Calcium oxide

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Crystal structure
Structure of calcium oxide
__ Ca 2+      __ O 2−
Crystal system


Space group

Fm 3 m (No. 225)Template: room group / 225

Coordination numbers

Ca [6], O [6]

Surname Calcium oxide
other names
  • quick lime
  • Quicklime
  • unslaked lime
  • Quicklime
  • Free lime
  • Limestone
  • Calcium oxide
  • lime
Ratio formula CaO
Brief description

white, odorless powder

External identifiers / databases
CAS number 1305-78-8
EC number 215-138-9
ECHA InfoCard 100.013.763
PubChem 14778
Wikidata Q185006
Molar mass 56.08 g mol −1
Physical state



3.37 g cm −3 (20 ° C)

Melting point

2580 ° C

boiling point

2850 ° C (100 h Pa )


heavy in water (1.65 g l −1 at 20 ° C, violent reaction)

Refractive index


safety instructions
GHS labeling of hazardous substances
05 - Corrosive 07 - Warning


H and P phrases H: 315-318-335
P: 261-280-305 + 351 + 338

Switzerland: 2 mg m −3 (measured as inhalable dust )

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

Calcium oxide , also quick lime , quicklime , unslaked lime , lime earth , quick lime or free lime , chemical formula CaO, is a white crystalline substance that reacts with water while generating a lot of heat . In this reaction, calcium hydroxide ( slaked lime ) is formed according to the reaction equation

educated. Quick lime is divided into soft, medium and hard quick lime .


Calcium oxide is produced in larger quantities by burning lime ( calcination ) in the lime kiln . From a temperature of around 800 ° C, calcium carbonate (usually calcite in the form of limestone ) is deacidified, i.e. carbon dioxide is expelled and calcium oxide is formed.

CaCO 3 deacidification in the DTA

A distinction is made between burnt lime u. a. according to their reaction rate (reactivity) of the lime with water during the extinguishing process:

<2 min reaction time: soft quicklime
2–6 min reaction time: medium quicklime
> 6 min reaction time: hard burnt lime

However, there are no more precise definitions.

The reactivity therefore decreases with a higher firing temperature. The differences are a consequence of the crystallite size of the CaO, pore volume and specific surface, which are influenced by the firing temperature and duration (these parameters of the firing process are collectively referred to as the degree of firing ).

Soft-burned lime is produced at temperatures of 900 to 1000 ° C, hard-burned lime at up to 1400 ° C, whereby the duration of the temperature exposure and, above all, the petrographic formation of the limestone play a role.

Calcium oxide is also obtained by thermal decomposition of calcium hydroxide, which breaks down into calcium oxide and water at 550 ° C under atmospheric pressure. The calcium oxide produced under these conditions is not very crystalline and therefore highly reactive.

Calcium oxide (in addition to calcium nitride) is also formed when calcium is burned in the air. Since elemental calcium is rare, this reaction is only carried out as part of laboratory tests.

Natural occurrence

Under the conditions of the sanidinite facies , pure limestone can be decomposed to calcium oxide if, for example, under low pressure (i.e. near the surface of the earth) it is immersed in basaltic lava of high temperature. Such occurrences are very rare, and due to the high reactivity of calcium oxide, only pseudomorphoses are observed in natural rocks .


Burnt (unslaked, formerly also "living") lime is very corrosive, so contact with the eyes can lead to blindness.

The course of the slaking process and the properties of the slaked lime formed depend on the composition and nature of the quicklime. The extinguishing reaction can be completed after a few seconds, but can also last more than a quarter of an hour, depending on the inner surface ( porosity ), the primary crystal size, the degree of sintering and the composition of the quicklime. While the course of the calcination is largely controllable and predictable when burning limestone to quicklime, it is not yet possible to make a satisfactory prognosis about the reactivity of the resulting quicklime solely by knowing the composition of the starting material. In particular, the exact conditions under which the burning process takes place also play a role.


Burnt and then (with water) slaked lime is used in the construction industry as an admixture to mortar and plaster , as well as for the industrial production of sand-lime bricks . It is also a subordinate component of cement clinker . In chemistry , the substance is also used as a drying agent and for absorbing carbon dioxide. Other areas of application of quicklime are z. B. fertilizer lime , the production of calcium carbide and its use for the production of lime mortar , lime plaster and lime paint and for use as a neutralizing agent .

One of the main areas of application is the desulphurisation of pig iron, in which the sulfur [S] occurs as a companion [FeS] and has to be extracted on the way to the steel (converter). Lime (CaO) is either blown into the pig iron or mixed in with a stirrer. The lime combines with the sulfur to form calcium sulfide [CaS], rises to the surface and is deposited there as slag . After desulfurization, this is removed with a scraping machine.

The slaked lime is used, among other things, as an alternative to limestone in flue gas desulphurisation . The amount used is around 1.8 times less than that of limestone. The gypsum (calcium sulfate) obtained from quicklime has a whiteness of 80% and can be used commercially. Due to its high reactivity, lower consumption quantities are required. The disadvantage is its significantly higher price compared to limestone.

By reacting with chlorine , calcium hydroxide can be converted into chlorinated lime .

Calcium oxide is added to foods as an acidity regulator . It serves primarily as a so-called technical auxiliary that is no longer present in the finished food. It is generally approved for food in the EU as a food additive under the designation E 529 without maximum quantity restrictions, but with the stipulation that only the technically required amount is used ( quantum satis ).

Earlier calcium became the causticization of soda and potash used what for soap making was of great importance. Calcium oxide was also used as the caustic lime charge of guns and blind pots thrown with catapults .

Quick lime can also be used in open field toilets ( outhouse ) thanks to its odor-binding effect .

It was also used to disinfect marketplaces and burial grounds.

Unslaked lime is occasionally used instead of slaked lime for disinfecting stables (the "lime" of stables). However, it should not come into contact with straw or similar highly flammable material, as the temperature of around 180 ° C, which is created when reacting with moisture, can in rare cases be sufficient to ignite.

According to at least one eyewitness report, caustic lime was distributed on the floor of train wagons during the Nazi era, at least in the area of ​​the Izbica transit ghetto , which transported victims of the Holocaust to the extermination camps . When it came into contact with damp human skin, it caused dangerous burns to prisoners.

Web links

Individual evidence

  1. a b c d e f Entry on calcium oxide in the GESTIS substance database of the IFA , accessed on February 14, 2017(JavaScript required) .
  2. Data sheet calcium oxide (PDF) from Merck , accessed on January 19, 2011.
  3. David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Index of Refraction of Inorganic Crystals, pp. 10-245.
  4. Swiss Accident Insurance Fund (Suva): Limit values ​​- current MAK and BAT values (search for 1305-78-8 or calcium oxide ), accessed on November 2, 2015.
  5. Dipl.-Ing. Bonar Marbun: Kinetics of the hydration of CaO and MgO , pp. 2 and 4ff, dissertation, February 2006, Faculty of Natural and Material Sciences, Clausthal University of Technology
  6. Walter Ehrenreich Tröger: Optical determination of the rock-forming minerals . 2nd Edition. tape 2 . Swiss beard, Stuttgart 1969, p. 69 .
  7. Jürgen Martin: The 'Ulmer Wundarznei'. Introduction - Text - Glossary on a monument to German specialist prose from the 15th century. Königshausen & Neumann, Würzburg 1991 (= Würzburg medical-historical research. Volume 52), ISBN 3-88479-801-4 (also medical dissertation Würzburg 1990), p. 141.
  8. Dipl.-Ing. Bonar Marbun: Kinetics of the hydration of CaO and MgO , pp. 1 and 7f, dissertation, February 2006, Faculty of Natural and Material Sciences, Clausthal University of Technology
  9. University of Regensburg: Chemistry of (house) construction ( Memento of the original from April 27, 2015 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  10. Hermann von Tappeiner : Textbook of Drug Science and Drug Prescription Theory: With special consideration of the German and Austrian Pharmacopoeia, Springer-Verlag, 15th edition, 1922, p. 126. ( limited preview in Google book search)
  11. Disinfectants, antiseptics . In: Theodor Husemann : Handbook of the entire drug theory. 2 volumes, Berlin 1873–1875; 2nd Edition. Springer, Berlin 1883, Volume 1, p. 251. ( limited preview in the Google book search)
  12. ORF: Unslaked lime as a fire risk
  13. Madeleine Janssen: Polish secret agent Karski: "As if I were fighting my way through a mass of death and decay" . In: Spiegel Online . May 16, 2018 ( online [accessed May 17, 2018]).