The action of mass, that is, the quantity of a compound in unit volume, is well illustrated by the action of water on antimonious chloride. A solution of this salt in hydrochloric acid gives a precipitate on adding water: SbCl3.nHCl.Aq + H2O = OSbCl + (n + 2)HCl.Aq. Here the increase in the number of molecules of water causes the precipitation of the basic chloride; on adding more hydrochloric acid, however, so as to increase its active mass, the reaction is reversed, and the precipitate re-dissolves : OSbCl+(n + 2)HCl.Aq = SbCl3.Aq.nHCl + H2O. Above a certain concentration of water SbOCl is stable; above a certain concentration of hydrogen chloride, SbCl3.
The higher halides of molybdenum, tungsten, and uranium, themselves prepared by the action of halogen on the element, yield the lower halides on heating. They are volatile, coloured bodies, soluble in water; the higher ones are decomposed by water.
The elements chromium, manganese, iron, cobalt, and nickel, although not all belonging to the same series, show, nevertheless, a gradation of properties. The dihalides of all are known in the dry state; they are most readily obtained by heating the metal in a current of hydrogen halide, if required anhydrous; or if in solution or crystallised with water, by dissolving the oxide or carbonate in the halogen acid and evaporating until crystallisation ensues. As examples: Fe + 2HCI = FeCl2 + H2O ; MnCO3 + 2HBr. Aq = MnBr2. Aq + H2O + CO2.
The trihalides are best made by heating the elements in a current of halogen, if required anhydrous; if in solution, by dissolving the oxide or hydroxide in the halogen acid. The trihalides of manganese and cobalt are very unstable; and if the corresponding oxides be treated with halogen acid, a portion of the halogen is evolved, thus: Fe9O3 + 6HCJ.Aq = 2FeCl3.Aq + 3H2O ; Mn2O3 + 6HC1.Aq = 2MnCl3Aq + 3H2O. But MnCJ3.Aq gradually decomposes, especially if temperature is raised, thus : 2MnCl3.Aq= 2MnCl2.Aq + Cl2. And if MnO2 be employed, chlorine is evolved from the outset: 2MnO2 + 8HCl.Aq=2MnCl3.Aq + 4H2O + CJ2; the MnCl3 decomposing further on standing or on rise of temperature. With Co2O3 a transient brown coloration is noticeable on adding hydrochloric acid, implying the momentary formation of CoCig. Aq ; but it is at once resolved into CoCl0.Aq and free chlorine.