The concept of electrolysis

Electrolysis is the redox process that takes place at the electrodes when direct current passes through a solution or melt of an electrolyte. The negative electrode is the cathode: cations accept electrons and are reduced there; the positive electrode is the anode: anions give up electrons and are oxidised. A melt contains only the ions of the salt (or oxide, alkali): 2NaCl → 2Na + Cl₂; aluminium is obtained by electrolysing a melt of Al₂O₃ in cryolite. In an aqueous solution water also takes part. At the cathode: active metals from Li to Al do not form the metal, H₂ is released instead; medium-activity metals from Mn to Pb give both the metal and H₂; metals after H (Cu, Ag, Au) are deposited. At an inert anode (graphite, Pt): oxygen-containing acid anions and F⁻ are not oxidised, water is oxidised and O₂ is released; Cl⁻, Br⁻, I⁻ and S²⁻ form Cl₂, Br₂, I₂ and S. These rules are simplified: the real result also depends on concentration and electrode material. A soluble anode (copper, nickel) dissolves itself; electroplating is based on this.