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

Hint: When copper is kept in air for a long time, then the effect which is observed, is very similar to the rusting of iron. The copper reacts with the oxygen, water vapour and carbon dioxide of the air and to form the respective products which make it appear green.

Complete step by step answer:
- In order to answer the question, we need to learn about the d block elements and especially the properties of copper. Before that, we also need to know about corrosion. In many cases, metals do not occur in the free state, they exist as minerals. Now, the pure metal is extracted from these minerals by using different methods. However, this pure metal, which is refined, is not chemically stable. These metals will try to go into their oxide, sulphide or hydroxide state, which are comparatively more stable, chemically. The natural phenomenon by which a pure refined metal reacts with oxygen or other element to go to their more chemically stable state is called corrosion.
- Copper metal falls in the d block of the modern periodic table and this metal is also subject to corrosion. Copper tries to go into its hydroxide form. Air contains a mixture of multiple gases. When a metal like copper is placed in air without any coating on its surface, then copper reacts with the water vapour and carbon dioxide of the air to form its respective hydroxide, which is green in colour. The end products of the reaction are copper hydroxide and copper carbonate, and the reaction is represented below:
\[2Cu+{{H}_{2}}O+C{{O}_{2}}+{{O}_{2}}\to Cu{{(OH)}_{2}}+CuC{{O}_{3}}\]
Due to the formation of the copper carbonate layer, copper becomes green when put in air for a very long time. So, the correct answer is “Option A”.

Note: Corrosion of metals is not good for industries and makes the metal weak. So a lot of money is spent to prevent corrosion. Some examples by which corrosion can be prevented are by electroplating, painting, greasing and galvanizing.