Metals are listed in decreasing ease of oxidation, or increasing ease of reduction. A metal can replace metals listed below it in the activity series, but not above. For example, sodium is highly active and thus able to replace hydrogen from water:

2 Na (s) + 2 H₂O (l) -> 2 NaOH (aq) + H₂ (g)

Metals that can replace hydrogen within acids but not water are listed in the middle of the activity series, for example zinc replaces hydrogen in sulfuric acid:

Zn (s) + H₂SO₄ (aq) _> ZnSO₄ (aq) + H₂ (g)

The activity series has applications in electrochemistry, where two dissimilar metals are chosen as electrodes of a battery.