Solubility of sulfides

The alkali metals and alkaline-earth metals are the only sulfides that have any appreciable water solubility and that appear to be primarily ionic. In contrast, the sulfides of the copper and zinc families are some of the least-soluble compounds known. When water-soluble metal sulfides are heated in aqueous solution with elemental sulfur, solutions of so-called polysulfides are formed. These solutions consist primarily of S₄²⁻ and S₃²⁻ anions. Sulfides are an important component of high-density power sources such as lithium and sodium sulfide batteries. The sulfides that have been utilized in these power sources are M₂S, M₂S₂, M₂S₄, and M₂S₅.