What happens
A more reactive metal takes the place of a less reactive metal in a solution of its salt. Here magnesium, zinc or iron displaces copper from blue copper sulphate solution. A brown layer of copper forms on the metal.
Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
NCERT activity 1.9 uses iron. Zinc is in NCERT equation 1.17. Copper placed in copper sulphate gives no reaction.
The reactivity series
KNaCaMgAlZnFePbHCuAgAuMost reactive on the left, least reactive on the right. A metal displaces any metal to its right from a solution of its salt.
| Metal | Speed | Solution at the end | Notes |
|---|
| Mg | Fastest | Colourless | Warms the beaker. Fine bubbles. Brown-black copper powder. |
| Zn | Medium | Colourless | Slightly warm. Brown copper layer. |
| Fe | Slow | Pale green | Brown copper layer on the nail. |
| Cu | No reaction | Stays blue | Nothing is displaced. |
What the ions do
Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)
Sulphate ions do not change. They are spectator ions.
- The metal atoms lose electrons and go into solution as ions. This is oxidation.
- Copper ions gain two electrons and become copper atoms on the metal. This is reduction.
- Blue Cu2+ ions are used up, so the blue fades. Mg2+ and Zn2+ are colourless. Fe2+ is pale green.
Energy and bubbles
The reaction is exothermic. The more reactive the metal, the more energy is released, so magnesium warms the beaker most. Copper sulphate solution is slightly acidic, so very reactive magnesium also gives off a few fine bubbles of hydrogen.
Why the set-up is designed this way
- The metal is rubbed with emery paper so the oxide or rust layer does not stop the reaction.
- A start-colour swatch lets you judge how far the blue has faded.
- A spare piece shows how the metal looked before dipping.
- Using one beaker and changing only the metal makes it a fair test.
Safety
- Copper sulphate is harmful if swallowed and irritates eyes and skin. Wear goggles and gloves.
- Wash your hands after. Dispose of the solution as your teacher says.
Write it like the mark scheme
- Give the observations: brown solid forms, blue solution fades or changes colour.
- Name the reaction type: displacement.
- Explain using the reactivity series.
- Write the equation with state symbols, and the ionic equation.
Syllabus links
CBSE class 10 science, chapter 1, activity 1.9 and displacement reactions, then chapter 3 on the reactivity series. IGCSE 0620: reactivity series, displacement reactions of metals and ionic equations.