Calcium sulfide is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with the formula Ca S. This white material crystallizes in cubes like rock salt. CaS has been studied as a component in a process that would recycle

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

, a product of flue-gas desulfurization. Like many salts containing

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

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 Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

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 In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...

of six Ca²⁺ ions, and complementarily, each Ca²⁺ ion surrounded by six S²⁻ ions.

Production

CaS is produced by carbothermic reduction of

calcium sulfate Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula . It occurs in several hydrated forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in evaporite deposits. Its dihydrate ...

, which entails the conversion of carbon, usually as charcoal, to

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

: :CaSO₄ + 2 C → CaS + 2 CO₂ and can react further: :3 CaSO₄ + CaS → 4

CaO Cao or CAO may refer to: Mythology *Cao (bull), a legendary bull in Meitei mythology Companies or organizations * Air China Cargo, ICAO airline designator CAO * CA Oradea, Romanian football club * CA Osasuna, Spanish football club * Canadian ...

+ 4 SO₂ In the second reaction the

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

(+6

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

) oxidizes the

(−2 oxidation state) to

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

(+4 oxidation state), while it is being reduced to sulfur dioxide itself (+4 oxidation state). CaS is also a byproduct in the

Leblanc process The Leblanc process was an early industrial process for making ''soda ash'' ( sodium carbonate) used throughout the 19th century, named after its inventor, Nicolas Leblanc. It involved two stages: making sodium sulfate from sodium chloride, fol ...

, a once major industrial process for producing sodium carbonate. In that process sodium sulfide reacts with

calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

: :Na₂S + CaCO₃ → CaS + Na₂CO₃ Millions of tons of this calcium sulfide byproduct was discarded, causing extensive pollution and controversy. Milk of lime, Ca(OH)₂, reacts with elemental sulfur to give a " lime-sulfur", which has been used as an

insecticide Insecticides are pesticides used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. The major use of insecticides is in agriculture, but they are also used in home and garden settings, i ...

. The active ingredient is probably a calcium

polysulfide Polysulfides are a class of chemical compounds derived from anionic chains of sulfur atoms. There are two main classes of polysulfides: inorganic and organic. The inorganic polysulfides have the general formula . These anions are the conjugate bas ...

, not CaS.

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

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

s such as

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

to release toxic

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

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. As a noxious byproduct of the

, it can be converted to calcium carbonate and

, the latter of which can be used as a sulfur source for the lead chamber process to produce the

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

necessary for the Leblanc process: :

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.

References

{{Meteorites Monosulfides Calcium compounds Corrosive substances Meteorite minerals Phosphors and scintillators Rock salt crystal structure

Production

Reactivity and uses

Natural occurrence

See also

References