Calcium hydroxide

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Calcium hydroxide
Identifiers
CAS number1305-62-0 YesY
PubChem14777
ChemSpider14094 YesY
UNIIPF5DZW74VN YesY
EC number215-137-3
KEGGD01083 YesY
ChEBICHEBI:31341 YesY
RTECS numberEW2800000
Jmol-3D imagesImage 1
Image 2
Properties
Molecular formulaCa(OH)2
Molar mass74.093 g/mol
Appearancewhite powder
Odorodorless
Density2.211 g/cm3, solid
Melting point

580 °C (loses water)

Solubility in water0.189 g/100 mL (0 °C)
0.173 g/100 mL (20 °C)
0.066 g/100 mL (100 °C)
Solubility product, Ksp4.68×10−6
SolubilitySoluble in glycerol and acids.
Insoluble in alcohol.
Acidity (pKa)12.4
Basicity (pKb)2.37
Refractive index (nD)1.574
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−987 kJ·mol−1[1]
Standard molar
entropy
So298
83 J·mol−1·K−1[1]
Hazards
MSDSExternal MSDS
EU IndexCorrosive (C), Irritant (Xi)
R-phrasesR22, R34
S-phrases(S2), S24
NFPA 704
NFPA 704.svg
0
3
0
Flash pointNon-flammable
LD507340 mg/kg (oral, rat)
7300 mg/kg (mouse)
Related compounds
Other cationsMagnesium hydroxide
Strontium hydroxide
Barium hydroxide
Related basesCalcium oxide
Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid, liquid, gas
Spectral dataUV, IR, NMR, MS
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references
 
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Calcium hydroxide
Identifiers
CAS number1305-62-0 YesY
PubChem14777
ChemSpider14094 YesY
UNIIPF5DZW74VN YesY
EC number215-137-3
KEGGD01083 YesY
ChEBICHEBI:31341 YesY
RTECS numberEW2800000
Jmol-3D imagesImage 1
Image 2
Properties
Molecular formulaCa(OH)2
Molar mass74.093 g/mol
Appearancewhite powder
Odorodorless
Density2.211 g/cm3, solid
Melting point

580 °C (loses water)

Solubility in water0.189 g/100 mL (0 °C)
0.173 g/100 mL (20 °C)
0.066 g/100 mL (100 °C)
Solubility product, Ksp4.68×10−6
SolubilitySoluble in glycerol and acids.
Insoluble in alcohol.
Acidity (pKa)12.4
Basicity (pKb)2.37
Refractive index (nD)1.574
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−987 kJ·mol−1[1]
Standard molar
entropy
So298
83 J·mol−1·K−1[1]
Hazards
MSDSExternal MSDS
EU IndexCorrosive (C), Irritant (Xi)
R-phrasesR22, R34
S-phrases(S2), S24
NFPA 704
NFPA 704.svg
0
3
0
Flash pointNon-flammable
LD507340 mg/kg (oral, rat)
7300 mg/kg (mouse)
Related compounds
Other cationsMagnesium hydroxide
Strontium hydroxide
Barium hydroxide
Related basesCalcium oxide
Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid, liquid, gas
Spectral dataUV, IR, NMR, MS
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca(OH)2. It is a colorless crystal or white powder and is obtained when calcium oxide (called lime or quicklime) is mixed, or "slaked" with water. It has many names including hydrated lime, builders' lime, slack lime, Chuna (Word used in India, Pakistan and Afghanistan), cal, or pickling lime. Calcium hydroxide is used in many applications, including food preparation. Limewater is the common name for a saturated solution of calcium hydroxide.

Contents

Properties

When heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101 kPa, which decomposes calcium hydroxide into calcium oxide and water.[3]

Ca(OH)2 → CaO + H2O

A suspension of fine calcium hydroxide particles in water is called milk of lime. The solution is called limewater and is a medium strength base that reacts with acids and attacks many metals. Limewater turns milky in the presence of carbon dioxide due to formation of calcium carbonate, a process called carbonatation:

Ca(OH)2 + CO2 → CaCO3 + H2O

Structure, preparation, occurrence

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 the related hydroxides of the alkaline earth metals. The packing resembles the cadmium iodide motif with layers of octahedral Ca centres. Strong hydrogen bonds exist between the layers.[4]

The structure of calcium hydroxide, with the hydrogen atoms omitted (purple = O centres).

Calcium hydroxide is produced commercially by treating lime with water:

CaO + H2O → Ca(OH)2

In the laboratory it can be prepared by mixing an aqueous solutions of calcium chloride and sodium hydroxide. The mineral form, portlandite, is relatively rare but can be found in some volcanic, plutonic, and metamorphic rocks. It has also been known to arise in burning coal dumps.

Uses

One significant application of calcium hydroxide is as a 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 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 used in the production of sodium hydroxide. This conversion is a component of the Kraft process.[4]

Niche uses

Because it is produced on a large scale, is easily handled, and is cheap, myriad niche and even large-scale applications have been described. A partial listing follows:

For making a fungicide or a dip for treating mange, it is boiled with sulfur, then diluted. One recipe for the concentrate is 36 lb quicklime, 80 lb sulfur, and 50 gal water boiled for 1 hour—the authors suggesting 1/3 more lime if slaked lime is used.

Food industry

Because of its low toxicity and the mildness of its basic properties, it is widely used in the food industry to:

Native American uses

Dry untreated corn (left), and treated corn (right) after boiling in water with calcium hydroxide (1 Tbsp lime for 500 g of corn) for 15 minutes

In Spanish, calcium hydroxide is called cal. Corn cooked with cal (nixtamalization) becomes hominy (nixtamal), which significantly increases the bioavailability of niacin, and it is also considered tastier and easier to digest.

In chewing areca nut or coca leaves, calcium hydroxide is usually chewed alongside to keep the alkaloid stimulants chemically available for absorption by the body. Similarly, Native Americans traditionally chewed tobacco leaves with calcium hydroxide derived from burnt 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.[6]

Afghan uses

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), and wood ash. It is consumed most in the Pathan diaspora, Afghanistan, Pakistan, India and also in Sweden, Norway. Villagers also use calcium hydroxide as a paint on to their mud made houses all over Afghanistan and Pakistan.

Ancient Celtic use

According to Diodorus Siculus:

The Gauls are tall of body with rippling muscles and white of skin and their hair is blond, and not only naturally so for they also make it their practice by artificial means to increase the distinguishing colour which nature has given it. For they are always washing their hair in limewater and they pull it back from the forehead to the nape of the neck, with the result that their appearance is like that of Satyrs and Pans since the treatment of their hair makes it so heavy and coarse that it differs in no respect from the mane of horses. —Diodorus Siculus

Health risks

Unprotected exposure to Ca(OH)2 can pose health risks, so should be limited. It can cause severe skin irritation, chemical burns, blindness, or lung damage. See MSDS.[2]

See also

References

  1. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed.. Houghton Mifflin Company. p. A21. ISBN 0-618-94690-X.
  2. ^ a b "MSDS Calcium hydroxide". http://www.avantormaterials.com/documents/MSDS/usa/English/C0407_msds_us_Default.pdf. Retrieved 2011-06-21.
  3. ^ Halstead, P.E.; Moore, A.E. (1957). "The Thermal Dissociation Of Calcium Hydroxide". Journal of the Chemical Society 769: 3873. doi:10.1039/JR9570003873.
  4. ^ a b Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  5. ^ O,Driscoll, Catherine (21 July 2008). "A dash of lime -- a new twist that may cut CO2 levels back to pre-industrial levels". Chemistry. PhysOrg.com. http://www.physorg.com/news135820173.html. Retrieved 20 November 2010.
  6. ^ A multidisciplinary overview of intoxicating snuff rituals in the Western Hemisphere, Peter A. G. M. de Smet, doi:10.1016/0378-8741(85)90060-1

External links