(a) Air contains traces of hydrogen sulphide `(H_(2)S)` gas. It slowly reacts with silver present in the silver article to form silver sulphide (black in colour). A coating of silver sulphide `(Ag_(2)S)` gets deposited on the surface of the metal. The phenomenon is known as corrosion. Tooth pastes generally contain traces of hydrogen peroxide as one of the contitutents. It reacts with silver sulphide to form silver sulphate which is white in colour. Therefore, whiteness is restored to the article.
`underset("(Black)")(Ag_(2)S) + 4H_(2)O_(2) to underset("(White)")(Ag_(2)SO_(4)) + 4H_(2)O`