Calcium sulfate

Calcium sulphate

Names
Other names Plaster of Paris Drierite Gypsum
Identifiers
CAS Number	7778-18-9 Y 10034-76-1 (hemihydrate) N 10101-41-4 (dihydrate) N
3D model (Jmol)	Interactive image
ChEBI	CHEBI:31346 Y
ChemSpider	22905 Y
ECHA InfoCard	100.029.000
E number	E516 (acidity regulators, ...)
KEGG	D09201 N
PubChem	24928
RTECS number	WS6920000
UNII	E934B3V59H Y
InChI InChI=1S/Ca.H2O4S/c;1-5(2,3)4/h;(H2,1,2,3,4)/q+2;/p-2 Y Key: OSGAYBCDTDRGGQ-UHFFFAOYSA-L Y InChI=1/Ca.H2O4S/c;1-5(2,3)4/h;(H2,1,2,3,4)/q+2;/p-2 Key: OSGAYBCDTDRGGQ-NUQVWONBAU
SMILES [Ca+2].[O-]S([O-])(=O)=O
Properties
Chemical formula	CaSO4
Molar mass	136.14 g/mol (anhydrous) 145.15 g/mol (hemihydrate) 172.172 g/mol (dihydrate)
Appearance	white solid
Odor	odorless
Density	2.96 g/cm3 (anhydrous) 2.32 g/cm3 (dihydrate)
Melting point	1,460 °C (2,660 °F; 1,730 K) (anhydrous)
Solubility in water	0.21g/100ml at 20 °C (anhydrous) 0.24 g/100ml at 20 °C (dihydrate)
Solubility product (Ksp)	4.93 × 10−5 mol2L−2 (anhydrous) 3.14 × 10−5 (dihydrate)
Solubility in glycerol	slightly soluble (dihydrate)
Acidity (pKa)	10.4 (anhydrous) 7.3 (dihydrate)
Magnetic susceptibility (χ)	-49.7·10−6 cm3/mol
Structure
Crystal structure	orthorhombic
Thermochemistry
Std molar entropy (So298)	107 J·mol−1·K−1
Std enthalpy of formation (ΔfHo298)	-1433 kJ/mol
Hazards
Safety data sheet	See: data page ICSC 1589
NFPA 704	0 1 0
Flash point	Non-flammable
US health exposure limits (NIOSH):
PEL (Permissible)	TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp) [for anhydrous form only]
REL (Recommended)	TWA 10 mg/m3 (total) TWA 5 mg/m3 (resp) [anhydrous only]
IDLH (Immediate danger)	N.D.
Related compounds
Other cations	Magnesium sulfate Strontium sulfate Barium sulfate
Related desiccants	Calcium chloride Magnesium sulfate
Related compounds	Plaster of Paris Gypsum
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).
N  (what is YN ?)
Infobox references

Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO₄ and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.

The compound exists in three levels of hydration:

The main use of calcium sulfate is to produce Plaster of Paris and stucco. These applications exploit the fact that calcium sulfate forms a moldable paste upon hydration and hardens as a hemihydrate. It is also convenient that calcium sulfate is very poorly soluble in water, so structures do not dissolve.

With judicious heating, gypsum converts to the partially dehydrated mineral called calcium sulfate hemihydrate, calcined gypsum, and plaster of Paris.This material has the formula CaSO₄·(nH₂O), where 0.5 ≤ n ≤ 0.8. Temperatures between 100 °C and 150 °C (302 °F) are required to drive off the water within its structure. The details of the temperature and time depend on ambient humidity. Temperatures as high as 170 °C are used in industrial calcination, but at these temperatures γ-anhydrite begins to form. The heat energy delivered to the gypsum at this time (the heat of hydration) tends to go into driving off water (as water vapor) rather than increasing the temperature of the mineral, which rises slowly until the water is gone, then increases more rapidly. The equation for the partial dehydration is:

The endothermic property of this reaction is relevant to the performance of drywall, conferring fire resistance to residential and other structures. In a fire, the structure behind a sheet of drywall will remain relatively cool as water is lost from the gypsum, thus preventing (or substantially retarding) damage to the framing (through combustion of wood members or loss of strength of steel at high temperatures) and consequent structural collapse. But at higher temperatures, calcium sulfate will release oxygen and act as an oxidizing agent. This property is used in aluminothermy. In contrast to most minerals, which when rehydrated simply form liquid or semi-liquid pastes, or remain powdery, calcined gypsum has an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice: