Calcium sulfide

Properties and structure

Calcium sulfide is an ionic solid with strong electrostatic attraction between cations and anions. It is commonly described by the empirical formula CaS. The crystal geometry leads to high lattice energy, which is reflected in a high melting point (reported around 2,525 °C) and a generally high thermal stability under dry conditions; see thermal and melting data here and related thermodynamic summaries here. In contact with water or acids CaS undergoes hydrolysis, producing hydrogen sulfide (H2S) and soluble calcium species.

Preparation and occurrence

CaS can be produced industrially by high‑temperature reactions such as the direct combination of elemental calcium and sulfur or by reduction of calcium sulfate (CaSO4) with carbon or other reducing agents. It is not a common stable mineral in typical oxidizing Earth surface conditions because sulfide is readily oxidized; in practice CaS is encountered in synthetic materials, metallurgical slags, and some specialized chemical products.

Uses, reactions and safety

Applications include use as a laboratory source of sulfide ions, and historically CaS doped with trace activators has been used in phosphorescent pigments and glow‑in‑the‑dark materials. Chemically, it reacts with acids to release toxic H2S gas and hydrolyzes in water with a characteristic foul odor, so storage and handling require precautions to avoid exposure. Its role in metallurgy is usually as an impurity or transient phase rather than a primary product.

Notable distinctions

CaS should not be confused with calcium sulfate (CaSO4), which contains sulfate (SO4 2−) rather than sulfide and has quite different solubility and environmental behavior. When discussing sulfide chemistry, note the tendency of S2− to form hydrogen sulfide in acidic or aqueous conditions and the environmental and safety implications of that conversion.

• Key identifiers and composition: CaS
• Structure overview: rock‑salt type
• Element references: calcium, sulfide
• Thermal data: melting point, thermodynamics