Cuprous sulphide, Cu₂S

The Cuprous sulphide, Cu₂S, occurs in nature as the mineral chalcocite or copper glance. It is synthesized by heating copper with sulphur, and fusing the product to decompose the cupric sulphide simultaneously produced. It can also be prepared from cupric sulphide by heating with hydrogen at 265° C., or with carbon dioxide at 130° to 150° C.; and from copper by the action of sulphuric acid at 124° C., or of a solution of ammonium sulphide in absence of air. It is also formed by heating cupric sulphide with sodium arsenite, sodium mono- thioarsenate being produced simultaneously:

2CuS + Na₃AsO₃ = Cu₂S + Na₃AsSO₃.

The mineral crystallizes in the rhombic system, its density being 5.7 to 5.8. The artificial substance forms dark-grey octahedra belonging to the cubic system, its density being given as 5.52 to 5.58, and 5.785 (20° C.). An evolution of 0.9 Cal. per gram-molecule attends the transformation of the rhombic into the cubic form, the transition-point being 103° C. Its melting-point is given as 1057° C. (in sulphur-vapour), 1091° C., 1096° C. (in an atmosphere of hydrogen sulphide), 1114° C., and 1135° C., and its mean specific heat as 0.1212 between 9° and 97° C. It is very slightly soluble in water, but is soluble in concentrated nitric acid and sulphuric acid with decomposition. The heat of formation evolved in the direct combination of the elements is 19.0 Cal.

When suspended in a solution containing both ammonia and ammonium (or other) salts, air at atmospheric pressure oxidizes cuprous sulphide to cupric sulphate and thiosulphate, the reaction being slower than with cupric sulphide. In suspension in neutral or acidic solutions, cupric sulphate is produced, the reaction being less energetic than in presence of ammonia, and up to 160° C. requiring compressed air.

With sodium monosulphide cuprous sulphide forms a double salt of the formula Na₂S,Cu₂S. This compound melts at 700° C.