Question

Copper becomes green when exposed to moist air for a long period of time due to the
A ) formation of a layer of cupric oxide on the surface of copper.
B ) formation of a layer of basic carbonate on the surface of copper.
C ) formation of a layer of cupric hydroxide on the surface of copper.
D ) none of the above.

Answer 1

Hint: Air is a mixture of nitrogen, oxygen, carbon dioxide, moisture and some other gases. Some of these gases react with copper to form different products. These products cover the surface of copper metal and give green colour to it.

Complete answer:
Upon exposure to air, the copper metal gains a green colour. The formation of green colour is due to corrosion.
During monsoon, copper metal, upon exposure to the atmosphere forms a mixture of hydroxide and carbonate of copper. This mixture is obtained due to the reaction of copper metal with water (from moisture present in the air) and oxygen and carbon dioxide gases (present in the atmosphere). The surface of copper metal is coated with a green layer, which contains hydroxide and carbonate of copper. The mixture of hydroxide and carbonate of copper can also be called as basic carbonate.
Write a balanced chemical reaction for the conversion of copper into a mixture of hydroxide and carbonate of copper.
\[{\text{2 Cu + }}{{\text{H}}_2}{\text{O + C}}{{\text{O}}_2}{\text{ + }}{{\text{O}}_2}{\text{ }} \to {\text{ Cu}}{\left( {{\text{OH}}} \right)_2}{\text{ + CuC}}{{\text{O}}_3}\]

Hence, the correct option is the option B ) formation of a layer of basic carbonate on the surface of copper.

Note: Cupric oxide is black in colour. Cupric hydroxide is pale blue in colour. Basic cupric carbonate is green in colour.