Silver sulphide, Ag₂S

The Silver sulphide, Ag₂S, occurs in nature as argentite, and also in combination with many other sulphides. It is formed by direct synthesis from its elements at ordinary temperatures, and also by immersion of the metal in solutions of alkali-metal poly sulphides. It can be prepared in cubic crystals by passing sulphur-vapour over heated silver, or by the action of hydrogen trisulphide, H₂S₃, on silver oxide. It is the only definite compound produced by fusion of mixtures of silver and sulphur in different proportions. The black, amorphous form is precipitated by the action of hydrogen sulphide or a soluble sulphide on a solution of a silver salt. It is also produced by the action of hydrogen sulphide on metallic silver, a reversible reaction.

The melting-point of the sulphide is given as 812° C., 815° C., and 834° C. It is the most insoluble of the silver salts, its solubility corresponding with 1.2×10^-16 gram-atom of silver per litre. When heated with Silver sulphate at 300° C., both salts are reduced to metallic silver. In the fused state it is miscible with molten silver in all proportions. When heated in vacuum, it decomposes into its elements rapidly at 810° C.