Acid in the rain reacted with the copper to turn it green. That is why there is sometimes green on pennies. Acid in the rain reacted with the copper to turn it green. That is why there is sometimes green on pennies. Acid in the rain reacted with the copper to turn it green. That is why there is sometimes green on pennies. Acid in the rain reacted with the copper to turn it green. That is why there is sometimes green on pennies.

The word is "oxidation", which in the case of copper is a three-step process where the copper oxidizes to copper oxide, then to cuprous or cupric sulfide, and then to copper carbonate. Copper carbonate is the green colored copper or patina that forms over time. The old name for this green patina is "verdigris".

copper tarnish and turn green because of present of iron in it.

Copper, monovalent: blue

Copper, divalent, in halides: blue-green

Copper, divalent, in non-halide compounds: green

yes copper turns green when exposed to moist air for a long time as green metal is a mixture of copper hydroxide and copper carbonate

2CU+H2O+O2 --> CU(OH)2+CUCO3