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]

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Solution

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]
$2 Ag (s) + H_{2} S (g) \to {Ag}_{2} S (s) + H_{2} (g)$

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

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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] 2Ag(s)+H2S(g)→Ag2S(s)+H2(g) Green coating is due to the formation of basic copper carbonate (CuCO3). [0.5 Marks]

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]
$2 Ag (s) + H_{2} S (g) \to {Ag}_{2} S (s) + H_{2} (g)$

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