When electrical power is applied to two plates placed inside the water, electrolysis occurs and hydrogen appear on the cathode and Oxygen on anode. It is due to the ideal faradaic efficiency, the amount of hydrogen generated is twice the number of moles of oxygen.
If you look at the equation , reduction occurs at cathode i.e
2 H+ + 2e− → H2
Oxidation occurs at anode
2 H2O → O2 + 4 H+ + 4e−
Overall reaction
2 H2O(l) → 2 H2(g) + O2(g)
Therefore, volume of gas collected over one electrode double the volume of gas collected over the other electrode.
