Somebody takes somebody else's place
A zinc strip is put in blue copper sulphate solution. The blue fades.
- Zinc: free. Copper sulphate: one group, blue.
- Zinc moves into the group. Copper is pushed out.
- Zinc sulphate forms. Copper sits free, coppery. Zn + CuSO₄ → ZnSO₄ + Cu.
Pick a metal strip and a solution. Does the metal push?
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The rule
In displacement, a free element takes another's place in a compound. The pushed-out element becomes free. Whether it happens is measured, not guessed.
A free element is not joined to another element. The blue fades because blue copper sulphate becomes colourless zinc sulphate.
Tests show this order for three metals: zinc, then iron, then copper. Among these three, a metal pushes out any metal below it.
| Strip in solution | Result |
|---|---|
| Zinc in CuSO₄ | Pushes. Zn + CuSO₄ → ZnSO₄ + Cu |
| Iron in CuSO₄ | Pushes. Fe + CuSO₄ → FeSO₄ + Cu |
| Zinc in FeSO₄ | Pushes. Zn + FeSO₄ → ZnSO₄ + Fe |
| Copper in ZnSO₄ | Nothing happens |
| Copper in FeSO₄ | Nothing happens |
| Iron in ZnSO₄ | Nothing happens |
Zinc also pushes hydrogen out of dilute hydrochloric acid. Dilute means mixed with much water. The fizz is hydrogen leaving.
Zn + 2HCl → ZnCl₂ + H₂
Swapping partners
In double displacement, two compounds swap partners. A solid that does not dissolve falls out as a precipitate.
Magnesium sulphate and sodium hydroxide are mixed in water:
MgSO₄(aq) + 2NaOH(aq) → Mg(OH)₂(s) + Na₂SO₄(aq)
Magnesium hydroxide is white and does not dissolve. It falls out of the clear mixture: it sinks as a solid. A solid that forms in a solution is a precipitate. Such a reaction is a precipitation reaction.
Swapping does not always make a solid. NaCl and KNO₃ solutions stay clear together.