Oxygen exists alone or is chemically combined with other elements. An oxide is a compound usually made of two elements; one is oxygen and the other usually a metal. Many useful mineral materials are oxides. Ordinary sand is silicon dioxide, SiO2. Chinese white clay (kaolin) is aluminum oxide; and lime is calcium oxide.
Some oxides are nonmetallic compounds. Our body cells make a gas, carbon dioxide, and slow-burning fuel forms poisonous carbon monoxide.
Mineral oxides are used to obtain the metals with which they are combined in natural ores. The required removal of oxygen, called reduction, is technically difficult. In reducing common hematite iron ore (which is Fe2O3), carbon monoxide from coked coal is the reducing agent used: Fe2O3 + SCO —» 2 Fe (iron) + 3CO2.
Aluminum is reduced from its ore, bauxite, by a different method. First, the BAUXITE is treated with soda lye to obtain pure aluminum oxide; this is melted with cryolite (Na3Al F6) and reduced by electrolysis, to yield pure molten aluminum.
Some of the nonmetallic oxides are very unstable. Sulfur dioxide, for example, is the choking gas formed when sulfur burns in oxygen or air. It will react with water to form sulfurous acid.