Calcium hydroxide: Wikis


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Calcium hydroxide
CAS number 1305-62-0 Yes check.svgY
PubChem 14777
ChemSpider 14094
RTECS number EW2800000
Molecular formula Ca(OH)2
Molar mass 74.093 g/mol
Appearance soft white powder/colourless liquid
Odor odorless
Density 2.211 g/cm3, solid
Melting point

512 °C (decomp.)

Solubility in water 0.189 g/100 mL (0 °C)
0.173 g/100 mL (20 °C)
Solubility product, Ksp 4.68 × 10−6
Basicity (pKb) 2.37
MSDS External MSDS
EU Index Corrosive (C)
R-phrases R22, R34
S-phrases (S2), S24
NFPA 704
NFPA 704.svg
Flash point Non-flammable
LD50 7340 mg/kg (oral, rat)
Related compounds
Other cations Magnesium hydroxide
Strontium hydroxide
Barium hydroxide
Related bases Calcium oxide
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 Yes check.svgY (what is this?)  (verify)
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, hydrated lime, slack lime, or pickling lime, is a chemical compound with the chemical formula Ca(OH)2. It is a colourless crystal or white powder, and is obtained when calcium oxide (called lime or quicklime) is mixed, or "slaked" with water. It can also be precipitated by mixing an aqueous solution of calcium chloride and an aqueous solution of sodium hydroxide. The name of the natural, mineral form is portlandite. It is a relatively rare mineral, known from some volcanic, plutonic, and metamorphic rocks. It has also been known to arise in burning coal dumps.

When heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101 kPa and decomposes into calcium oxide and water.[1]

A suspension of fine calcium hydroxide particles in water is called milk of lime. The solution is called lime water and is a medium strength base that reacts violently with acids and attacks many metals in presence of water. It turns milky if carbon dioxide is passed through, due to precipitation of calcium carbonate.



Calcium hydroxide is produced commercially by treating lime with water:

CaO + H2O → Ca(OH)2

In the laboratory it may be prepared by treating an aqueous solution of any calcium salt with an alkali.


Because of its weak basic properties, calcium hydroxide has many and varied uses:

  • A flocculant, in water and sewage treatment and improvement of acid soils
  • An ingredient in whitewash, mortar, and plaster
  • An alkali used as a lye substitute in no-lye hair relaxers
  • A chemical depilatory agent found in Nair
  • A calcium supplement in mineral fortified baby formulas
  • A chemical reagent
    • In Bordeaux mixture to neutralise the solution and form a long lasting fungicide
    • In the reef aquarium hobby for adding bio-available calcium in solution for calcium-using animals such as algae, snails, hard tube worms, and corals (often referred to as Kalkwasser mix), and also to increase the alkalinity of the water
    • In the tanning industry for neutralization of acid, the liming of hides and skins and the flocculation of wastewater
    • In the petroleum refining industry for the manufacture of additives to oils (salicatic, sulphatic, fenatic)
    • In the chemical industry for manufacture of calcium stearate
    • In the food industry for processing water (for alcoholic and soft drinks)
    • Used in the separation of sugar from sugar cane in the sugar industry
    • Used in the processing of Norwegian lutefisk. Dried cod fish is soaked in a mixture of slaked lime and soda to produce a soft-fleshed fish fillet that is steamed or baked and served with potato lefse
    • For clearing a brine of carbonates of calcium and magnesium in the manufacture of salt for food and pharmacopoeia
    • In Native American and Mesoamerican cooking, calcium hydroxide is called "cal". Corn cooked with cal becomes nixtamal which significantly increases its nutrition value, and is also considered tastier and easier to digest.
    • In chewing betel 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.
    • In Chinese cuisine, for making century eggs
  • A filler
  • In dentistry, it is used as dressing in paste form used for anti-microbial effect during a dental root canal procedure. Calcium hydroxide is known to have a strong anti-microbial effect and is a bone-regeneration stimulant.[2]
  • It has been proposed to add it to sea water in great quantities to reduce atmospheric CO2 and fight the greenhouse effect[1]. However, since the production of calcium oxide (the main starting material of calcium hydroxide) involves calcination of limestone, which involves great deal of heat, it would be not very productive if fossil fuel is used in the production process. The calcination of limestone in itself releases the same amount of carbon dioxide as calcium hydroxide will absorb, so this method is at best carbon neutral.
  • In the production of metals, lime is injected into the waste gas stream to neutralise acids such as fluorides and chlorides prior to being released to atmosphere.
  • Used in relaxers for permanently straightening very curly hair.

Health risks

As with many chemicals, exposure may pose health risks:[3]

  • Inhalation: Respiratory tract irritation. Coughing, shortness of breath, chemical bronchitis.
  • Ingestion: Internal bleeding, possible perforation of esophagus, severe pain, vomiting, diarrhea, and collapse.
  • Eyes: severe irritation, pain, ulceration, blindness.
  • Skin: burns, blistering.
  • Chronic Exposure: dermatitis or severe irritation to skin.


  1. ^ Halstead, P.E.; Moore, A.E. (1957). "The Thermal Dissociation Of Calcium Hydroxide". Journal of the Chemical Society 769: 3873. doi:10.1039/JR9570003873. 
  2. ^ Gomes, Brenda; et al. (2002). "In Vitro Antimicrobial Activity of Calcium Hydroxide Pastes and Their Vehicles Against Selected Microorganisms". Brazilian Dental Journal 13 (3). doi:10.1590/S0103-64402002000300002. Retrieved 2008-02-05. 
  3. ^ "MSDS Calcium hydroxide". Retrieved 2008-12-26. 

External links


Simple English

Calcium hydroxide

Calcium hydroxide, also known as slaked lime, is a chemical compound with the formula Ca(OH)2. It dissolves slightly in water. It is a base. It can be made by the chloralkali process. It can be dehydrated (remove the water) to calcium oxide by heat. It can be used to neutralize acidic soils.

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