Close-up of the electrodes during a demonstration of the electrolysis of water, showing the different amounts of gas produced at each. These electrodes are in an aqueous solution of sodium hydroxide. The cathode (negative electrode) is seen first, and the anode (positive) second. At the cathode, water molecules are broken up by electrons, producing hydroxide ions and hydrogen gas, which bubbles off. At the anode, hydroxide ions combine to produce water, electrons and oxygen gas, which bubbles off. Water has the formula H2O, and contains twice as many hydrogen atoms as oxygen. As such, twice as much hydrogen gas is produced, as is evident from the production of bubbles.