Question

Copper becomes green when exposed to moist air for a long period. This is due to:

• A. the formation of a layer of cupric oxide on the surface of copper
• B. the formation of a layer of basic carbonate of copper on the surface of copper
• C. the formation of a layer of cupric hydroxide on the surface of copper
• D. the formation of basic copper sulphate layer on the surface of the metal

Answer 1

Answer: (B)

the formation of a layer of basic carbonate of copper on the surface of copper

When a copper vessel is exposed to moist air for a long time it develops a green layer on its surface. Copper corrodes by oxidation in which it reacts with oxygen in the air to form copper oxide.

Copper oxide then combines with carbon dioxide to make copper carbonate, which gives it a green colour. This process is called corrosion of copper.

The green material is a mixture of copper hydroxide ($Cu(OH{)}_2$) and copper carbonate ($CuC{O}_3$). The following is the reaction:

$2Cu\left(s\right)+H_{2}O\left(g\right)+C{O}_2+O_2\to Cu(OH{)}_2+CuC{O}_3(s)$

Copper(II) carbonate is a blue-green compound.

Hence, option $B$ is correct.