In which example does a colour change occur along with the formation of new substances?

Correct answer: Placing an iron nail in copper sulphate solution

When an iron nail is placed in blue copper sulphate solution, iron displaces copper from copper sulphate. Green iron sulphate and copper are formed: \(\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\). Thus, the colour change is accompanied by the formation of new substances. In contrast, folding paper is only a physical change and produces no new substance.