Question

why does the colour of copper sulphate solution change when an iron nail is dipped in it..!?

Answer 1

Because iron replaces copper from its salt solution and forms iron sulphate thus resulting in the change of color from blue to brownish

Answer 2

When iron nail is dipped in the copper sulphate $\tt{(CuSO_4)}$ solution, being more reactive, iron displaces copper from its solution and form iron sulphate $\tt{(FeSO_4)}$ solution. Due to the formation of iron sulphate solution, blue colour changes to $<b>green</b>$ colour.

The reaction is –

$\tt{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}$

This reaction is categorised under $<b>Displacement reaction.</b>$