Electrolysis, the process of decomposing a solution or a molten compound by passing an electric current through it. Electrolysis is used in refining gold, silver, copper, and tin, and for obtaining aluminum, sodium, magnesium, and other metals from their ores. Electroplating is the use of electrolysis to coat a metallic object with a thin layer of another metal. Anodizing is the use of electrolysis to coat a metallic object with a thin layer of oxide for protection against corrosion and wear. Electrolysis is also used for the production of oxygen, hydrogen, chlorine, caustic soda, and other substances.
Electrolysis is carried out in an electrolytic cell. In the cell, the electrolytic solution or molten electrolyte is in contact with two solid conductors called electrodes. A simple example of electrolysis is the decomposition of a solution of hydrogen chloride and water, yielding hydrogen gas at one electrode and chlorine gas at the other. When the product of decomposition is a metal, it is either deposited as a coating on one of the electrodes or precipitated as sludge.
The electrodes are connected to opposite poles of a battery. The electrode connected to the positive pole is called the anode and the electrode connected to the negative pole is the cathode.
Substances that undergo electrolysis are either solutions of electrolytes or electrolytes that have been melted. When dissolved or molten, electrolytes separate into ions, atoms or groups of atoms that bear an electrical charge from having lost or gained one or more electrons.
During electrolysis, ions with a positive electrical charge move toward the cathode, where they take up electrons; ions with a negative charge move toward the anode, where they give up electrons. This transfer of electrons causes chemical changes to take place at each electrode.
Much of the original research on electrolysis was done by an English physicist and chemist, Michael Faraday, who published his results in 1833. Faraday's laws of electrolysis state:
1. The amount of chemical change that takes place in a solution during electrolysis is proportional to the amount of current passed through the solution.
2. The amount of a substance that is deposited is proportional to its equivalent weight (its atomic weight divided by its valence).