Calcium sulfide

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 S^2âˆ' ion is surrounded by an octahedron of six Ca²⁺ ions, and complementarily, each Ca²⁺ ion surrounded by six S^2âˆ' 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 â†' CaS + 2 CO₂

and can react further:

3 CaSO₄ + CaS â†' 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 â†' Ca(SH)(OH)

Ca(SH)(OH) + H₂O â†' 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.

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

CaS + 2 HCl â†' 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

• Glossary of meteoritics

References

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