walls of the container in form of a silver mirror. This reaction founds its application in qualitative sugar analysis, for proof of a sugar structure. All sugars that possesses free aldehyde group are know as reducing sugar and form silver mirror when react with diamminesilver(I) solution. The reaction is known as Tollens’ test.
It is worth to mention that modern silvering process is quite different from the process established by Liebig in nineteenth century.