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.

Explanation

(a) Silver article turns black when kept in the air because silver article with sulphur compounds such as hydrogen sulphide `(H_(2)S)` present in air. The phenomenon is called corrosion. For silver particularly, it is called tarnishing of silver.
(b) The black substance formed is silver sulphide`(Ag_(2)S)`.
`underset("Silver")(2Ag)+underset(underset("sulphide")("Hydrogen"))(H_(2)S) to underset(underset(underset("(black)")("sulphide"))("Silver"))(Ag_(2)S)+underset("Hydrogen")(H_(2))`