Electrolysis of Water, Producing Hydrogen & Oxygen. Electrolysis is the use of an electrical current to decompose a chemical, in this case water. Reactions at the two electrodes (black hooks dipped in the beaker of water) are powered by the electric current from a battery (not shown). Oxygen and hydrogen gas bubbles are evolved at the anode and cathode 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 (upper right). Use of a burning splint will ignite the hydrogen gas, while the oxygen will re-light a glowing splint.