Question

The coating formed on the metals such as iron, silver and copper after corrosion will be:
(A) Brown, black and green in colour
(B) Black, brown and green in colour
(C) Brown in colour in all the metals
(D) Black in colour in all the metals

Answer 1

Hint :When metals are exposed to air or some chemicals, then they get converted into its stable oxide, hydroxide or sulphides and a coloured coating is formed on their surface. This process is known as corrosion. It can be destructive as well as productive based on the conditions and the type of the metal.

Complete Step By Step Answer:
When metals like iron, silver and undergo corrosion, then following changes are observed:
Iron: The following reaction takes place when iron metal is exposed to air-
 $ 4Fe + 3{O_2} + 6{H_2}O \to 4Fe{(OH)_3} $
The hydride formed undergoes dehydration reaction as follows:
 $ 2Fe{(OH)_3} \to F{e_2}{O_3} + 3{H_2}O $
Thus, after the reaction formation of ferric oxide takes place which is reddish brown in colour.
 Silver: When silver metal is exposed to air, it first forms silver oxide as per following reaction-
 $ 4Ag + {O_2} \to 2A{g_2}O $
Then the oxide formed reacts with the traces of hydrogen sulphide present in the atmosphere to give the product as follows:
 $ A{g_2}O + {H_2}S \to A{g_2}S + {H_2}O $
Thus, after the reaction formation of silver sulphide takes place which is black in colour.
Copper: When copper metal is exposed to air, it reacts with various gases present in the atmosphere and its surface acquires a coloured coating. The reaction is as follows:
 $ 2Cu + {H_2}O + C{O_2} + {O_2} \to Cu{(OH)_2} + CuC{O_3} $
Thus, after the reaction a mixture of copper carbonate and copper hydroxide is formed which is green in colour.
Hence, the coating formed on the metals such as iron, silver and copper after corrosion will be brown, black and green respectively.
So, option (A) is the correct answer.

Note :
Make sure to not to get confused between the terms corrosion and rusting. Corrosion is oxidation of any metal in the presence of air or a chemical whereas rusting is specifically corrosion of iron metals and its alloys in the presence of air and moisture.