Question

Why does the iron nail become brownish in colour and the blue colour of copper sulphate solution fade?

Answer 1

iron nail become brownish in colour because rust is formed as it comes in contact to moisture.

Answer 2

This is an example of "double displacement reaction" of Fe with copper sulphate solution.

The chemical formula for copper sulphate is $CuSO_4$. It is a blue color solution. When an "iron nail is dipped" in this solution it reacts with Fe to form pale green color $FeSO_4$ and Cu deposited on nail that turns nail brownish in color.

Hence, the chemical equation can be written as:

$\mathrm{Fe}(\mathrm{s})+\mathrm{CuSO}_{4}(\mathrm{l}) \rightarrow \mathrm{Cu}(\mathrm{s})+\mathrm{FeSO}_{4}(\mathrm{l})$