Calcium oxide

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Calcium oxide
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Ionic crystal structure of calcium oxide
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Powder sample of white calcium oxide
Names
IUPAC name
Calcium oxide
Other names
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  • Lime
  • Quicklime
  • Burnt lime
  • Unslaked lime
  • Free lime (building)
  • Caustic lime
  • Pebble lime
  • Calcia
  • Oxide of calcium
Identifiers
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3D model (JSmol)
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485425
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UN number 1910
  • InChI=1S/Ca.O checkY
    Key: ODINCKMPIJJUCX-UHFFFAOYSA-N checkY
  • O=[Ca]
Properties
CaO
Molar mass 56.0774 g/mol
Appearance White to pale yellow/brown powder
Odor Odorless
Density 3.34 g/cm3[1]
Melting point 2,613 °C (4,735 °F; 2,886 K)[1]
Boiling point 2,850 °C (5,160 °F; 3,120 K) (100 hPa)[2]
Reacts to form calcium hydroxide
Solubility in methanol Insoluble
Solubility in diethyl ether Insoluble
Solubility in octanol Insoluble
Acidity (pKa) 12.8
−15.0×10−6 cm3/mol
Structure
Cubic, cF8
Thermochemistry
40 J·mol−1·K−1[3]
−635 kJ·mol−1[3]
Pharmacology
QP53AX18 (WHO)
Hazards
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NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
3
0
2
W
Flash point Non-flammable[4]
Lethal dose or concentration (LD, LC):
>2000 mg/kg oral, female rat [5]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 5 mg/m3[4]
REL (Recommended)
 TWA 2 mg/m3[4]
IDLH (Immediate danger)
 25 mg/m3[4]
Safety data sheet (SDS) ICSC 0409
Related compounds
Other anions
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Other cations
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Template:Chembox Footer/trackingTemplate:Short description

Calcium oxide (formula: CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term lime connotes calcium-containing inorganic compounds, in which carbonates, oxides, and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, quicklime specifically applies to the single compound calcium oxide. Calcium oxide that survives processing without reacting in building products, such as cement, is called free lime.[6]

Quicklime is relatively inexpensive. Both it and the chemical derivative calcium hydroxide (of which quicklime is the base anhydride) are important commodity chemicals.

Preparation

Calcium oxide is usually made by the thermal decomposition of materials, such as limestone or seashells, that contain calcium carbonate (Page Module:Chem2/styles.css has no content.CaCO3; mineral calcite) in a lime kiln. This is accomplished by heating the material to above 825 °C (1,517 °F),[7][8] a process called calcination or lime-burning, to liberate a molecule of carbon dioxide (CO2), leaving quicklime behind. This is also one of the few chemical reactions known in prehistoric times.[9]

Page Module:Chem2/styles.css has no content.CaCO3(s) → CaO(s) + CO2(g)

The quicklime is not stable and, when cooled, will spontaneously react with CO2 from the air until, after enough time, it will be completely converted back to calcium carbonate unless slaked with water to set as lime plaster or lime mortar.

Annual worldwide production of quicklime is around 283 million tonnes. China is by far the world's largest producer, with a total of around 170 million tonnes per year. The United States is the next largest, with around 20 million tonnes per year.[10]

Uses

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Page Module:Chem2/styles.css has no content.CaO (s) + H2O (l) ⇌ Ca(OH)2 (aq) (ΔHr = −63.7 kJ/mol of CaO)
As it hydrates, an exothermic reaction results and the solid puffs up. The hydrate can be reconverted to quicklime by removing the water by heating it to redness to reverse the hydration reaction. One litre of water combines with approximately 3.1 kilograms (6.8 lb) of quicklime to give calcium hydroxide plus 3.54 MJ 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, cooking, and heating water without open flames. Several companies sell cooking kits using this heating method.[12]
  • It is a food additive used as an acidity regulator, a flour treatment agent and a leavener.[13] It has E number E529.
  • Light: When quicklime is heated to 2,400 °C (4,350 °F), it emits an intense glow. This form of illumination is known as a limelight, and was used broadly in theatrical productions before the invention of electric lighting.[14]
  • Cement: Calcium oxide is a key ingredient for the process of making cement.
  • As a cheap and widely available alkali.[15]
  • Petroleum industry: Water detection pastes contain a mix of calcium oxide and phenolphthalein. Should this paste come into contact with water in a fuel storage tank, the CaO reacts with the water to form calcium hydroxide. Calcium hydroxide has a high enough pH to turn the phenolphthalein a vivid purplish-pink color, thus indicating the presence of water.
  • Chemical pulping: Calcium oxide is used to make calcium hydroxide, which is used to regenerate sodium hydroxide from sodium carbonate in the chemical recovery at kraft pulp mills.
  • Plaster: There is archeological evidence that Pre-Pottery Neolithic B humans used limestone-based plaster for flooring and other uses.[16][17][18] Such Lime-ash floor remained in use until the late nineteenth century.
  • Chemical or power production: Solid sprays or slurries of calcium oxide can be used to remove sulfur dioxide from exhaust streams in a process called flue-gas desulfurization.
  • Carbon capture and storage: Calcium oxide can be used to capture carbon dioxide from flue gases in a process called calcium looping.
  • Mining: Compressed lime cartridges exploit the exothermic properties of quicklime to break rock. A shot hole is drilled into the rock in the usual way and a sealed cartridge of quicklime is placed within and tamped. A quantity of water is then injected into the cartridge and the resulting release of steam, together with the greater volume of the residual hydrated solid, breaks the rock apart. The method does not work if the rock is particularly hard.[19][20][21]
  • Disposal of corpses: Historically, it was mistakenly thought that quicklime was efficacious in accelerating the decomposition of corpses. The application of quicklime can, in fact, promote preservation. Quicklime can aid in eradicating the stench of decomposition, which may have led people to the erroneous conclusion.[22]
  • It has been determined that the durability of ancient Roman concrete is attributed in part to the use of quicklime as an ingredient. Combined with hot mixing, the quicklime creates macro-sized lime clasts with a characteristically brittle nano-particle architecture. As cracks form in the concrete, they preferentially pass through the structurally weaker lime clasts, fracturing them. When water enters these cracks it creates a calcium-saturated solution which can recrystallize as calcium carbonate, quickly filling the crack.[23]
  • The thermochemical heat storage mechanism is greatly impacted by the sintering of CaO and Page Module:Chem2/styles.css has no content.CaCO3. It demonstrates that the storage materials become less reactive and denser at increasing temperatures. It also pinpoints particular sintering processes and variables influencing the efficiency of these materials in heat storage.

Weapon

Quicklime is also thought to have been a component of Greek fire. Upon contact with water, quicklime would increase its temperature above 150 °C (302 °F) and ignite the fuel.[24]

David Hume, in his History of England, recounts that early in the reign of Henry III, the English Navy destroyed an invading French fleet by blinding the enemy fleet with quicklime.[25] Quicklime may have been used in medieval naval warfare – up to the use of "lime-mortars" to throw it at the enemy ships.[26]

Substitutes

Limestone is a substitute for lime in many applications, which include agriculture, fluxing, and sulfur removal. Limestone, which contains less reactive material, is slower to react and may have other disadvantages compared with lime, depending on the application; however, limestone is considerably less expensive than lime. Calcined gypsum is an alternative material in industrial plasters and mortars. Cement, cement kiln dust, fly ash, and lime kiln dust are potential substitutes for some construction uses of lime. Magnesium hydroxide is a substitute for lime in pH control, and magnesium oxide is a substitute for dolomitic lime as a flux in steelmaking.[27]

Safety

Because of vigorous reaction of quicklime with water, quicklime causes severe irritation when inhaled or placed in contact with moist skin or eyes. Inhalation may cause coughing, sneezing, and labored breathing. It may then evolve into burns with perforation of the nasal septum, abdominal pain, nausea and vomiting. Although quicklime is not considered a fire hazard, its reaction with water can release enough heat to ignite combustible materials.[28][better source needed]

Mineral

Calcium oxide is also a separate mineral species (with the unit formula CaO), named 'Lime'.[29][30] It has an isometric crystal system, and can form a solid solution series with monteponite. The crystal is brittle, pyrometamorphic, and is unstable in moist air, quickly turning into portlandite (Page Module:Chem2/styles.css has no content.Ca(OH)2).[31][32]

References

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  1. ^ a b Page Module:Citation/CS1/styles.css has no content.Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, Florida: CRC Press. p. 4.55. ISBN 1-4398-5511-0.
  2. ^ Calciumoxid (Script error: No such module "webarchive".). GESTIS database
  3. ^ a b Page Module:Citation/CS1/styles.css has no content.Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A21. ISBN 978-0-618-94690-7.
  4. ^ a b c d Page Module:Citation/CS1/styles.css has no content.NIOSH Pocket Guide to Chemical Hazards. "#0093". National Institute for Occupational Safety and Health (NIOSH).
  5. ^ Page Module:Citation/CS1/styles.css has no content."Safety Data Sheet: Calcium Oxide". ThermoFisher Scientific. p. 5. Retrieved 12 September 2025.
  6. ^ Page Module:Citation/CS1/styles.css has no content."free lime". DictionaryOfConstruction.com. Archived from the original on 2017-12-09.
  7. ^ Merck Index of Chemicals and Drugs, 9th edition monograph 1650
  8. ^ Page Module:Citation/CS1/styles.css has no content.Kumar, Gupta Sudhir; Ramakrishnan, Anushuya; Hung, Yung-Tse (2007), "Lime Calcination", in Wang, Lawrence K.; Hung, Yung-Tse; Shammas, Nazih K. (eds.), Advanced Physicochemical Treatment Technologies, Handbook of Environmental Engineering, vol. 5, Totowa, NJ: Humana Press, pp. 611–633, doi:10.1007/978-1-59745-173-4_14, ISBN 978-1-58829-860-7, retrieved 2022-07-26
  9. ^ Page Module:Citation/CS1/styles.css has no content."Lime throughout history | Lhoist - Minerals and lime producer". Lhoist.com. Retrieved 10 March 2022.
  10. ^ Page Module:Citation/CS1/styles.css has no content.Miller, M. Michael (2007). "Lime". Minerals Yearbook (PDF). U.S. Geological Survey. p. 43.13. Archived from the original (PDF) on 2017-05-14. Retrieved 2009-03-31.
  11. ^ Page Template:Citation/styles.css has no content.US patent 3955554, Collie, Robert L., "Solar heating system", issued Script error: No such module "auto date formatter". 
  12. ^ Page Module:Citation/CS1/styles.css has no content.Gretton, Lel. "Lime power for cooking - medieval pots to 21st century cans". Old & Interesting. Retrieved 13 February 2018.
  13. ^ Page Module:Citation/CS1/styles.css has no content."Compound Summary for CID 14778 - Calcium Oxide". PubChem.
  14. ^ Page Module:Citation/CS1/styles.css has no content.Gray, Theodore (September 2007). "Limelight in the Limelight". Popular Science: 84. Archived from the original on 2008-10-13. Retrieved 2009-03-31.
  15. ^ Page Module:Citation/CS1/styles.css has no content.Tony Oates (2007), "Lime and Limestone", Ullmann's Encyclopedia of Industrial Chemistry (7th ed.), Wiley, pp. 1–32, doi:10.1002/14356007.a15_317, ISBN 978-3527306732
  16. ^ Page Module:Citation/CS1/styles.css has no content.Tel Aviv University (August 9, 2012). "Neolithic man: The first lumberjack?". phys.org. Retrieved 2023-02-02.
  17. ^ Page Module:Citation/CS1/styles.css has no content.Karkanas, P.; Stratouli, G. (2011). "Neolithic Lime Plastered Floors in Drakaina Cave, Kephalonia Island, Western Greece: Evidence of the Significance of the Site". The Annual of the British School at Athens. 103: 27–41. doi:10.1017/S006824540000006X. S2CID 129562287.
  18. ^ Page Module:Citation/CS1/styles.css has no content.Connelly, Ashley Nicole (May 2012). Analysis and Interpretation of Neolithic Near Eastern Mortuary Rituals from a Community-Based Perspective (PDF) (Thesis). Texas: Baylor University. Archived from the original (PDF) on 2015-03-09.
  19. ^ Page Module:Citation/CS1/styles.css has no content.Walker, Thomas A (1888). The Severn Tunnel Its Construction and Difficulties. London: Richard Bentley and Son. p. 92.
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  21. ^ Page Template:Citation/styles.css has no content.US Patent 255042, issued Script error: No such module "auto date formatter". 
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  23. ^ Page Module:Citation/CS1/styles.css has no content."Riddle solved: Why was Roman concrete so durable?", MIT News, January 6, 2023
  24. ^ Page Module:Citation/CS1/styles.css has no content.Croddy, Eric (2002). Chemical and biological warfare: a comprehensive survey for the concerned citizen. Springer. p. 128. ISBN 0-387-95076-1.
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  26. ^ Page Module:Citation/CS1/styles.css has no content.Sayers, W. (2006). "The Use of Quicklime in Medieval Naval Warfare". The Mariner's Mirror. 92 (3): 262–269. doi:10.1080/00253359.2006.10657001.
  27. ^ Page Module:Citation/CS1/styles.css has no content.Lime (PDF). U.S. Geological Survey, Mineral Commodity Summaries (Report). February 2019. p. 96. Archived from the original (PDF) on 2021-12-19. Retrieved 2022-03-10.
  28. ^ Page Module:Citation/CS1/styles.css has no content.Mallinckrodt Baker Inc. - Strategic Services Division (December 8, 1996). "Hazards". ww25.hazard.com. Archived from the original on May 1, 2012. Retrieved 2023-02-02.
  29. ^ Page Module:Citation/CS1/styles.css has no content."List of Minerals". Ima-mineralogy.org. 21 March 2011.
  30. ^ Page Module:Citation/CS1/styles.css has no content.Fiquet, G.; Richet, P.; Montagnac, G. (Dec 1999). "High-temperature thermal expansion of lime, periclase, corundum and spinel". Physics and Chemistry of Minerals. 27 (2): 103–111. Bibcode:1999PCM....27..103F. doi:10.1007/s002690050246. S2CID 93706828. Retrieved 9 February 2023.
  31. ^ Page Module:Citation/CS1/styles.css has no content.Tian, X.K.; Lin, S.C.; Yan, J.; Zhao, C.Y. (2022). "Sintering mechanism of calcium oxide/calcium carbonate during thermochemical heat storage process". Chemical Engineering Journal. 428 131229. Bibcode:2022ChEnJ.42831229T. doi:10.1016/j.cej.2021.131229.
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