Calcium oxide

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Calcium oxide (CaO), commonly known as burnt lime, lime or quicklime, is a widely used chemical compound. It is a white, caustic and alkaline crystalline solid. As a commercial product, lime often also contains magnesium oxide, silicon oxide and smaller amounts of aluminium oxide and iron oxide.

Calcium oxide is usually made by the thermal decomposition of materials such as limestone, that contain calcium carbonate (CaCO3; mineral name: calcite) in a lime kiln. This is accomplished by heating the material to above 825°C,[1] a process called calcination or lime-burning, to liberate a molecule of carbon dioxide (CO2); leaving CaO. This process is reversible, since once the quicklime product has cooled, it immediately begins to absorb carbon dioxide from the air, until, after enough time, it is completely converted back to calcium carbonate. Calcination of limestone is one of the first chemical reactions discovered by man and was known in prehistory.

Up until the 20th century, quicklime was used as a disinfectant, usually in a 10% solution called milk of lime. However, it had the disadvantage of decomposing rapidly on exposure to air and moisture, and the burnt lime had to be fresh and unslaked.[2]


As hydrated or slaked lime, Ca(OH)2 (mineral name: portlandite), it is used in mortar and plaster to increase the rate of hardening as well as to improve adhesion[citation needed]. Hydrated lime is very simple to make as lime is a basic anhydride and reacts vigorously with water. Lime is also used in glass production and its ability to react with silicates is also used in modern metal production (steel, magnesium, aluminium and other metals) industries to remove impurities as slag.

It is also used in water and sewage treatment to reduce acidity, to soften, as a flocculant, and to remove phosphates and other impurities; in paper making to dissolve lignin, as a coagulant, and in bleaching; in agriculture to improve acidic soils; and in pollution control, in gas scrubbers to desulfurize waste gases and to treat many liquid effluents. It has traditionally been used in the burial of bodies in open graves, to hide the smell of decomposition, as well as in forensic science, to reveal fingerprints. It is a refractory and a dehydrating agent and is used to purify citric acid, glucose, dyes and as a CO2 absorber. It is also used in pottery, paints and the food industry. Furthermore, quicklime is used in epidemics, plagues, and disasters to disintegrate bodies in order to help fight the spread of disease. CaO is a key ingredient in the nixtamalization process used to create corn hominy and masa or tortilla dough.

A relatively inexpensive substance, CaO produces heat energy by the formation of the hydrate, as in the following equation:[3]

CaO + H2O ↔ Ca(OH)2 + 63.7kJ/mol of CaO

The hydrate can be reconverted to calcium oxide by removing the water in the reversible equation. If the hydrated lime is heated to redness, the CaO will be regenerated to reverse the reaction. As it hydrates, an exothermic reaction results. One litre of water combines with approximately 3.1kg of calcium oxide to give calcium hydroxide plus 3.54GJ of energy. This process can be used to provide a convenient portable source of heat, as for on-the-spot food warming in a self-heating can.

World lime annual production is around 130 million metric tons, with the USA and China the largest producers, each producing around 20 million metric tons.[1]


  1. Merck Index of chemicals and Drugs , 9th ed. monograph 1650
  2. A Complete Handbook for the Sanitary Troops of the U.S. Army and Navy (1917)
  3. US Patent 3,955,554, Solar heating system.

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