Question

Why the blue colour of copper sulphate solution disappears when an iron nail dipped into it?

Answer 1

When Iron nail (Ferrum) is dipped in Copper sulphate (CuSO4) there takes reaction between them and coppersulphate change its color from blue to light green. This shows Iron is more reactive than Copper, it can replace Copper from CuSO4.
Fe + CuSO4 → FeSO4 + Cu
CuSO4 is in blue Color and FeSO4 is in light green color.
So, there will be change in the composition and color of the solution changes from blue to light green.

Answer 2

The blue colour disappears because rusting of iron occurs and the surface of iron metal gets rough and corrosive. The Copper Sulphate solution changes into Iron Sulphate solution and Copper changes to Copper metal
Copper Sulphate Solution + Iron Nail= Iron Sulphate Solution + Copper Metal
OR
CuSO4 + Fe = FeSO4 + Cu