Calcium hydroxide

Calcium hydroxide(traditionally calledslaked lime) is aninorganic compoundwith 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.
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 solutionof calcium hydroxide.
Calcium hydroxide is modestly 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.Itssolubility productKspof 5.02×10at 25 °C,its dissociation in water is large enough that its solutions are basic according to the following dissolution reaction:
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 pressureof water in equilibrium with calcium hydroxide reaches 101kPa (normal atmospheric pressure), whichdecomposescalcium hydroxide intocalcium oxideand water:
When carbon dioxide is passed through limewater, the solution takes on a milky appearance due toprecipitationof insolublecalcium carbonate:
If excess CO2is added: the following reaction takes place:
The milkiness disappears since calcium bicarbonate is water-soluble.
Calcium hydroxide reacts withaluminium.  This reaction is the basis ofaerated concrete.It does not corrodeironandsteel, owing topassivationof their surface.
Calcium hydroxide reacts withhydrochloric acidto givecalcium hydroxychlorideand thencalcium chloride.
In a process calledsulfation, sulphur dioxide reacts with limewater:
Limewater is used in a process known aslime softeningto reduce water hardness.  It is also used as a neutralizing agent in municipal waste water treatment.
Calcium hydroxide adopts apolymericstructure, as do all metal hydroxides.  The structure is identical to that of Mg(OH)2(brucite structure); i.e., thecadmium iodidemotif. Stronghydrogen bondsexist between the layers.
Calcium hydroxide is produced commercially by treating (slaking)quicklimewith water:
Alongside the production of quicklime fromlimestonebycalcination, this is one of the oldest known chemical reactions; evidence ofprehistoricproduction dates back to at least 7000 BCE.
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.
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 thecausticizingstep in theKraft processfor making pulp. In the causticizing operation, burned lime is added togreen liquor, which is a solution primarily ofsodium carbonateandsodium sulfateproduced by dissolvingsmelt, which is the molten form of these chemicals from the recovery furnace.
Inorchardcrops, 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.
Calcium hydroxide is used in dentistry, primarily in the specialty of endodontics.
Because of its lowtoxicityand the mildness of its basic properties, slaked lime is widely used in thefood industry,
InNahuatl, the language of theAztecs, the word for calcium hydroxide isnextli. In a process callednixtamalization,maizeis 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.
Limewater is used in the preparation of maize forcorn tortillasand other culinary purposes using a process known asnixtamalization. Nixtamalization makes the niacin nutritionally available and preventspellagra.Traditionally lime water was used inTaiwanandChinato preservepersimmonand to removeastringency.
In chewingcoca leaves, calcium hydroxide is usually chewed alongside to keep thealkaloidstimulantschemically available forabsorptionby 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 someAnadenantheraspecies.
Calcium hydroxide, locally known aschuna,choonaorsoon, is typically added to a bundle ofareca nutandbetelleaf called "paan" to keep thealkaloidstimulantschemically available to enter the bloodstream viasublingualabsorption.
Choonais a key ingredient inPetha, contributing to its characteristic crunchy and firm texture.
It is used in makingnaswar(also known asnassorniswar), a type of dipping tobacco made from fresh tobacco leaves,Choona, and wood ash.  It is consumed most in thePathandiaspora,Afghanistan,Pakistan,IndiaandBangladesh. Villagers also use calcium hydroxide topaint their mud housesin Afghanistan, Pakistan and India.
In buonfrescopainting, limewater is used as the colour solvent to apply on fresh plaster. Historically, it is known as the paintwhitewash.
Limewater is widely used by marine aquarists as a primary supplement ofcalciumandalkalinityfor reef aquariums.Coralsof orderScleractiniabuild theirendoskeletonsfromaragonite(apolymorphof calcium carbonate). When used for this purpose, limewater is usually referred to asKalkwasser. It is also used intanningand makingparchment. The lime is used as a dehairing agent based on its alkaline properties.
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 Siculusdescribed theCeltsas 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...".
Calcium hydroxide is also applied in a leather process calledliming.
The ion CaOHhas been detected in the atmosphere ofS-type stars.
Limewateris a saturated aqueous solution of calcium hydroxide. Calcium hydroxide is sparsely soluble at room temperature in water (1.5 g/L at 25 °C). "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 apHof 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.
Unprotected exposure to Ca(OH)2, as with any strongbase, can cause skin burns, but it is not acutely toxic.