Question

Which of the following are examples of corrosion?

• A. Rusting of iron
• B. Tarnishing of copper
• C. Tarnishing of silver
• D. Rusting of stainless steel

Answer 1

Answer: (A), (B), (C)

The correct options are
A Rusting of iron
B Tarnishing of copper
C Tarnishing of silver

Corrosion is a process of conversion of a metal into its compound by continuous interaction with atmospheric air and water vapour.

Iron ($\text{Fe}$) on continuous exposure to moisture and oxygen forms a reddish brown layer on its surface (${\text{Fe}}_2{\text{O}}_3.{\text{nH}}_2\text{O}$). This is called rusting of iron.

Copper ($\text{Cu}$) develops a greenish coating of basic copper carbonate (${\text{CuCO}}_3.{\text{Cu(OH)}}_2$), when exposed to carbon dioxide (${\text{CO}}_2$), oxygen (${\text{O}}_2$) and water (${\text{H}}_2\text{O}$). It is called tarnishing of copper.

Silver ($\text{Ag}$) forms a black layer of silver sulphide, (${\text{Ag}}_2\text{S}$), when silver is exposed to an atmosphere containing hydrogen sulphide (${\text{H}}_2\text{S}$) gas. It is called tarnishing of silver.

Stainless steel does not rust.