Calcium hydroxide (traditionally called slaked lime) is an

inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

with the chemical formula

( OH)₂. It is a colorless crystal or white powder and is produced when quicklime (

calcium oxide Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms f ...

) is mixed or slaked with

water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water

. It has many names including hydrated lime, caustic lime, builders' lime, slacked lime, cal, and pickling lime. Calcium hydroxide is used in many applications, including food preparation, where it has been identified as

E number E numbers ("E" stands for "Europe") are codes for substances used as food additive Food is any substance consumed to provide nutritional support for an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμ ...

E526.

Limewater Limewater is the common name for a saturated aqueous solution of calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula calcium, Ca(Hydroxide, OH)2. It is a colorless crystal ...

, also called milk of lime, is the common name for a

saturated solution In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence i ...

of calcium hydroxide.

Properties

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 ''K''_sp of 5.5 at T = ?. its dissociation in water is large enough that its solutions are basic according to the following dissolution reaction: : Ca(OH)₂ → Ca²⁺ + 2 OH⁻ At ambient temperature, calcium hydroxide (

portlandite Portlandite is an oxide mineral The oxide mineral class includes those mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the ...

) dissolves in pure water to produce an alkaline solution with a pH of about 12.5. Calcium hydroxide solutions can cause chemical burns. At high pH value due to a common-ion effect with the hydroxide anion , its solubility drastically decreases. This behavior is relevant to cement pastes. Aqueous solutions of calcium hydroxide are called

limewater Limewater is the common name for a saturated aqueous solution of calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula calcium, Ca(Hydroxide, OH)2. It is a colorless crystal ...

and are medium-strength bases, which reacts with

acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special case of , proton donors form the H3O+ and are ...

s and can attack some

metal A metal (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

metal

s such as

aluminium Aluminium (aluminum in and ) is a with the Al and 13. Aluminium has a density lower than those of other common , at approximately one third that of . It has a great affinity towards , and of on the surface when exposed to air ...

(amphoteric hydroxide dissolving at high pH), while protecting other metals, such as

iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

iron

and

steel Steel is an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appe ...

steel

, from corrosion by passivation of their surface. Limewater turns milky in the presence of

carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

due to formation of

calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togethe ...

, a process called

carbonatation Carbonatation is a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter ...

: : Ca(OH)₂ + CO₂ → CaCO₃ + H₂O When heated to 512 °C, the

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mix ...

of water in equilibrium with calcium hydroxide reaches 101kPa (normal atmospheric pressure), which

decomposes

calcium hydroxide into

and water: : Ca(OH)₂ → CaO + H₂O

Structure, preparation, occurrence

Calcium hydroxide adopts a

polymeric A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical ...

structure, as do all metal hydroxides. The structure is identical to that of Mg(OH) (''brucite structure''); i.e., the

cadmium iodide Cadmium iodide, CdI2, is a chemical compound of cadmium Cadmium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure subs ...

motif. Strong

hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

s exist between the layers. Calcium hydroxide is produced commercially by treating lime with water: :CaO + H₂O → Ca(OH)₂ In the laboratory it can be prepared by mixing

aqueous An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute an ...

solutions of

calcium chloride Calcium chloride is an inorganic compound, a salt with the chemical formula CaCl2. It is a white coloured crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calc ...

and

sodium hydroxide Sodium hydroxide, also known as lye A lye is a metal hydroxide traditionally obtained by leaching wood ashes, or a strong alkali In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chem ...

. The mineral form,

, is relatively rare but can be found in some volcanic,

plutonic Intrusive rock is formed when magma Magma () is the molten or semi-molten natural material from which all igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The thr ...

, and metamorphic rocks. It has also been known to arise in burning coal dumps. The positively charged ionized species CaOH⁺ has been detected in the atmosphere of

S-type star An S-type star (or just S star) is a cool giant star, giant with approximately equal quantities of carbon and oxygen in its atmosphere. The class was originally defined in 1922 by Paul W. Merrill, Paul Merrill for stars with unusual absorption li ...

Retrograde solubility

The solubility of calcium hydroxide at 70 °C is about half of its value at 25 °C. According to Hopkins and Wulff (1965), the decrease of calcium hydroxide solubility with temperature is known since a long time by the works of

Marcellin Berthelot Pierre Eugène Marcellin Berthelot (; 1827–1907) was a French chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist A scientist is a person who conducts Scientific m ...

(1875)Berthelot, M. (1875). Dissolution des acides et des alcalis. issolution of acids and alkalis In: Annales de Chimie et de Physique. Vol. 4, pp. 445–536. and Julius Thomsen (1883)Thomsen J. (1883). Thermochemische untersuchungen hermochemical studies Vol. III, Johann Ambrosius Barth Verlag, Leipzig. (see

Thomsen–Berthelot principleIn thermochemistry, the Thomsen–Berthelot principle is a hypothesis in the history of chemistry which argued that all chemical process, chemical changes are accompanied by the production of heat and that processes which occur will be ones in which ...

), when the presence of ions in aqueous solutions was still questioned. Since, it has been studied in detail by many authors, a.o., Miller and Witt (1929) or Johnston and Grove (1931) and refined many times (''e.g.'', Greenberg and Copeland (1960); Hopkins and Wulff (1965); Seewald and Seyfried (1991); Duchesne and Reardon (1995)). The reason for this rather uncommon behavior is that the dissolution of calcium hydroxide in water is an exothermic process. Thus, according to

Le Chatelier's principle Le Chatelier's principle (pronounced or ), also called Chatelier's principle, is a principle of chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, mole ...

, a lowering of temperature favours the elimination of the heat liberated through the process of dissolution and increases the equilibrium constant of dissolution of Ca(OH)₂, and so increase its solubility at low temperature. This counter-intuitive temperature dependence of the solubility is referred to as "retrograde" or "inverse" solubility. The variably hydrated phases of

calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that cove ...

(

gypsum Gypsum is a soft sulfate mineral The sulfate minerals are a class of mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with ...

gypsum

bassanite Bassanite is a calcium sulfate mineral with formula calcium sulfate, CaSO4·water of crystallization, H2O or 2CaSO4·H2O. In other words it has half a water per CaSO4 unit hence its synonym ''calcium sulfate hemihydrate''. Bassanite was first des ...

and

anhydrite Anhydrite, or anhydrous calcium sulfate Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrate In chemistry Chemistry is the science, scientific study of the properties and behavior o ...

) also exhibit a retrograde solubility for the same reason because their dissolution reactions are exothermic.

Uses

Calcium hydroxide is commonly used to prepare

lime mortar Lime mortar is composed of lime Lime refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may ...

. One significant application of calcium hydroxide is as a

flocculant

, in water and

sewage treatment Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment Wastewater treatment is a process used to remove contaminants from wastewater or sewage Sewage, or domestic/municip ...

. 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. It is also used in the preparation of ammonia gas (NH₃), using the following reaction: : Ca(OH)₂ + 2

NH₄Cl

→ 2

NH₃

CaCl₂

+ 2 H₂O 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 The kraft process (also known as kraft pulping or sulfate process) is a process for conversion of into , which consists of almost pure fibers, the main component of . The kraft process entails treatment of wood chips with a hot mixture of water, ...

for making pulp.Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. . In the causticizing operation, burned lime is added to '' green liquor'', which is a solution primarily of

sodium carbonate Sodium carbonate, ·10, (also known as Natrium Carbonate, washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield modera ...

and

sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compou ...

produced by dissolving ''smelt'', which is the molten form of these chemicals from the recovery furnace.

Food industry

Because of its low

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacteria, bacterium, or plant, as well as the effect on ...

and the mildness of its basic properties, slaked lime is widely used in the

food industry The food industry is a complex, global network of diverse business Business is the activity of making one's living or making money by producing or buying and selling products (such as goods and services). Simply put, it is "any activity ...

: * In USDA certified food production in plants and livestock * To clarify raw juice from

sugarcane Sugarcane or sugar cane is a species of (often hybrid) tall, perennial A perennial plant or simply perennial is a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, ...

sugar beet A sugar beet is a plant whose root contains a high concentration of sucrose Sucrose is a type of sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Table sugar, granulated ...

s in the

sugar industry The sugar industry subsumes the production, processing and marketing of sugars Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, incl ...

, (see

) * To process water for alcoholic beverages and soft drinks * Pickle cucumbers and other foods * To make Chinese

century egg Century eggs (), also known as preserved eggs, hundred-year eggs, thousand-year eggs, thousand-year-old eggs, millennium eggs, skin eggs, or black eggs, are a Chinese Chinese can refer to: * Something related to China China, officiall ...

s * In maize preparation: removes the cellulose hull of maize kernels (see

nixtamalization Nixtamalization () is a process for the preparation of maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American North America is a continent in the Northern Hemispher ...

) * To clear a

brine Brine is a high-concentration solution of salt (NaCl) in water (H2O). In different contexts, ''brine'' may refer to salt solutions ranging from about 3.5% (a typical concentration of seawater, on the lower end of solutions used for brining food ...

brine

carbonate In chemistry, a carbonate is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline mi ...

s of calcium and

magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

in the manufacture of salt for food and pharmaceutical uses * In fortifying (Ca supplement) fruit drinks, such as orange juice, and

infant formula Infant formula, baby formula or just formula (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United ...

* As a digestive aid (called Choona, used in India in ''

paan Paan (from sa, पर्ण, parṇá, lit. ''leaf'', cognate with English ''fern'') is a preparation combining betel The betel (''Piper betle'') is a vine of the family Piperaceae, which includes Black pepper, pepper and kava. Betel lea ...

paan

'', a mixture of

areca nuts

, calcium hydroxide and a variety of seeds wrapped in

betel The betel (''Piper betle'') is a vine of the family Piperaceae, which includes Black pepper, pepper and kava. Betel leaf is mostly consumed in Asia, and elsewhere in the world by some Asian emigrants, as betel ''quid'' or in paan, with areca nut ...

betel

leaves) * As a substitute for

baking soda Sodium bicarbonate (IUPAC name: sodium hydrogencarbonate), commonly known as baking soda or bicarbonate of soda, is a chemical compound with the formula NaHCO3. It is a salt (chemistry), salt composed of a sodium cation (Sodium, Na+) and a bica ...

in making ''

papadam

'' * In the removal of carbon dioxide from controlled atmosphere produce storage rooms * In the preparation of mushroom growing substrates

Native American uses

Nixtamalized Corn maize El Salvador recipe

In Spanish, calcium hydroxide is called . Maize cooked with (in a process of

) becomes

hominy Hominy (Spanish: maíz molido; literally meaning "milled corn") is a food produced from dried maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American North America is ...

(), which significantly increases the bioavailability of

niacin Niacin, also known as nicotinic acid, is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, ...

niacin

(vitamin B3), and is also considered tastier and easier to digest. In chewing

coca leaves

, calcium hydroxide is usually chewed alongside to keep the

alkaloid Alkaloids are a class of basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming language In computer science Computer science deals with the theoretical foundations of ...

stimulant Stimulants (also often referred to as psychostimulants or colloquially as uppers) is an overarching term that covers many drug Uncoated tablets, consisting of about 90% acetylsalicylic acid, along with a minor amount of inert fillers and b ...

s chemically available for

absorption Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the body through the skin *Absorption (pharmacolo ...

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 ''Anadenanthera'' is a genus of South America South America is a entirely in the and mostly in the , with a relatively small portion in the . It can also be described as the southern of a single continent called (see the ). The ref ...

'' species.

Asian uses

Calcium hydroxide is typically added to a bundle of

areca nut The areca nut ( or ) is the seed of the areca ''Areca'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Vir ...

and

leaf called "

" to keep the

s chemically available to enter the bloodstream via

sublingual Sublingual (abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full version of the word or phrase; for e ...

absorption. It is used in making ''

naswar Naswār ( ps, نسوار, : насва́р), also called nās (ناس; на́с) or nasvay (نسوای; насвай), is a moist, powdered tobacco consumed mostly in , , , , , , , , and . Naswar is stuffed in the floor of the mouth under the low ...

'' (also known as ''nass'' or ''niswar''), a type of dipping tobacco made from fresh tobacco leaves, calcium hydroxide (''chuna'' or ''soon''), and wood ash. It is consumed most in the

Pathan Pashtuns (, or ; ps, پښتانه, ; Pakhtuns or Pathans), historically known as Afghans Afghan (Pashto Pashto (,; / , ), sometimes spelled Pukhto or Pakhto, is an Eastern Iranian language The Eastern Iranian languages are a s ...

Pathan

diaspora,

Afghanistan Afghanistan (; Pashto Pashto (,; / , ), sometimes spelled Pukhto or Pakhto, is an Eastern Iranian language The Eastern Iranian languages are a subgroup of the Iranian languages The Iranian or Iranic languages are a branch of t ...

Pakistan Pakistan, . Pronounced variably in English as , , , and . officially the Islamic Republic of Pakistan, is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a popul ...

India India, officially the Republic of India (Hindi Hindi (Devanagari: , हिंदी, ISO 15919, ISO: ), or more precisely Modern Standard Hindi (Devanagari: , ISO 15919, ISO: ), is an Indo-Aryan language spoken chiefly in Hindi Belt, ...

India

and

Bangladesh Bangladesh (, bn, বাংলাদেশ, ), officially the People's Republic of Bangladesh, is a country in South Asia South Asia is the southern region of Asia, which is defined in both geography, geographical and culture, ethno-c ...

. Villagers also use calcium hydroxide to paint their mud houses in Afghanistan, Pakistan and India.

Health risks

Unprotected exposure to Ca(OH)₂ can cause severe skin irritation, chemical burns, blindness, lung damage or rashes.

References

External links

* *
CDC – NIOSH Pocket Guide to Chemical Hazards – Calcium Hydroxide
*
MSDS Data Sheet
{{DEFAULTSORT:Calcium Hydroxide

Building materials Building materials Building material is material used for construction Construction is a general term meaning the art and science to form Physical object, objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford ...

Calcium compounds Dental materials Hydroxides Inorganic compounds Intoxication E-number additives