(a) There are traces of hydrogen sulphide (H2S) gas in the air. Silver sulphide is formed as H2S slowly reacts with the silver in the silver article which is black in colour. A layer of silver sulphide (Ag2S) is deposited on the metal’s surface. Corrosion is the term for this phenomenon.
Hydrogen peroxide is commonly found in toothpaste as one of the constituents. It forms silver sulphate, which is white when it reacts with silver sulphide. As a result, the article’s whiteness is restored.
Ag2S + 4H2O2 → Ag2SO4 + 4H2O
(b) The black colour compound formed is Silver Sulphide
2Ag + H2S → Ag2S + H2