Electrolysis

Perhaps the best known example of electrolysis is the electrolytic decomposition of water to produce hydrogen and oxygen:

Because water is such a stable compound, we can only make this reaction go by pumping energy into it—in this case, in the form of an electric current. Pure water, which does not conduct electricity very well, must first be made into an electrolyte by dissolving an acid, base, or salt in it. Then an anode and a cathode, usually made of graphite or some non-reacting metal such as platinum, can be inserted and connected to a battery or other source of direct current.

At the cathode, where electrons are being pumped into the water by the battery, they are taken up by water molecules to form hydrogen gas:

At the anode, electrons are being removed from water molecules:

The net result of these two electrode reactions added together is

(Note that when these two equations are added together, the four H⁺ions and four OH^-ions on the right-hand side are combined to form four H₂O molecules, which then cancel four of the H₂O molecules on the left-hand side.) Thus, every two molecules of water have been decomposed into two molecules of hydrogen and one molecule of oxygen.

The acid, base, or salt that made the water into an electrolyte was chosen so that its particular ions cannot be oxidized or reduced (at least at the voltage of the battery), so they do not react chemically and serve only to conduct the current through the water. Sulfuric acid, H₂SO₄, is commonly used.