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a derivative of hydrogen polysulfide, H2Sx where x = 2–9. Polysulfides are solids, ranging in color from light yellow to brownish red, with a specific, unpleasant odor.
Polysulfides of ammonia, alkali metals, and alkaline-earth metals are known—for example, potassium forms K2S2, K2S3, K2S4, K2S5, and K2S6. Sulfur atoms are arranged in an —S—S—S— chain in polysulfide molecules. The melting point of polysulfides is lower than that of sulfides. For example, the melting point of K2S is 835°C, whereas that of K2S6 is 183°C. Polysulfides are chemically very unstable; they decompose upon heating to form sulfides. In the presence of acids they liberate H2Sx which decomposes immediately into H2S and S.
Polysulfides are produced by fusing sulfides, hydroxides, or carbonates with sulfur; sulfur may be added to an aqueous sulfide solution. Polysulfides of calcium and barium are used against agricultural pests. Barium polysulfide is an effective depilatory.