On exposure to moist air, copper gains a green coat on its surface due

On exposure to moist air, copper gains a green coat on its surface due to formation of which one of the following compounds ?

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Copper carbonate

Copper oxide

Copper sulphate

Copper nitrate

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This question was previously asked in

UPSC NDA-1 – 2019

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On exposure to moist air, copper gains a green coat on its surface due to the formation of copper carbonate (more specifically, basic copper carbonate).

When copper is exposed to moist air containing carbon dioxide, it reacts to form a complex mixture of copper carbonates and hydroxides, often referred to as patina. The simplified reaction involves copper reacting with oxygen, water, and carbon dioxide to form basic copper carbonate, typically Cu₂(OH)₂CO₃ (malachite), which has a characteristic green colour.

The reaction is often represented as: 2Cu(s) + H₂O(g) + CO₂(g) + O₂(g) → Cu₂(OH)₂CO₃(s) (basic copper carbonate). While the coating is technically a basic carbonate, “Copper carbonate” (referring to the presence of the carbonate group) is the closest option provided. Pure copper(II) carbonate (CuCO₃) is also green, but the patina is usually a mixture of CuCO₃ and Cu(OH)₂.

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