Electrolysis of water, illustration. Electrolysis is the use of an electrical current to decompose a chemical, in this case water. Reactions at the two electrodes are powered by the electric current from the battery when the switch is closed. Oxygen and hydrogen gas bubbles are evolved at the anode (left electrode) and cathode (right electrode) respectively. As water molecules consist of two hydrogen atoms and one oxygen atom, twice as much hydrogen as oxygen is trapped in the test tubes. Use of a burning splint will ignite the hydrogen gas, while the oxygen will relight a glowing splint.