Question
Answers

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 Verified Verified
Hint: This is a similar process as iron forms an orange-reddish layer when left outside for too long. A similar reaction occurs and a similar product is formed during Copper’s reaction with moist air resulting in it turning green.

Step-by-Step Solution:
Let us analyse this phenomenon in detail before offering an answer.
Copper turns green because of chemical reactions with the atmosphere. Iron metal if put in an open environment for a long time period, then it would form a flaky orange-red outer layer, copper when it gets exposed to the environment undergoes a series of chemical reactions that give the shiny metal with a pale green outer layer. This layer is also called patina.
The copper metal reacts with oxygen, resulting in the formation of an outer layer of copper oxide, which appears green or bluish-green in colour. This layer is known as the patina.
\[2Cu+{{O}_{2}}\to 2CuO\]
Unlike other destructive oxidation processes, the patina acts as a protective layer, and it does not cause any weakness in the metal. Thus, actually copper is considered as an element that is resistant to corrosion. The layer of copper oxide prevents the inner layer of copper metal from getting oxidised. Very often, this layer is seen on the rooftops of old buildings, and acts as a waterproofing and sun-proofing coat.

Thus, we can safely conclude that the answer to this question is (A) The formation of a layer of cupric oxide on the surface of copper.

Note: Copper does not react with water, but it does slowly react with atmospheric oxygen to form a layer of brown-black copper oxide which eventually turns greenish. While in case of iron, the rust forms on iron in moist air but it cannot protect it from further corrosion.