(a) Silver reacts with moisture present in air and forms a layer of black substance, called silver oxide. Finally, it forms silver sulphide with the reaction of oxygen and hydrogen sulphide gas present in atmosphere, because of which silver metal turns black. This phenomenon is called Tarnish or Tarnishing of Silver.
4Ag + O2 + 2H2S → 2Ag2S + 2H2O
The Silver sulphide so formed can be cleaned using toothpaste. Toothpastes have many abrasive substances. Aluminium hydroxide Al (OH)3 is one of them.
When toothpaste is rubbed over tarnished silver metal, the silver sulphide reactors with aluminium and aluminiumsulhide is formed leaving silver metal shiny.'The cleansing speed ups with the use of hot water and sodium hydrogen carbonate.
3Ag2S + 2Al → 6Ag + Al2S3
(b)The black substance formed over the silver metal is silver sulphide. The chemical formula of silver sulphide is Ag2S.