Question

Why blue colour of copper sulphate change to green when a piece of iron is added to this solution? Give reason.

Answer 1

• Copper salts are blue in colour and iron salts are green in colour. When the iron piece is added to the blue solution of copper sulphate (CuSO₄), a displacement reaction takes place.
• Iron is a more active metal than copper so it displaces the copper from the copper sulphate solution.
• As a result, the blue colour of copper sulphate fades and the green colour of iron sulphate (FeSO₄) is formed.
• $$\begin{gathered} \mathrm{Fe}\left(\mathrm{s}\right)+{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\to \mathrm{FeSO}{\left(\mathrm{aq}\right)}_4+\mathrm{Cu}\left(\mathrm{s}\right) \\ \mathrm{Iron}\mathrm{Copper}\mathrm{sulphate}\mathrm{Ferrous}\mathrm{sulphate}\mathrm{Copper} \end{gathered}$$
• This is a displacement reaction.
• Displacement reaction - A reaction in which more active metal displaces less active metal from its salt solution.