Question

copper artical when exposed to air devlops a green coating why

Answer 1

Heya....

Copper when kept in open reacts with the atmospheric moisture and forms a coating of copper carbonate i.e. CuCO3.Cu(OH)2..
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Answer 2

copper articles, just like aluminum articles, lose their shine due to the formation of a layer of copper oxide on them on prolonged exposure to moist air. it reacts with co2, o2 add h2o if air to form basic copper carbonate due to which it develops a green coating