The starting-point for these compounds is the hypohalite ; it is produced by the action of a hydroxide on the element in cold aqueous solution, thus : 2NaOH.Aq + Cl₂, Br₂, or I₂ = NaCl, NaBr, or Nal.Aq + H₂O + NaOCl, NaOBr, or NaOI.Aq.

Chlorine monoxide, C1₂O, is formed on passing over dry mercuric oxide, prepared by precipitation from mercuric chloride with caustic soda, dry chlorine gas; the tube containing the oxide must be cooled with ice, for the monoxide is a dark brown, very unstable liquid, boiling at 6°. The equation is: 2HgO + 2Cl₂ = Hg₂Cl₂O + CI-O-CI. Its density at io° corresponds with the formula given. If the mercuric oxide be made into an emulsion with water, and chlorine be passed through, the acid is produced in aqueous solution ; it is a pale yellow liquid, with a not unpleasant smell, recalling that of chlorine. If concentrated, it decomposes into water, chlorine, and oxygen. It reacts at once with hydrochloric acid, forming water and chlorine: H-O-Cl.Aq + H-Cl.Aq = CL + H₂O.Aq.

The most important hypochlorite is a double compound, obtained by the action of chlorine on slaked lime, termed "chloride of lime" or "bleaching-powder." It is a white, non-crystalline powder, smelling of hypochlorous acid. Its formula is CI-Ca-O-CI. That it is a compound, and not a mixture of calcium chloride and hypochlorite, is proved by the fact that bleaching-powder is not deliquescent, whereas calcium chloride is a very deliquescent salt; calcium chloride and hypochlorite are both very soluble salts, but bleaching-powder is only sparingly soluble, but if a saturated solution of bleaching-powder be cooled, crystals of hypochlorite separate out, thus proving that it is dissociated in aqueous solution into these two salts. Its smell, as well as that of other hypochlorites, is due to the fact that hypo-chlorous acid is a very feeble acid, and is only slightly ionised; hence the calcium and other salts are hydrolysed by the ions of water, and the solution contains free base and free acid; and the latter reveals its presence by its smell. No ion has a smell ; hence one does not smell solutions of salts, but only volatile non-ionised compounds.

When bleaching-powder is distilled with just enough acid to liberate the hypochlorous acid, that acid comes over ; but if excess of such an acid as sulphuric or hydrochloric be added, chlorine is liberated, owing to the reaction between hydrochloric and hypochlorous acids. The addition of a trace of a salt of cobalt to bleaching-powder results in the liberation of oxygen when it is warmed ; this reaction, which is termed "catalytic," is supposed to be due to the alternate formation and decomposition of an oxide of cobalt of the formula Co₄O₇ ; but the reaction is still obscure.