Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
When an iron nail is dipped in copper sulphate solution, the blue colour of the solution fades. This happens because iron is more reactive than copper and displaces copper from the copper sulphate solution.
The following displacement reaction takes place: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Iron displaces or removes copper from copper sulphate solution, forming iron sulphate (FeSO₄). As copper sulphate gets consumed and iron sulphate is formed, the blue colour of the solution fades. The displaced copper deposits on the iron nail, making it brownish in colour.
Explanation
The textbook explains this phenomenon through Activity 1.9, where iron nails are immersed in copper sulphate solution. The reaction Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) is a displacement reaction. Since iron is more reactive than copper, it displaces copper from its compound. The blue colour of copper sulphate fades because it is being converted to iron sulphate, while copper metal deposits on the iron nail giving it a brownish colour.
Solution Steps
Step 1: Identify that iron is more reactive than copper.
Step 2: Write the displacement reaction: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).
Step 3: Explain that copper sulphate (blue) is consumed and iron sulphate is formed, causing the colour change.