Milk of lime
|Jmol 3D model||Interactive image
|Molar mass||74.093 g/mol|
|Density||2.211 g/cm3, solid|
|Melting point||580 °C (1,076 °F; 853 K) (loses water, decomposes)|
|0.189 g/100 mL (0 °C)
0.173 g/100 mL (20 °C)
0.066 g/100 mL (100 °C)
Solubility product (Ksp)
|Solubility||Soluble in glycerol and acids.
Insoluble in alcohol.
Refractive index (nD)
Std enthalpy of
|Safety data sheet||See: data page
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|7340 mg/kg (oral, rat)
7300 mg/kg (mouse)
|US health exposure limits (NIOSH):|
|TWA 15 mg/m3 (total) 5 mg/m3 (resp)|
|TWA 5 mg/m3|
IDLH (Immediate danger)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
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, caustic lime, builders' lime, slack lime, 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.
Calcium hydroxide is relatively soluble in water, with a solubility product Ksp of 5.5 × 10−6. It is large enough that it will partially dissolve and release hydroxyl anions (OH-) in solution according to the following reaction:
- Ca(OH)2 → Ca2+ + 2 OH-
At ambient temperature, calcium hydroxide (portlandite) can dissolve in pure water to produce an alkaline solution with a pH of about 12.4. Calcium hydroxide solutions can therefore cause severe chemical burns. The solubility of calcium hydroxide also strongly depends on pH value. At high pH value, in the presence of alkali hydroxides (NaOH, KOH), such as in fresh cement water, calcium hydroxide solubility drastically drops.
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 can attack some metals such as aluminium (amphoteric hydroxide dissolving at high pH) while protecting other metals from corrosion such as iron and steel by passivation of their surface. 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
When heated to 512 °C, the partial pressure of water in equilibrium with calcium hydroxide reaches 101 kPa (normal atmospheric pressure), which decomposes calcium hydroxide into calcium oxide and water.
- Ca(OH)2 → CaO + H2O
Structure, preparation, occurrence
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.
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 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. CaOH has been detected in the atmosphere of S-type stars.
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 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, using the following reaction:
Ca(OH)2 + 2NH4Cl → 2NH3 + CaCl2 + 2H2O
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 cauasticizing step in the Kraft process for making pulp. In the causticizing operation burned lime is added to green liquor which is a solution primarily of sodium carbonate and sodium sulfate produced by dissolving smelt, which is the molten form of these chemicals from the recovery furnace.
- clarify raw juice from sugarcane or sugar beets in the sugar industry, (see carbonatation)
- process water for alcoholic beverages and soft drinks
- pickle cucumbers and other foods
- make Chinese century eggs
- make corn tortillas (it helps the corn flour (masa) bind together) (see nixtamalization)
- clear a brine of carbonates of calcium and magnesium in the manufacture of salt for food and pharmaceutical uses
- fortify (Ca supplement) fruit drinks, such as orange juice, and infant formula
- aid digestion (called Choona, used in India in paan, a mixture of areca nuts, calcium hydroxide and a variety of seeds wrapped in betel leaves)
- substitute for baking soda in making papadam.
Native American uses
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 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 mollusk 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.
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, Bangladesh and also in Sweden and Norway. Villagers also use calcium hydroxide to paint their mud houses in Afghanistan, Pakistan and India.
- Baralyme (carbon dioxide absorbent)
- Lime mortar
- Lime plaster
- Magnesium hydroxide (less alkaline due to a lower solubility product)
- Soda lime (carbon dioxide absorbent)
- Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A21. ISBN 0-618-94690-X.
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- "NIOSH Pocket Guide to Chemical Hazards #0092". National Institute for Occupational Safety and Health (NIOSH).
- Halstead, P.E.; Moore, A.E. (1957). "The Thermal Dissociation Of Calcium Hydroxide". Journal of the Chemical Society. 769: 3873. doi:10.1039/JR9570003873.
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- CDC – NIOSH Pocket Guide to Chemical Hazards – Calcium Hydroxide
- MSDS Data Sheet