A gas, H₂S, with an odour of rotten eggs; r.d. 1.54 (liquid); m.p. –85.5°C; b.p. –60.7°C. It is soluble in water and ethanol and may be prepared by the action of mineral acids on metal sulphides, typically hydrochloric acid on iron(II) sulphide (see Kipp’s apparatus). Solutions in water (known as hydrosulphuric acid) contain the anions HS⁻ and minute traces of S²⁻ and are weakly acidic. Acid salts (those containing the HS⁻ ion) are known as hydrogensulphides (formerly hydrosulphides). In acid solution hydrogen sulphide is a mild reducing agent. Hydrogen sulphide has an important role in traditional qualitative chemical analysis, where it precipitates metals with insoluble sulphides. Hydrogen sulphide is exceedingly poisonous (more toxic than hydrogen cyanide). See also Claus process.