Category: Q&A

Why does copper turn green after some time when exposed to the atmosphere?

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Copper reacts with oxygen in the air to form a copper(II) oxide coating.
2Cu (s) + O2 (g) –> 2CuO (s)
Copper(II) oxide reacts with moist carbon dioxide to form a mixture of copper(II) carbonate and copper(II) hydroxide. The green copper(II) carbonate causes the green appearance of copper.
2CuO (s) + H2O (l) + CO2 (g) –> CuCO3 (s) + Cu(OH)2 (s)

The mixture of copper(II) carbonate and copper(II) hydroxide forms a protective coating, preventing further corrosion of copper underneath the coating.
Whereas in the corrosion of iron or steel, the coating of rust (hydrated iron(III)oxide) flakes off easily, exposing the iron/steel underneath, causing corrosion of iron/steel to continue until the whole piece of iron/steel is corroded to iron(III)oxide.