electrolysis

Passing an electric current through acidified water (such as diluted sulphuric acid) breaks down the water into its constituent elements - hydrogen and oxygen.

Copper purification by electrolysis. Pure copper dissolves from the impure copper anode, which decreases in size. The copper is deposited at the cathode, which increases in size. When the purification process is complete the cathode is melted down and cast into bars to be sold to manufacturers, such as producers of electrical wiring.

In chemistry, the production of chemical changes by passing an electric current through a solution or molten salt (the electrolyte), resulting in the migration of ions to the electrodes: positive ions (cations) to the negative electrode (cathode) and negative ions (anions) to the positive electrode (anode).

During electrolysis, the ions react with the electrode, either receiving or giving up electrons. The resultant atoms may be liberated as a gas, or deposited as a solid on the electrode, in amounts that are proportional to the amount of current passed, as discovered by English chemist Michael Faraday. For instance, when acidified water is electrolysed, hydrogen ions (H⁺) at the cathode receive electrons to form hydrogen gas; hydroxide ions (OH⁻) at the anode give up electrons to form oxygen gas and water.

One application of electrolysis is electroplating, in which a solution of a salt, such as silver nitrate (AgNO₃), is used and the object to be plated acts as the negative electrode, thus attracting silver ions (Ag⁺). Electrolysis is used in many industrial processes, such as coating metals for vehicles and ships, refining bauxite into aluminium, and the chlor-alkali industry, in which brine (sodium chloride solution) is electrolysed to produce chlorine, hydrogen, and sodium hydroxide (caustic soda); it also forms the basis of a number of electrochemical analytical techniques, such as polarography.

When acidified water is electrolysed, the chemical changes that occur at the electrodes are as follows:

negative electrode: 4H+ + 4e− → 2H2 (reduction)

positive electrode: 4OH− − 4e− → 2H2O + O2 (oxidation)