Calcium hydroxide (traditionally calledslaked lime) is aninorganic compound with the chemical formulaCa(OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed withwater. Annually, approximately 125 million tons of calcium hydroxide are produced worldwide.[8]
Calcium hydroxide has many names includinghydrated lime,caustic lime,builders' lime,slaked lime,cal, andpickling lime. Calcium hydroxide is used in many applications, including food preparation, where it has been identified asE numberE526.Limewater, also calledmilk of lime, is the common name for asaturated solution of calcium hydroxide.
Calcium hydroxide is moderately soluble in water, as seen for many dihydroxides. Its solubility increases from 0.66 g/L at 100 °C to 1.89 g/L at 0 °C.[8] Itssolubility productKsp of 5.02×10−6 at 25 °C,[1] its dissociation in water is large enough that its solutions are basic according to the following dissolution reaction:
Ca(OH)2 → Ca2+ + 2 OH−
The solubility is affected by thecommon-ion effect. Its solubility drastically decreases upon addition of hydroxide or calcium sources.
When heated to 512 °C, thepartial pressure of water in equilibrium with calcium hydroxide reaches 101kPa (normal atmospheric pressure), whichdecomposes calcium hydroxide intocalcium oxide and water:[9]
Ca(OH)2 → CaO + H2O
When carbon dioxide is passed through limewater, the solution takes on a milky appearance due toprecipitation of insolublecalcium carbonate:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
If excess CO2 is added: the following reaction takes place:
In a process calledsulfation, sulphur dioxide reacts with limewater:
Ca(OH)2(aq) + SO2(g) → CaSO3(s) + H2O(l)
Limewater is used in a process known aslime softening to reduce water hardness. It is also used as a neutralizing agent in municipal waste water treatment.
SEM image of fractured hardened cement paste, showing plates of calcium hydroxide and needles ofettringite (micron scale)
Calcium hydroxide adopts apolymeric structure, as do all metal hydroxides. The structure is identical to that of Mg(OH)2 (brucite structure); i.e., thecadmium iodide motif. Stronghydrogen bonds exist between the layers.[10]
Calcium hydroxide is produced commercially by treating (slaking)quicklime with water:
CaO + H2O → Ca(OH)2
Alongside the production of quicklime fromlimestone bycalcination, this is one of the oldest known chemical reactions; evidence ofprehistoric production dates back to at least 7000 BCE.[11]
Calcium hydroxide is commonly used to preparelime mortar.
One significant application of calcium hydroxide is as aflocculant, in water andsewage 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.[citation needed]
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 thecausticizing step in theKraft process for making pulp. In the causticizing operation, burned lime is added togreen liquor, which is a solution primarily ofsodium carbonate andsodium sulfate produced by dissolvingsmelt, which is the molten form of these chemicals from the recovery furnace.[10]
Inorchard crops, calcium hydroxide is used as a fungicide. Applications of 'lime water' prevent the development of cankers caused by the fungal pathogenNeonectria 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.[12]
Calcium hydroxide is used in dentistry, primarily in the specialty of endodontics.
Dry untreated maize (left), and treated maize (right) after boiling in water with calcium hydroxide (15ml, or 1tbsp, lime for 500g of corn) for 15 minutes
InNahuatl, the language of theAztecs, the word for calcium hydroxide isnextli. In a process callednixtamalization,maize is cooked with nextli to becomenixtamal, also known ashominy. Nixtamalization significantly increases the bioavailability ofniacin (vitamin B3), and is also considered tastier and easier to digest. Nixtamal is often ground into a flour, known asmasa, which is used to make tortillas and tamales.[citation needed]
Limewater is used in the preparation of maize forcorn tortillas and other culinary purposes using a process known asnixtamalization. Nixtamalization makes the niacin nutritionally available and preventspellagra.[16] Traditionally lime water was used inTaiwan andChina to preservepersimmon and to removeastringency.[17]: 623
In chewingcoca leaves, calcium hydroxide is usually chewed alongside to keep thealkaloidstimulants chemically available forabsorption 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 South American tribes as an ingredient inyopo, a psychedelic snuff prepared from the beans of someAnadenanthera species.[18]
Calcium hydroxide, locally known aschuna,choona orsoon, is typically added to a bundle ofareca nut andbetel leaf called "paan" to keep thealkaloidstimulants chemically available to enter the bloodstream viasublingual absorption.
Choona is a key ingredient inPetha, contributing to its characteristic crunchy and firm texture.
It is used in makingnaswar (also known asnass orniswar), a type of dipping tobacco made from fresh tobacco leaves,Choona, and wood ash. It is consumed most in thePathan diaspora,Afghanistan,Pakistan,India andBangladesh. Villagers also use calcium hydroxide topaint their mud houses in Afghanistan, Pakistan and India.
In buonfresco painting, limewater is used as the colour solvent to apply on fresh plaster. Historically, it is known as the paintwhitewash.
Closeup of cast ofThe Dying Gaul, showing distinctive hairstyle, supposedly derived from washing in limewater
Limewater is widely used by marine aquarists as a primary supplement ofcalcium andalkalinity for reef aquariums.Corals of orderScleractinia build theirendoskeletons fromaragonite (apolymorph of calcium carbonate). When used for this purpose, limewater is usually referred to asKalkwasser. It is also used intanning and makingparchment. The lime is used as a dehairing agent based on its alkaline properties.[19]
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 theCelts 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...".[20][21]
Calcium hydroxide is also applied in a leather process calledliming.
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[23]). "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 apH of 12.4. Limewater is named afterlimestone, not thelime fruit. Limewater may be prepared by mixing calcium hydroxide (Ca(OH)2) 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 asmilk of lime.
^Halstead, P. E.; Moore, A. E. (1957). "The Thermal Dissociation of Calcium Hydroxide".Journal of the Chemical Society.769: 3873.doi:10.1039/JR9570003873.
^abGreenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann.ISBN0-7506-3365-4.
^Pesticide Research Institute for the USDA National Organic Program (23 March 2015)."Hydrated Lime: Technical Evaluation Report"(PDF).Agriculture Marketing Services. Retrieved17 July 2019.
^de Smet, Peter A. G. M. (1985). "A multidisciplinary overview of intoxicating snuff rituals in the Western Hemisphere".Journal of Ethnopharmacology.3 (1):3–49.doi:10.1016/0378-8741(85)90060-1.PMID3887041.
^The Nature and Making of Parchment by Ronald Reed[ISBN missing]
^Jørgensen, Uffe G. (1997),"Cool Star Models", in van Dishoeck, Ewine F. (ed.),Molecules in Astrophysics: Probes and Processes, International Astronomical Union Symposia. Molecules in Astrophysics: Probes and Processes, vol. 178, Springer Science & Business Media, p. 446,ISBN079234538X.
^'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 missing]
National Organic Standards Board Technical Advisory Panel (4 April 2002)."NOSB TAP Review: Calcium Hydroxide"(PDF). Organic Materials Review Institute. Archived fromthe original(.PDF) on 31 October 2007. Retrieved5 February 2008.{{cite journal}}:Cite journal requires|journal= (help)
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