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When copper reacts with moist carbon dioxide (CO₂) in air, it forms a

June 1, 2025 by rawan239

When copper reacts with moist carbon dioxide (CO₂) in air, it forms a green coating of which one of the following compounds ?

Cupric carbonate

Cuprous oxide

Cupric oxide

Copper sulphate

This question was previously asked in

UPSC CDS-2 – 2021

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When copper is exposed to the atmosphere, it reacts with components in the air, including moisture (H₂O), oxygen (O₂), and carbon dioxide (CO₂). This reaction leads to the formation of a green layer known as patina. The primary chemical compound responsible for this green colour is basic copper carbonate, with the formula generally represented as Cu₂(OH)₂CO₃ (malachite) or Cu₃(OH)₂(CO₃)₂ (azurite, blue, often found mixed with malachite) or a mixture of Cu(OH)₂ and CuCO₃. The term “cupric carbonate” refers to Copper(II) carbonate, CuCO₃. While the patina is technically *basic* copper carbonate, Cu₂(OH)₂CO₃, which is a compound containing both hydroxide and carbonate, among the given options, “Cupric carbonate” is the closest description of the carbonate component that gives the green colour, often formed via the reaction:
2Cu(s) + H₂O(g) + CO₂(g) + O₂(g) → Cu₂(OH)₂CO₃(s)
Cupric means Copper(II). Option A correctly identifies the compound as a form of copper carbonate.

– The green coating on copper exposed to air is called patina.
– Patina is formed by the reaction of copper with moisture, oxygen, and carbon dioxide.
– The main component of green patina is basic copper carbonate.

Other compounds listed: Cuprous oxide (Cu₂O) is reddish-brown. Cupric oxide (CuO) is black. Copper sulphate (CuSO₄) is a blue crystalline solid or white powder and is soluble in water; it wouldn’t typically form a persistent green coating from atmospheric exposure alone unless sulphur dioxide is also present, forming basic copper sulfate.

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