Q. 524.2( 20 Votes )

When a copper object is exposed to moist air for a long t e, then a green coating is formed on its surface.

(a) What is the material of the green coating?

(b) State whether the green coating is acidic or basic.

Answer :

(a) When 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 (CuCO3). The following is the reaction

2Cu+H2O+CO2+O2Cu (OH)2 + CuCO3


moist air


(b) The green coating will be basic in nature. Green coating is made of copper oxide which is an oxide of metal. Metals react with oxygen to form metallic oxides. These metallic oxides are basic because they react with water to form bases.


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