Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/506

494 CHEMISTRY [CHLORINE Compounds of (Jte Halogens with Oxygen. Oxides of fluorine are unknown, and the oxides of the remaining halogens are very imperfectly investigated. Three oxides of chlorine have been prepared, viz. : C1 2 0, Hypochlorous anhydride. C1 2 O 3 , Chlorous anhydride. and CIO., Chlorine peroxide. At ordinary temperatures these compounds are greenish- yellow gases, remarkable on account of their instability, being decomposed with explosive violence by very slight elevations of temperature. Their instability is accounted for by the circumstance that their decomposition is attended with the development of a very considerable amount of heat ; thus, according to Thomsen, the formation of gaseous hypo- chlorous anhydride from its elements would involve the absorption of 18,040 units of heat per molecule, and con sequently this amount is developed when the anhydride is resolved into its elements. The explanation of the absorption of heat in the formation of the oxides of chlorine would appear to be that more energy must be expended in separating the atoms of oxygen from each other, and in separating the atoms of chlorine from each other, than is produced by the combination of the chlorine and oxygen atoms. Oxides of bromine are also unknown, and since thermo- chemical investigation has shown that oxygen has less affinity to bromine than to chlorine, their non-formation in reactions corresponding to those by which the oxides of chlorine are produced is no longer surprising. Iodine differs entirely from chlorine and bromine with regard to its affinity for oxygen. Only one oxide of iodine is known, viz., iodic anhydride, I 2 O 5 , but this is a white crystalline substance of considerable stability, and, accord ing to Thomson s determination, the formation of this com pound from its elements would involve the development of no less than 44,860 units of heat per molecule. It has not been directly produced from its elements, although its formation in this manner is not improbable. Chlorous anhydride is prepared by passing dry chlorine gas over dry mercuric oxide 2HgO + 2C1 2 = C1 2 O + Hg 2 OCl 2 . It may be condensed by surrounding the receiver with a mixture of ice and salt, and thus obtained as a deep red liquid which emits a vapour of a deeper colour than that of chlorine, having a peculiar suffocating chlorous smell. It is with the greatest readiness decomposed into chlorine and oxygen, the warmth of the hand being sufficient to cause it to explode. Water dissolves about 200 times its bulk of the gas, forming a pale yellow solution of hypo- chlorous acid, which has an acid but not sour taste C1 2 + H 2 = 2HOC1 . By neutralizing this solution with metallic hydroxides or carbona es corresponding metallic hypochlorites are formed, but these salts are usually prepared by passing chlorine into water in which the metallic hydroxide or carbonate is suspended or dissolved, the liquid being carefully cooled 2KOH + C1 2 = KOC1 + KC1 + OH 2 . Potassium r>>,i Potassium Potassium , , hydroxide. Chlorine. hypochlorlte . chloride. A concentrated solution of hypochlorous acid decom poses rapidly, even when kept in ice ; a dilute solution is more stable, but is decomposed by boiling into chloric acid, water, chlorine, and oxygen. These substances are probably the products of several distinct changes, such as are represented by the following equations 2HOC1 O 9 + 2HC1 3HOC1 - HO 3 C1 + 2HC1. Hypochlorous acid. Chloric acid. Hydrogen chloride. Hypochlorous acid, however, readily decomposes hydro chloric acid, and the chlorine produced is probably the result of reaction between these bodies Hypochlorous acid is a very powerful oxidizing agent, and rapidly converts many of the elements into their highest oxides, at the same time liberating chlorine; indeed, in many respects it is not unlike ozone and hydrogen dioxide in its chemical behaviour, and it enters into reaction with the latter in the manner represented by the equation HOC1 + H 2 O 2 = O 2 + HC1 + H 2 O . Like chlorine, it possesses powerful bleaching properties. The hypochlorites also act as bleaching agents, probably by yielding oxygen to the substance submitted to their action, but less powerfully than the free acid. The bleaching powder or chloride of lime of commerce is prepared by exposing slaked lime or calcium hydroxide to the action of chlorine gas. There has been much dis cussion among chemists as to the constitution of the compound so produced, and it is only quite recently that evidence has been obtained which enables us to pronounce a decision. The composition of bleaching powder is represented by the formula CaCl 2 O, whereas calcium hypochlorite would be represented by the formula CaCl/) 2 ; it was therefore supposed that the bleaching powder was a mixture of calcium chloride and calcium hypochlorite, since CaCl 2 + CaCl 2 O 2 = 2CaCl 2 O. Appar ently, however, it is a distinct compound intermediate between calcium chloride and calcium hypochlorite, thus nJCl c (Cl CaJ( C1 >- a { Cl a (OC1) } (OC1) Calcium chloride Ctt!dum chlorohypochlorite. Calcium (Bleaching powder). hypochlorite. and its formation from calcium hydroxide may be repre sented by the following equation On dissolving in water, according to Kingzett, it is resolved into calcium chloride and calcium hypochlorite, which may be obtained in crystals by carefully con centrating the solution in vacuo, so that a solution of bleaching powder is correctly regarded as a solution of calcium hypochlorite f Cl ^_ . _ f OC1 2Ca |5a CaC1 * + Ca )ocr Bleaching powder. Calcium chloride. Calcium hypochlorite. Solutions of hypochlorites are very unstable, and when boiled they furnish a mixture of chloride and chlorate ; thus 3KOC1 K0 3 C1 + 2KC1. Potassium hypochlorite. Potassium chlorate. Potassium chloride. It is on this account that it is necessary in preparing hypochlorites to act on a cold solution of the hydroxide, since if the solution be heated chlorate is formed by the decomposition of the hypochlorite. The chlorates are very stable compounds as compared with the hypochlorites. They may be obtained well crys tallized, and are all soluble in water. They are destitute of bleaching properties. The most important chlorate is potassium chlorate, KC1O 3 , which is chiefly interesting as a source of oxygen, and is largely employed in the manufacture of fireworks ; the resolution of this salt into oxygen and potassium chloride is attended with a develop ment of heat amounting to 97UO units per molecule decomposed. By exactly decomposing a solution of barium chlorate with sulphuric acid, a solution of chloric acid, HC1O 3 , is

obtained, which may be separated from the insoluble