Electrolysis of water. Brownlee electrolysis apparatus is used to demonstrate electrolysis of water, a decomposition of water into oxygen and hydrogen due to electric current. Here current is supplied by a 6 V battery through two electrodes, anode (red, positive electrode) and cathode (black, negative electrode). Anode half-reaction: H2O -> O2 + H+ + e- (oxidation), cathode half-reaction: H+ + e- -> H2 (reduction). Complete balanced reaction: 2 H2O -> 2 H2 + O2. Oxygen and hydrogen gasses bubble up from the anode and the cathode, respectively, and are collected over water in inverted test tubes. The volume of hydrogen produced is twice the volume of oxygen, as follows from the balanced equation. A small amount of sulfuric acid has been added to water to act as an electrolyte and speed up the process.