A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.(i) Why do they turn black? Name the phenomenon involved.(ii) Name the black substance formed and write its formula.
(i) Silver article reacts with sulphur compounds such as H2S present in air. The phenomenon is called corrosion. For silver particularly, it is called tarnishing of silver.(ii) The black substance is silver sulphide (Ag2S).