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Electrolysing water splits the water molecules (H₂O) into hydrogen (H₂) and oxygen (O₂) molecules according to the following equation:

water → hydrogen + oxygen

2H₂O(l) → 2H₂(g) + O₂(g)

From the diagram and equations, we can see that the volume of hydrogen produced is double the volume of oxygen. This confirms that there are twice as many hydrogen atoms as oxygen atoms in water.

At the negative electrode (cathode) a reduction reaction occurs.

4H⁺(aq) + 4e^– → 2H₂(g)

[Higher tier only]

At the positive electrode (anode) an oxidation reaction occurs.

4OH^–(aq) → 2H₂O(l) + O₂(g) + 4e^–