Electrolysis

The idea

Electrolysis uses electricity to break down an ionic compound that is molten or dissolved (the electrolyte). Positive ions move to the cathode (negative electrode); negative ions move to the anode (positive electrode). In molten compounds the metal forms at the cathode and the non-metal at the anode. Aluminium is extracted from aluminium oxide mixed with cryolite to lower its melting point; the carbon anodes burn away. In aqueous solutions, hydrogen forms at the cathode if the metal is more reactive than hydrogen, and oxygen forms at the anode unless halide ions are present.

1. Name the ions in the electrolyte, including H⁺ and OH⁻ from water.
2. Cathode: the less reactive of the metal and hydrogen.
3. Anode: the halogen if a halide is present, otherwise oxygen.

Watch out In aqueous sodium chloride, hydrogen, not sodium, forms at the cathode.