Question: Rahul has been collecting copper coins and silver coins. One day he observed a green coating on copper coins and a black coating on silver coins. State the chemical phenomenon responsible for these coating and also write chemical names of each coating. [CBSE, Term 1, Set 1, 2016]
Answer: The chemical phenomenon responsible for the coatings is corrosion. Copper and silver metals in presence of moist air gets oxidised forming protective layer over them. Silver turns black due to the formation of silver sulphide (Ag₂S) while copper turns green due to the formation of a mixture of copper carbonate and copper hydroxide [CuCO₃⋅Cu(OH)₂].
