Iron filings react with copper sulphate solution to form copper and iron sulphate.

The chemical reaction is:

Fe + CuSO4 --> FeSO4 + Cu

At first, the copper sulphate solution is green, but as the reaction proceeds, the solution turns colourless because the copper ions are being replaced by iron ions. The solution turns colorless because ferrous sulfate (FeSO4) is formed, which is a pale green color.