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Calcium sulfide

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Title: Calcium sulfide  
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Subject: Calcium oxide, Calcium bromate, Uranium disulfide, Galoter process, Calcium bisulfite
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Calcium sulfide

Calcium sulfide
Calcium sulfide
CAS number  YesY
ChemSpider  YesY
EC number
Jmol-3D images Image 1
Image 2
Molecular formula CaS
Molar mass 72.143 g/mol
Appearance white crystals
Density 2.59 g/cm3
Melting point 2,525 °C (4,577 °F; 2,798 K)
Solubility in water slightly soluble
Solubility insoluble in alcohol
reacts with acid
Refractive index (nD) 2.137
Crystal structure Halite (cubic), cF8
Space group Fm3m, No. 225
Octahedral (Ca2+); octahedral (S2–)
EU Index 016-004-00-0
EU classification Irritant (Xi)
Dangerous for the environment (N)
R-phrases R50
S-phrases (S2), S28, S61
Main hazards H2S source
NFPA 704
Related compounds
Other anions Calcium oxide
Other cations Magnesium sulfide
Strontium sulfide
Barium sulfide
Related sulfides Sodium sulfide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N   YesY/N?)

Calcium sulfide is the chemical compound with the formula CaS. This white material crystallizes in cubes like rock salt. CaS has been studied as a component in a process that would recycle gypsum, a product of flue-gas desulfurization. Like many salts containing sulfide ions, CaS typically has an odour of H2S, which results from small amount of this gas formed by hydrolysis of the salt.

In terms of its atomic structure, CaS crystallizes in the same motif as sodium chloride indicating that the bonding in this material is highly ionic. The high melting point is also consistent with its description as an ionic solid. In the crystal, each S2− ion is surrounded by an octahedron of six Ca2+ ions, and complementarily, each Ca2+ ion surrounded by six S2− ions.


CaS is produced by "carbothermic reduction" of calcium sulfate, which entails the conversion of carbon, usually as charcoal, to carbon dioxide:

CaSO4 + 2 C → CaS + 2 CO2

and can react further:

3 CaSO4 + CaS → 4 CaO + 4 SO2

In the second reaction the sulfate (+6 oxidation state) oxidizes the sulfide (-2 oxidation state) to sulfur dioxide (+4 oxidation state), while it is being reduced to sulfur dioxide itself (+4 oxidation state).

CaS is also a byproduct in the Leblanc process.

Reactivity and uses

Calcium sulfide decomposes upon contact with water, including moist air, giving a mixture of Ca(SH)2, Ca(OH)2, and Ca(SH)(OH).

CaS + H2O → Ca(SH)(OH)
Ca(SH)(OH) + H2O → Ca(OH)2 + H2S

Milk of lime, Ca(OH)2, reacts with elemental sulfur to give a "lime-sulfur", which has been used as an insecticide. The active ingredient is probably a calcium polysulfide, not CaS.[1]

It reacts with acids such as hydrochloric acid to release toxic hydrogen sulfide gas.

CaS + 2 HCl → CaCl2 + H2S

Natural occurrence

Oldhamite is the name for mineralogical form of CaS. It is a rare component of some meteorites and has scientific importance in solar nebula research. Burning of coal dumps can also produce the compound.

See also


  1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
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