View source for Calcium oxide

{{Chembox new
|   Name = Calcium oxide
|   ImageFile = Calcium-oxide-3D-vdW.png
|   ImageSize = 200px
|   ImageName = Calcium oxide
|   IUPACName = Calcium Oxide
| Section2 = {{Chembox Properties
|   Formula = [[CaO]]
|   MolarMass = 56.077 g/mol
|   Appearance = White solid
|   Density = 3350 kg/m³, solid
|   Solubility = ''reacts''
|   MeltingPt = 2572 [[celsius|°C]] (2845 [[kelvin|K]])
|   BoilingPt = 2850 [[celsius|°C]] (3123 [[kelvin|K]])
  }}
| Section3 = {{Chembox Structure
|   Structure = [[Cubic (crystal system)|Face-Centered Cubic]]
  }}
}}
{{EH}}

==Overview==
'''Calcium oxide''' ('''CaO'''), commonly known as '''burnt lime''', '''[[Lime (mineral)|lime]]''' or '''quicklime''', is a widely used [[chemical compound]]. It is a white, [[Caustic (substance)|caustic]] and [[alkali]]ne [[crystal]]line 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]] (CaCO<sub>3</sub>; mineral name: [[calcite]]) in a [[lime kiln]].  This is accomplished by heating the material to above 825°[[celsius|C]],<ref name="merck">Merck Index of chemicals and Drugs , 9th ed. monograph 1650</ref> a process called [[calcination]] or ''lime-burning'', to liberate a molecule of [[carbon dioxide]] (CO<sub>2</sub>); 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.<ref name="sani">[http://books.google.com/books?id=RI2mGCexECAC A Complete Handbook for the Sanitary Troops of the U.S. Army and Navy (1917)]</ref>

==Usage==
As hydrated or [[slaked lime]], Ca(OH)<sub>2</sub> (mineral name: [[portlandite]]), it is used in [[mortar (masonry)|mortar]] and [[plaster]] to increase the rate of hardening as well as to improve adhesion{{Fact|date=October 2007}}. [[Hydrate]]d 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 [[acid]]ity, to soften, as a [[flocculation|flocculant]], and to remove [[phosphate]]s and other impurities; in [[paper]] making to dissolve [[lignin]], as a [[coagulant]], and in [[bleach]]ing; in agriculture to improve acidic [[soil]]s; and in [[pollution]] control, in gas scrubbers to [[sulfur|desulfur]]ize 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]], [[dye]]s and as a CO<sub>2</font> absorber. It is also used in [[pottery]], [[paint]]s 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 [[maize|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:<ref name="patent">US Patent 3,955,554, '' Solar heating system.''</ref>

::CaO + H<sub>2</sub>O ↔ Ca(OH)<sub>2</sub> + 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 3.54[[Joule|GJ]] 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.[http://indexmundi.com/en/commodities/minerals/lime/lime_t9.html]





==References==
<references/>

==External links==
* [http://minerals.usgs.gov/minerals/pubs/commodity/lime/limemyb01.pdf An authoritative discussion of lime and its uses (US context) by the US Geological Survey]
* [http://www.cheresources.com/quicklime.shtml Factors Affecting the Quality of Quicklime]
* [http://www.americanscientist.org/template/AssetDetail/assetid/17236/page/3 ''American Scientist''] (discussion of C<sup>14</sup> dating of mortar)
* [http://scifun.chem.wisc.edu/chemweek/lime/lime.html Chemical of the Week -- Lime]

<!--Categories-->
[[Category:Oxides]]
[[Category:Calcium compounds]]
[[Category:Bases]]
[[Category:Limestone]]
[[Category:Alchemical substances]]
[[Category:Disinfectants]]

<!--Interwiki-->

[[ar:أكسيد كالسيوم]]
[[ay:Q'atawi]]
[[bg:Негасена вар]]
[[ca:Calç]]
[[cs:Oxid vápenatý]]
[[da:Kalciumoxid]]
[[de:Calciumoxid]]
[[et:Kaltsiumoksiid]]
[[es:Cal]]
[[eo:Kalko]]
[[fr:Oxyde_de_calcium]]
[[gl:Cal]]
[[it:Ossido di calcio]]
[[he:סיד]]
[[lv:Kalcija oksīds]]
[[lt:Kalcio oksidas]]
[[hu:Kalcium-oxid]]
[[nl:Calciumoxide]]
[[ja:酸化カルシウム]]
[[pl:Tlenek wapnia]]
[[pt:Cal]]
[[ro:Oxid de calciu]]
[[ru:Оксид кальция]]
[[sk:Oxid vápenatý]]
[[fi:Kalkki]]
[[sv:Kalciumoxid]]
[[vi:Ôxít canxi]]
[[tr:Kalsiyum oksit]]
[[uk:Оксид кальцію]]
[[zh:氧化钙]]
{{WH}}
{{WS}}