Electrolysis

Process in which electric current passed through a substance causes a chemical change, usually the gaining or losing of electrons (&see; oxidation-reduction). It is carried out in an electrolytic cell consisting of separated positive and negative electrodes (anode and cathode, respectively) immersed in an electrolyte solution containing ions or in a molten ionic compound. Reduction occurs at the cathode, where electrons are added that combine with positively charged cations in the solution. Oxidation occurs at the anode, where negatively charged anions give up electrons.

Both thus become neutral molecules. For historical reasons, electric current is defined to flow in the opposite direction to the flow of electrons. Thus, current is said to flow from the cathode to the anode, even though electrons flow in the opposite direction. Electrolysis is used extensively in metallurgy to extract or purify metals from ores or compounds and to deposit them from solution (electroplating). Electrolysis of molten sodium chloride yields metallic sodium and chlorine gas; that of a strong solution of sodium chloride in water (brine) yields hydrogen gas, chlorine gas, and sodium hydroxide (in solution); and that of water (with a low concentration of dissolved sodium chloride or other electrolyte) yields hydrogen and oxygen.