When copper reacts with moist carbon dioxide (CO2) in air, it forms a green coating of which one of the following compounds ?
- A.Cupric carbonate
- B.Cuprous oxide
- C.Cupric oxide
- D.Copper sulphate
Correct answer
Explanation
The correct answer is A, Cupric carbonate. Copper exposed to moist air slowly gains a green layer of basic copper carbonate, a form of cupric carbonate.
Copper does not rust like iron, but it corrodes. Over time it reacts with oxygen, moisture and carbon dioxide in the air to form basic copper carbonate, CuCO3·Cu(OH)2. This green layer is called patina or verdigris, and it protects the metal underneath from further corrosion. It is why old copper vessels turn green, and why the copper-skinned Statue of Liberty in New York looks green instead of reddish brown. Such vessels are cleaned with lemon or tamarind, whose acids dissolve the carbonate.
B is wrong: cuprous oxide is red; it is not the green coating seen on copper.
C is wrong: cupric oxide is black, formed when copper is heated in air.
D is wrong: copper sulphate is blue and needs sulphuric acid, not carbon dioxide, to form.
Exam tip: copper turns green (basic copper carbonate), silver turns black (silver sulphide), iron turns reddish brown (rust).
Practice Questions
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