Calcium sulfide

Calcium sulfide
IUPAC name
Calcium sulfide
Other names
Calcium monosulfide,
Hepar calcies,
Sulfurated lime
20548-54-3 YesY
3D model (Jmol) Interactive image
Interactive image
ChemSpider 8373113 YesY
ECHA InfoCard 100.039.869
EC Number 243-873-5
KEGG C17392 N
PubChem 10197613
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)
slightly soluble
Solubility insoluble in alcohol
reacts with acid
Halite (cubic), cF8
Fm3m, No. 225
Octahedral (Ca2+); octahedral (S2−)
Main hazards H2S source
Irritant (Xi)
Dangerous for the environment (N)
R-phrases R31, R36/37/38, R50
S-phrases (S2), S28, S61
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored diamond
Related compounds
Other anions
Calcium oxide
Other cations
Magnesium sulfide
Strontium sulfide
Barium sulfide
Related sulfides
Sodium sulfide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YesYN ?)
Infobox references

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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