Electrolysis of water using a Hoffman apparatus. Electrolysis uses an electrical current to decompose molecules of water. Reactions at the two electrodes (black hooks dipped in the beaker of water) are powered by the electric current from the battery (lower left). Oxygen and hydrogen gas bubbles are evolved at the anode (left electrode) and cathode (right electrode) respectively. Water molecules are composed of two hydrogen atoms and an atom of oygen, leaving twice as much hydrogen as oxygen trapped in the test tubes (upper right). A burning splint ignites the hydrogen gas, while the oxygen relights a glowing splint.