Calcium sulfide

Calcium sulfide

Names
IUPAC name Calcium sulfide
Other names Calcium monosulfide, Hepar calcies, Sulfurated lime Oldhamite
Identifiers
CAS Number	20548-54-3 Y
3D model (JSmol)	Interactive image Interactive image
ChEBI	CHEBI:81055
ChemSpider	8373113 Y
ECHA InfoCard	100.039.869
EC Number	243-873-5
KEGG	C17392 Y
PubChem CID	10197613
UNII	1MBW07J51Q Y
CompTox Dashboard (EPA)	DTXSID30885140
InChI InChI=1S/Ca.S/q+2;-2 Y Key: AGVJBLHVMNHENQ-UHFFFAOYSA-N Y InChI=1/Ca.S/rCaS/c1-2 Key: JGIATAMCQXIDNZ-WSLZQIQFAE InChI=1/Ca.S/q+2;-2 Key: AGVJBLHVMNHENQ-UHFFFAOYAE
SMILES [Ca]=S [Ca+2].[S-2]
Properties
Chemical formula	CaS
Molar mass	72.143 g/mol
Appearance	white crystals hygroscopic
Density	2.59 g/cm3
Melting point	2,525 °C (4,577 °F; 2,798 K)
Solubility in water	Hydrolyses
Solubility	Insoluble in alcohol reacts with acid
Refractive index (nD)	2.137
Structure
Crystal structure	Halite (cubic), cF8
Space group	Fm3m, No. 225
Coordination geometry	Octahedral (Ca2+); octahedral (S2−)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Reacts with water to release H2S
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315, H319, H335, H400
Precautionary statements	P261, P273, P305+P351+P338
NFPA 704 (fire diamond)	2 0 3
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 YN ?) 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.

Production

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, a once major industrial process for producing sodium carbonate. In that process sodium sulfide reacts with calcium carbonate:^[1]

Na₂S + CaCO₃ → CaS + Na₂CO₃

Millions of tons of this calcium sulfide byproduct was discarded, causing extensive pollution and controversy.^[2]

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.^[3]

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

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

CaS + 2 HCl → CaCl₂ + H₂S

Calcium sulfide is phosphorescent, and will glow a blood red for up to an hour after a light source is removed.^[4]

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.^[5][6] Burning of coal dumps can also produce the compound.^[7]

See also

• Glossary of meteoritics

References