Slaked lime

Calcium hydroxide

Names
IUPAC name Calcium hydroxide
Other names Slaked lime Milk of lime Caustic lime Calcium(II) hydroxide Pickling lime Hydrated lime Portlandite Calcium hydrate
Identifiers
CAS Number	1305-62-0 Y
3D model (Jmol)	Interactive image Interactive image
ChEBI	CHEBI:31341 Y
ChemSpider	14094 Y
ECHA InfoCard	100.013.762
EC Number	215-137-3
E number	E526 (acidity regulators, ...)
Gmelin Reference	846915
KEGG	D01083 Y
PubChem	14777
RTECS number	EW2800000
UNII	PF5DZW74VN Y
InChI InChI=1S/Ca.2H2O/h;2*1H2/q+2;;/p-2 Y Key: AXCZMVOFGPJBDE-UHFFFAOYSA-L Y InChI=1/Ca.2H2O/h;2*1H2/q+2;;/p-2 Key: AXCZMVOFGPJBDE-NUQVWONBAD
SMILES [Ca+2].[OH-].[OH-] [OH-].[OH-].[Ca+2]
Properties
Chemical formula	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)
Solubility in water	1.89 g/L (0 °C) 1.73 g/L (20 °C) 0.66 g/L (100 °C)
Solubility product (Ksp)	5.5×10−6
Solubility	Soluble in glycerol and acids. Insoluble in alcohol.
Basicity (pKb)	1.37 (first OH–), 2.43 (second OH–)
Magnetic susceptibility (χ)	-22.0·10−6 cm3/mol
Refractive index (nD)	1.574
Structure
Crystal structure	Hexagonal, hP3
Space group	P3m1 No. 164
Lattice constant	a = 0.35853 nm, c = 0.4895 nm
Thermochemistry
Std molar entropy (So298)	83 J·mol−1·K−1
Std enthalpy of formation (ΔfHo298)	−987 kJ·mol−1
Hazards
Safety data sheet	See: data page
R-phrases	R22, R34
S-phrases	(S2), S24
NFPA 704	0 3 0
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)	7340 mg/kg (oral, rat) 7300 mg/kg (mouse)
US health exposure limits (NIOSH):
PEL (Permissible)	TWA 15 mg/m3 (total) 5 mg/m3 (resp)
REL (Recommended)	TWA 5 mg/m3
IDLH (Immediate danger)	N.D.
Related compounds
Other cations	Magnesium hydroxide Strontium hydroxide Barium hydroxide
Related bases	Calcium oxide
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
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 K_sp 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: