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'''Calcium hydroxide''' (traditionally called '''slaked lime''') is an [[inorganic compound]] with the chemical formula [[calcium|Ca]]([[Hydroxide|OH]])<sub>2</sub>. It is a colorless crystal or white powder and is produced when quicklime ([[calcium oxide]]) is mixed with [[water]].
Calcium hydroxide has many names including '''hydrated lime''', '''caustic lime''', '''builders' lime''', '''slaked lime''', '''cal''', and '''pickling lime'''. Calcium hydroxide is used in many applications, including food preparation, where it has been identified as [[E number]] '''E526'''. '''Limewater''', also called '''milk of lime''', is the common name for a [[saturated solution]] of calcium hydroxide.
Calcium hydroxide is poorly soluble in water, with a retrograde solubility increasing from 0.66 g/L at 100 °C to 1.89 g/L at 0 °C. With a [[Solubility product|solubility product ''K''<sub>sp</sub>]] of 5.02{{e|−6}} at 25 °C,<ref name="crc" />{{Clarify|date=June 2021}} its dissociation in water is large enough that its solutions are basic according to the following dissolution reaction:▼
==Solubility==
▲Calcium hydroxide is
: Ca(OH)<sub>2</sub> → Ca<sup>2+</sup> + 2 OH<sup>−</sup>
The solubility is affected by the [[common-ion effect]]. Its solubility drastically decreases upon addition of hydroxide or calcium sources.
==Reactions==
: Ca(OH)<sub>2</sub> + CO<sub>2</sub> → CaCO<sub>3</sub> + H<sub>2</sub>O▼
When heated to 512 °C, the [[partial pressure]] of water in equilibrium with calcium hydroxide reaches 101{{nbsp}}kPa (normal atmospheric pressure), which [[chemical decomposition|decomposes]] calcium hydroxide into [[calcium oxide]] and water:<ref>{{cite journal |last=Halstead |first=P. E. |author2=Moore, A. E. |title=The Thermal Dissociation of Calcium Hydroxide |journal=Journal of the Chemical Society |volume=769 |pages=3873 |year=1957 |doi=10.1039/JR9570003873}}</ref>
: Ca(OH)<sub>2</sub> → CaO + H<sub>2</sub>O
When carbon dioxide is passed through limewater, the solution takes on a milky appearance due to [[precipitation (chemistry)|precipitation]] of insoluble [[calcium carbonate]]:
Calcium hydroxide reacts with [[hydrogen chloride]] to first give [[calcium hydroxychloride]] and then [[calcium chloride]].▼
▲: Ca(OH)<sub>2</sub>{{sub|({{serif|''aq''}})}} + CO<sub>2</sub>{{sub|({{serif|''g''}})}} → CaCO<sub>3</sub>{{sub|({{serif|''s''}})}} + H<sub>2</sub>O{{sub|({{serif|''l''}})}}
If excess CO<sub>2</sub> is added: the following reaction takes place:
: CaCO<sub>3</sub>{{sub|({{serif|''s''}})}} + H<sub>2</sub>O{{sub|({{serif|''l''}})}} + CO<sub>2</sub>{{sub|({{serif|''g''}})}} → [[Calcium bicarbonate|Ca(HCO<sub>3</sub>)<sub>2</sub>]]{{sub|({{serif|''aq''}})}}
The milkiness disappears since calcium bicarbonate is water-soluble.
Calcium hydroxide reacts with [[aluminium]]. This reaction is the basis of [[aerated concrete]].<ref name=Ullmann/> It does not corrode [[iron]] and [[steel]], owing to [[passivation (chemistry)|passivation]] of their surface.
==Structure, preparation, occurrence==▼
[[File:CaOH2SEM.jpg|thumb|left|170px|[[Scanning electron microscopy|SEM]] image of fractured hardened cement paste, showing plates of calcium hydroxide and needles of [[ettringite]] (micron scale)]]▼
Calcium hydroxide adopts a [[polymeric]] structure, as do all metal hydroxides. The structure is identical to that of Mg(OH){{sub|2}} (''brucite structure''); i.e., the [[cadmium iodide]] motif. Strong [[hydrogen bond]]s exist between the layers.<ref name=G&W/>▼
▲Calcium hydroxide reacts with [[
Calcium hydroxide is produced commercially by treating (slaking) lime with water:▼
In a process called [[sulfation]], sulphur dioxide reacts with limewater:
: Ca(OH)<sub>2</sub>{{sub|({{serif|''aq''}})}} + SO<sub>2</sub>{{sub|({{serif|''g''}})}} → CaSO<sub>3</sub>{{sub|({{serif|''s''}})}} + H<sub>2</sub>O{{sub|({{serif|''l''}})}}
Limewater is used in a process known as [[lime softening]] to reduce water hardness. It is also used as a neutralizing agent in municipal waste water treatment.
The positively charged ionized species CaOH<sup>+</sup> has been detected in the atmosphere of [[S-type star]]s.<ref>{{citation▼
| first1=Uffe G. | last1=Jørgensen▼
| title=Molecules in Astrophysics: Probes and Processes▼
| chapter=Cool Star Models | volume=178▼
| series=International Astronomical Union Symposia. Molecules in Astrophysics: Probes and Processes▼
| editor-first=Ewine F. | editor-last=van Dishoeck ▼
| publisher=Springer Science & Business Media▼
| year=1997 | isbn=079234538X | page=446 | postscript=.▼
| chapter-url=https://books.google.com/books?id=VW50otz5v8sC&pg=PA446▼
}}</ref>▼
▲[[File:CaOH2SEM.jpg|thumb|left|170px|[[Scanning electron microscopy|SEM]] image of fractured hardened cement paste, showing plates of calcium hydroxide and needles of [[ettringite]] (micron scale)]]
▲Calcium hydroxide adopts a [[polymeric]] structure, as do all metal hydroxides. The structure is identical to that of Mg(OH){{sub|2}} (''brucite structure''); i.e., the [[cadmium iodide]] motif. Strong [[hydrogen bond]]s exist between the layers.<ref name=G&W/>
:{{chem2|CaO + H2O -> Ca(OH)2}}
Alongside the production of quicklime from [[limestone]] by [[calcination]], this is one of the oldest known chemical reactions; evidence of [[prehistoric]] production dates back to at least 7000 BCE.<ref>{{cite web|url=https://www.sciencelearn.org.nz/resources/1420-history-of-limestone-uses-timeline |title=History of limestone uses – timeline |website=Science Learning Hub – Pokapū Akoranga Pūtaiao |publisher=Curious Minds New Zealand|date=October 1, 2012|access-date=June 7, 2024}}</ref>
==Uses==
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One significant application of calcium hydroxide is as a [[flocculation|flocculant]], in water and [[sewage treatment]]. It forms a fluffy charged solid that aids in the removal of smaller particles from water, resulting in a clearer product. This application is enabled by the low cost and low toxicity of calcium hydroxide. It is also used in fresh-water treatment for raising the pH of the water so that pipes will not corrode where the base water is acidic, because it is self-regulating and does not raise the pH too much.{{cn|date=September 2023}}
Another large application is in the paper industry, where it is an intermediate in the reaction in the production of sodium hydroxide. This conversion is part of the ''causticizing'' step in the [[Kraft process#Recovery process|Kraft process]] for making pulp.
▲Another large application is in the paper industry, where it is an intermediate in the reaction in the production of sodium hydroxide. This conversion is part of the ''causticizing'' step in the [[Kraft process#Recovery process|Kraft process]] for making pulp.<ref name=G&W>Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. {{ISBN|0-7506-3365-4}}.</ref> In the causticizing operation, burned lime is added to ''[[green liquor]]'', which is a solution primarily of [[sodium carbonate]] and [[sodium sulfate]] produced by dissolving ''smelt'', which is the molten form of these chemicals from the recovery furnace.{{cn|date=September 2023}}
In [[orchard]] crops, calcium hydroxide is used as a fungicide. Applications of 'lime water' prevent the development of cankers caused by the fungal pathogen ''[[Neonectria galligena]]''. The trees are sprayed when they are dormant in winter to prevent toxic burns from the highly reactive calcium hydroxide. This use is authorised in the European Union and the United Kingdom under Basic Substance regulations.<ref>{{Cite web |last=European Union |date=13 May 2015 |title=COMMISSION IMPLEMENTING REGULATION (EU) 2015/762 of 12 May 2015 approving the basic substance calcium hydroxide in accordance with Regulation (EC) No 1107/2009 of the European Parliament and of the Council concerning the placing of plant protection products on the market, and amending the Annex to Commission Implementing Regulation (EU) No 540/2011 |url=https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32015R0762&from=RO |access-date=12 May 2022}}</ref>
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===Food industry===
Because of its low [[toxicity]] and the mildness of its basic properties, slaked lime is widely used in the [[food industry]]
* In USDA certified food production in plants and livestock<ref>{{Cite web |url=https://www.ams.usda.gov/sites/default/files/media/Lime%20Hydrated%20TR%202015.pdf |title=Hydrated Lime: Technical Evaluation Report |last=Pesticide Research Institute for the USDA National Organic Program |date=23 March 2015 |website=Agriculture Marketing Services |access-date=17 July 2019}}</ref>
* To clarify raw juice from [[sugarcane]] or [[sugar beet]]s in the [[sugar industry]] (see [[carbonatation]])
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==== Native American uses ====
[[File:Nixtamalized Corn maize El Salvador recipe.jpg|thumb|left|Dry untreated maize (left), and treated maize (right) after boiling in water with calcium hydroxide (15{{nbsp}}ml, or 1{{nbsp}}tbsp, lime for 500{{nbsp}}g of corn) for 15 minutes]] In [[Nahuatl]], the language of the [[Aztecs]], the word for calcium hydroxide is ''nextli''. In a process called ''[[nixtamalization]]'', [[maize]] is cooked with nextli to become {{lang|es|nixtamal}}, also known as [[hominy]]. Nixtamalization significantly increases the bioavailability of [[Niacin (nutrient)|niacin]] (vitamin B3), and is also considered tastier and easier to digest. Nixtamal is often ground into a flour, known as ''[[masa]]'', which is used to make tortillas and tamales.{{cn|date=September 2023}}
Limewater is used in the preparation of maize for [[corn tortillas]] and other culinary purposes using a process known as [[nixtamalization]]. Nixtamalization makes the [[niacin]] nutritionally available and prevents [[pellagra]].<ref>{{cite journal |url=https://www.researchgate.net/publication/228453826 |title=Nixtamalization, a Mesoamerican technology to process maize at small-scale with great potential for improving the nutritional quality of maize based foods |last=Wacher |first=Carmen |date=2003-01-01 |journal=Food Based Approaches for a Healthy Nutrition in Africa |url-status=live |archive-url=https://web.archive.org/web/20180305202539/https://www.researchgate.net/publication/228453826_Nixtamalization_a_Mesoamerican_technology_to_process_maize_at_small-scale_with_great_potential_for_improving_the_nutritional_quality_of_maize_based_foods |archive-date=2018-03-05}}</ref> Traditionally lime water was used in [[Taiwan]] and [[China]] to preserve [[persimmon]] and to remove [[wikt:astringency|astringency]].<ref>{{Cite book |last=Hu |first=Shiu-ying |url=https://www.worldcat.org/oclc/58840243 |title=Food plants of China |date=2005 |publisher=Chinese University Press |isbn=962-201-860-2 |location=Hong Kong |oclc=58840243}}</ref>{{Rp|page=623}}
In chewing [[coca leaves]], calcium hydroxide is usually chewed alongside to keep the [[alkaloid]] [[stimulant]]s chemically available for [[absorption (small intestine)|absorption]] by the body. Similarly, Native Americans traditionally chewed tobacco leaves with calcium hydroxide derived from burnt mollusc shells to enhance the effects. It has also been used by some indigenous American tribes as an ingredient in ''[[yopo]]'', a psychedelic snuff prepared from the beans of some ''[[Anadenanthera]]'' species.<ref>{{cite journal |author=de Smet, Peter A. G. M. |title=A multidisciplinary overview of intoxicating snuff rituals in the Western Hemisphere |doi=10.1016/0378-8741(85)90060-1 |pmid=3887041 |journal=Journal of Ethnopharmacology |volume=3 |issue=1 |year=1985 |pages=3–49}}</ref>
====Asian uses====
Calcium hydroxide is typically added to a bundle of [[areca nut]] and [[betel]] leaf called "[[paan]]" to keep the [[alkaloid]] [[stimulant]]s chemically available to enter the bloodstream via [[sublingual]] absorption.
It is used in making ''[[naswar]]'' (also known as ''nass'' or ''niswar''), a type of dipping tobacco made from fresh tobacco leaves, calcium hydroxide (''chuna''/''choona'' or ''soon''), and wood ash. <!--[[calcium oxide]] plays an important role in making a ''naswar''.--> It is consumed most in the [[Pashtun people|Pathan]] diaspora, [[Afghanistan]], [[Pakistan]], [[India]] and [[Bangladesh]]. Villagers also use calcium hydroxide to [[Whitewash|paint their mud houses]] in Afghanistan, Pakistan and India.
===Hobby uses===
In buon [[fresco]] painting, limewater is used as the colour solvent to apply on fresh plaster. Historically, it is known as the paint [[whitewash]].
[[File:Dying Gaul (casting in Pushkin museum) 04 by shakko.jpg|thumb|Closeup of cast of ''The Dying Gaul'', showing distinctive hairstyle, supposedly derived from washing in limewater.]]
Limewater is widely used by marine aquarists as a primary supplement of [[calcium]] and [[alkalinity]] for reef aquariums. [[Coral]]s of order [[Scleractinia]] build their [[endoskeleton]]s from [[aragonite]] (a [[Polymorphism (materials science)|polymorph]] of calcium carbonate). When used for this purpose, limewater is usually referred to as ''Kalkwasser''. It is also used in [[tanning (leather)|tanning]] and making [[parchment]]. The lime is used as a dehairing agent based on its alkaline properties.<ref>''The Nature and Making of Parchment'' by Ronald Reed {{ISBN?}}</ref>
=== Personal care and adornment ===
Treating one's hair with limewater causes it to stiffen and bleach, with the added benefit of killing any lice or mites living there. [[Diodorus Siculus]] described the [[Celts]] as follows:
"Their aspect is terrifying... They are very tall in stature, with rippling muscles under clear white skin. Their hair is blond, but not only naturally so: they bleach it, to this day, artificially, washing it in lime and combing it back from their foreheads. They look like wood-demons, their hair thick and shaggy like a horse's mane. Some of them are clean-shaven, but others – especially those of high rank, shave their cheeks but leave a moustache that covers the whole mouth...".<ref>{{Cite web|url=http://exploringcelticciv.web.unc.edu/diodorus-siculus-library-of-history/|title = Diodorus Siculus, Library of History | Exploring Celtic Civilizations}}</ref><ref>{{cite web|url=https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Diodorus_Siculus/5B*.html|title=Diodorus Siculus – Book V, Chapter 28|website=penelope.uchicago.edu|language=english|access-date=2017-11-12}}</ref>
Calcium hydroxide is also applied in a leather process called [[Liming (leather processing)|liming]].
==Interstellar medium==
▲The
▲ | first1=Uffe G. | last1=Jørgensen
▲ | title=Molecules in Astrophysics: Probes and Processes
▲ | chapter=Cool Star Models | volume=178
▲ | series=International Astronomical Union Symposia. Molecules in Astrophysics: Probes and Processes
▲ | editor-first=Ewine F. | editor-last=van Dishoeck
▲ | publisher=Springer Science & Business Media
▲ | year=1997 | isbn=079234538X | page=446 | postscript=.
▲ | chapter-url=https://books.google.com/books?id=VW50otz5v8sC&pg=PA446
▲}}</ref>
==Limewater==
'''Limewater''' is a saturated aqueous solution of [[calcium hydroxide]]. Calcium hydroxide is sparsely soluble at room temperature in water (1.5 g/L at 25 °C<ref>'Solubility of Inorganic and Metalorganic Compounds – A Compilation of Solubility Data from the Periodical Literature', A. Seidell, W. F. Linke, Van Nostrand (Publisher), 1953 {{ISBN?}}</ref>). "Pure" (i.e. less than or fully saturated) limewater is clear and colorless, with a slight earthy smell and an astringent/bitter taste. It is basic in nature with a [[pH]] of 12.4. Limewater is named after [[limestone]], not the [[Lime (fruit)|lime fruit]]. Limewater may be prepared by mixing calcium hydroxide (Ca(OH)<sub>2</sub>) with water and removing excess undissolved solute (e.g. by filtration). When excess calcium hydroxide is added (or when environmental conditions are altered, e.g. when its temperature is raised sufficiently), there results a milky solution due to the homogeneous suspension of excess calcium hydroxide. This liquid has been known traditionally as '''milk of lime'''.
==Health risks==
Unprotected exposure to Ca(OH)<sub>2</sub>, as with any strong [[Base (chemistry)|base]], can cause
==See also==
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