A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

(b) Name the black substance formed and give its chemical formula.

(a) When silver articles are kept in the open, then a layer of silver oxide is formed on the articles due to the reaction of silver with the oxygen present in the air. Silver oxide is black in colour. Finally, after sufficient time, it will form silver sulphide after coming in contact with oxygen and Hydrogen sulphide present in the atmospheric air. Silver sulphide is also black in colour.
4Ag + O₂ + 2H₂S → 2Ag₂S + 2H₂O
This process is termed as tarnishing of silver or corrosion.
As toothpastes consist of several abrasive substances (like, Aluminium hydroxide), this layer of silver sulphide can be removed by rubbing the article with toothpaste.
3Ag₂S + 2Al →6Ag + Al₂S₃
This reaction leaves the silver metal all shiny. This particular cleaning process can be improved by using hot water and sodium bicarbonate.

(b) The black substance that is formed over the silver metal on exposure to atmospheric air is silver sulphide. The chemical formula of silver sulphide is Ag₂S.