The Activity (Reactivity) Series of Metals and Its Uses
The activity series ranks metals by how easily they lose electrons (are oxidised) — the further left, the more reactive. It's the standard tool for predicting displacement reactions.
The series
K Ca Na Mg Al Zn Fe Sn Pb (H) Cu Hg Ag Pt Au
More reactive on the left (lose electrons more easily); hydrogen (H) is the reference divider.
Use 1: with dilute acids
Metals above H displace hydrogen from dilute HCl/H₂SO₄; those below H do not.
Zn + 2HCl → ZnCl₂ + H₂↑ (Cu does not react with dilute HCl)
Use 2: with salt solutions
A metal higher in the series displaces a lower one from its salt solution.
Fe + CuSO₄ → FeSO₄ + Cu
Use 3: with water/oxygen
K, Ca, Na are extremely reactive and react with water at room temperature; the more reactive, the more easily oxidised and harder to reduce.
Note: K, Ca and Na are too reactive — in a salt solution they react with the water first rather than displacing the metal. See equations in the reactions library.