Question

When an object made of metal A is kept in air for a considerable time, it loses its shine and becomes almost black due to the formation of a layer of substance B. When an object made of another metal C is kept in damp air for a considerable time, it gets covered with a green layer of substance D. Metal A is the best conductor of electricity whereas metal C is the next best conductor of electricity.
(a) What is metal A?
(b) What is metal C?
(c) Name the substance B.
(d) Name the substance D.

Answer 1

(a) Metal A is silver. Silver becomes black in the presence of hydrogen sulphide gas in air. Silver reacts with hydrogen sulphide gas in air and forms a thin black layer of silver sulphide (Ag₂S) on it. Therefore, silver jewellery or ornaments become dull due to the formation of silver sulphide coating on their surfaces. Silver is the best conductor of electricity.

(b) Metal C is copper. When copper is kept in moist air, it slowly reacts with carbon dioxide and moisture (water) in the air to form a green layer of basic copper carbonate (CuCO₃.Cu(OH)₂). This basic copper carbonate is a combination of copper carbonate and copper hydroxide. After silver, copper is the next best conductor of electricity.

(c) Substance B is silver sulphide (Ag₂S).

(d) Substance D is basic copper carbonate (CuCO₃.Cu(OH)₂).

(e) Dilute acid solution can be used to remove the green layer on copper. This acid solution dissolves the green layer of basic copper carbonate present on corroded copper. This helps copper to regain its shine.