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On prolonged exposure to moist air, copper develops a green coating. This green colour is caused by:

1. formation of a cupric oxide layer on the copper surface
2. formation of a basic copper sulphate layer on the metal surface
3. formation of a cupric hydroxide layer on the copper surface
4. formation of a layer of basic copper carbonate on the copper surface

Correct answer: formation of a layer of basic copper carbonate on the copper surface

Solution

The green patina is basic copper carbonate, CuCO3.Cu(OH)2, formed by reaction of copper with moisture, oxygen and CO2 in air.

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