Calcium hydroxide
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| Names | |
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IUPAC name
Calcium hydroxide
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| Other names
Slaked lime
Milk of lime Caustic lime Calcium(II) hydroxide Pickling lime Hydrated lime Portlandite Calcium hydrate |
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| Identifiers | |
1305-62-0 
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| 3D model (Jmol) |
Interactive image Interactive image |
| ChEBI |
CHEBI:31341
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| ChemSpider |
14094
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| ECHA InfoCard | 100.013.762 |
| EC Number | 215-137-3 |
| E number | E526 (acidity regulators, ...) |
| 846915 | |
| KEGG |
D01083
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| PubChem | 14777 |
| RTECS number | EW2800000 |
| UNII |
PF5DZW74VN
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| Properties | |
| Ca(OH)2 | |
| Molar mass | 74.093 g/mol |
| Appearance | white powder |
| Odor | odorless |
| Density | 2.211 g/cm3, solid |
| Melting point | 580 °C (1,076 °F; 853 K) (loses water, decomposes) |
| 1.89 g/L (0 °C) 1.73 g/L (20 °C) 0.66 g/L (100 °C) |
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Solubility product (Ksp)
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5.5×10−6 |
| Solubility | Soluble in glycerol and acids. Insoluble in alcohol. |
| Basicity (pKb) | 1.37 (first OH–), 2.43 (second OH–) |
| -22.0·10−6 cm3/mol | |
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Refractive index (nD)
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1.574 |
| Structure | |
| Hexagonal, hP3 | |
| P3m1 No. 164 | |
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a = 0.35853 nm, c = 0.4895 nm
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| Thermochemistry | |
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Std molar
entropy (S |
83 J·mol−1·K−1 |
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Std enthalpy of
formation (ΔfH |
−987 kJ·mol−1 |
| Hazards | |
| Safety data sheet |
See: data page |
| R-phrases | R22, R34 |
| S-phrases | (S2), S24 |
| NFPA 704 | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
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LD50 (median dose)
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7340 mg/kg (oral, rat) 7300 mg/kg (mouse) |
| US health exposure limits (NIOSH): | |
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PEL (Permissible)
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TWA 15 mg/m3 (total) 5 mg/m3 (resp) |
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REL (Recommended)
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TWA 5 mg/m3 |
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IDLH (Immediate danger)
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N.D. |
| Related compounds | |
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Other cations
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Magnesium hydroxide Strontium hydroxide Barium hydroxide |
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Related bases
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Calcium oxide |
| Supplementary data page | |
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Refractive index (n), Dielectric constant (εr), etc. |
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Thermodynamic
data |
Phase behaviour solid–liquid–gas |
| UV, IR, NMR, MS | |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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 (what is  ?) |
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| 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 "" 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 insoluble in water, with a solubility product Ksp of 5.5 × 10−6. It is large enough that its solutions are basic according to the following reaction:
At ambient temperature, calcium hydroxide (portlandite) dissolves in pure water to produce an alkaline solution with a pH of about 12.4. Calcium hydroxide solutions can cause chemical burns. At high pH value (see common ion effect), its solubility drastically decreases. This behavior is relevant to cement pastes. Its aqueous solutions 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:
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