Question

A numismatist (coin collector) has been collecting gold coins, silver coins, and copper coins for a long time. One day he observed a black coating on silver coins and green coating on copper coins.

Which chemical process is responsible for these coatings. Also name the chemical formula of the black and green coatings.
[2 Marks]

Answer 1

The coating occurred due to a chemical process called corrosion.
[1 Mark]
Black coating is due to deposit of silver sulphide (Ag₂S).
[0.5 Marks]
$2\text{Ag}\text{(s)}+{\text{H}}_2\text{S}\text{(g)}\to {\text{Ag}}_2\text{S}\text{(s)}+{\text{H}}_2\text{(g)}$

Green coating is due to the formation of basic copper carbonate (CuCO₃).
[0.5 Marks]