Who pushes whom out
An iron nail dipped in copper sulphate solution gets a copper coat. The solution fades.
- An iron nail, and a tube of copper sulphate solution.
- The nail goes in.
- The nail wears copper. The solution fades. Fe + CuSO₄ → FeSO₄ + Cu.
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The rule
A more reactive metal displaces a less reactive one from its salt. Reactive means quick to react.
A displacement reaction is one where a free element takes another's place. A free element is one not joined to other elements. The pattern is A + BC → AC + B. Here A and B are metals. C is the rest of the compound.
Worked example
Iron is more reactive than copper. So iron takes copper's place: Fe + CuSO₄ → FeSO₄ + Cu. The blue colour fades as copper leaves the solution.
Zinc is more reactive than iron. So zinc takes iron's place: Zn + FeSO₄ → ZnSO₄ + Fe.
Now dip a copper strip in zinc sulphate. Nothing happens. Copper is less reactive than zinc, so it cannot take zinc's place.