Note 30 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE
George Kamensky translation
Quicklime, CaO (or calcium carbonate, CaCO3), does not absorb chlorine when cold, but at a red heat, in a current of chlorine, it forms calcium chloride, with the evolution of oxygen. (This was confirmed in 1893 by Wells, at Oxford.) This reaction corresponds with the decomposing action of chlorine on methane, ammonia, and water. Slaked lime (calcium hydroxide, CaH2O2) also, when dry, does not absorb chlorine at 100°. The absorption proceeds at the ordinary temperature (below 40°). The dry mass thus obtained contains not less than three equivalents of calcium hydroxide to four equivalents of chlorine, so that its composition is [Ca(HO)2]5Cl4. In all probability a simple absorption of chlorine by the lime at first takes place in this case, as may be seen from the fact that even carbonic anhydride, when acting on the dry mass obtained as above, disengages all the chlorine from it, leaving only calcium carbonate. But if the bleaching powder be obtained by a wet method, or if it be dissolved in water (in which it is very soluble), and carbonic anhydride be passed into it, then chlorine is no longer disengaged, but chlorine oxide, Cl2O, and only half of the chlorine is converted into this oxide, while the other half remains in the liquid as calcium chloride. From this it may be inferred that calcium chloride is formed by the action of water on bleaching powder, and this is proved to be the case by the fact that small quantities of water extract a considerable amount of calcium chloride from bleaching powder. If a large quantity of water act on bleaching powder an excess of calcium hydroxide remains, a portion of which is not subjected to change. The action of the water may be expressed by the following formulæ: From the dry mass Ca3(HO)6Cl4 there is formed lime, Ca(HO)2, calcium chloride, CaCl2, and a saline substance, Ca(ClO)2. Ca3H6O6Cl4 = CaH2O2 + CaCl2O2 + CaCl2 + 2H2O. The resulting substances are not equally soluble; water first extracts the calcium chloride, which is the most soluble, then the compound Ca(ClO)2 and ultimately calcium hydroxide is left. A mixture of calcium chloride and hypochlorite passes into solution. On evaporation there remains Ca2O2Cl43H2O. The dry bleaching powder does not absorb more chlorine, but the solution is able to absorb it in considerable quantity. If the liquid be boiled, a considerable amount of chlorine monoxide is evolved. After this calcium chloride alone remains in solution, and the decomposition may be expressed as follows: CaCl2 + CaCl2O2 + 2Cl2 = 2CaCl2 + 2Cl2O. Chlorine monoxide may be prepared in this manner.
It is sometimes said that bleaching powder contains a substance, Ca(OH)2Cl2, that is calcium peroxide, CaO2, in which one atom of oxygen is replaced by (OH)2, and the other by Cl2; but, judging from what has been said above, this can only be the case in the dry state, and not in solutions.
On being kept for some time, bleaching powder sometimes decomposes, with the evolution of oxygen (because CaCl2O2 = CaCl2 + O2, see p. 163); the same takes place when it is heated.