THE HALOGENS.] cule of dissolved hydnodic acid from its elements 13,170 units of heat are evolved, and about 34,800 units of heat are developed in the production of a molecule of silver chloride ; whereas 39,320 units of heat are developed in the formation of a molecule of dissolved hydrochloric acid from its elements, and about 18,650 units in the pro duction of a molecule of silver iodide. But (34,800 + 13,170) - (18,650 + ?9,320) = - 10,000 that is to say, the action of hydriodic acid on silver chloride is attended with the development of no less than 10,000 units of heat. All metals which decompose water at a red heat or at lower temperatures decompose a solution of hydrogen chloride with evolution of hydrogen, and form the cor responding metallic chloride, but with very different degrees of readiness ; thus Zu + 2HC1 = H 2 + ZnCl 2 . Zinc. Hydrogen chloride. Hydrogen. Zinc chloride. According to Thomsen, the amounts of heat developed in the production of 2 grammes of hydrogen by the action of various metals on a dilute solution of hydrogen chloride are as follows : Lithium 125,860 heat-units. Potassium 123,700 Sodium 114,380 Magnesium 108,290 Aluminium 79,880 Manganese 49,360 Zinc 34,200 Iron , 21,310 Cadmium 17,610 Lead 4,130 Exact comparative observations of the degrees of readiness with which the various metals evolve hydrogen from a solution of hydrogen chloride have not as yet been made, but from ordinary observations it appears that those act most readily which develop the greatest amount of heat when dissolved. It is very difficult to compare the behaviour of different metals, however, since minute quan tities of impurity exert a most remarkable influence ; thus, ordinary zinc and iron dissolve with the greatest readiness in a dilute solution of hydrogen chloride, but the pure metals are only slowly dissolved. Hydrogen bromide and iodide closely resemble hydro gen chloride in their behaviour with metals ; mercury, however, which is not affected by hydrogen chloride or bromide, slowly dissolves in hydrogen iodide. Hydrogen fluoride not only acts with great readiness upon all metals which are dissolved by the other haloid acids, but likewise on many which these acids do not attack, copper and silver, for example ; and it also dissolves many elements which are insoluble in all other acids, such as silicon, boron, titanium, tantalum, and zirconium, with evolution of hydrogen. Gore has shown that anhydrous liquid hydrogen fluoride and chloride are, in most cases, much less energetic in their chemical reactions than their aqueous solutions ; this is probably because the products which are formed in the first instance, being insoluble or diffi cultly soluble in the acid, form a coating on the surface of the substance submitted to the action of the acid, and thus preserve it from further action, or cause the action to take place more slowly ; when water is present the product is dissolved, and thus a fresh surface is continu ally exposed. The haloid acids readily enter into reaction with most metallic oxides, forming water and the corresponding compound of the metal with the halogen; thus ZnO + 2HC1 = ZnCl 2 + OH 2 Zinc oxide. Hydrogen chloride. Zinc chloride. Water. Fe 2 O 3 + 6HC1 = Fe 2 Cl 6 + 30H 2 . Ferric oxide. Hydrogen chloride. Ferric chloride. Water. CHEMISTRY 493 Hydrogen fluoride is capable of dissolving some oxides, such as silica, SiO 2 , for example, which are not affected by the other haloid acids. It is in consequence easily dis tinguished from the other haloid acids by the powerful corrosive action which it exerts on glass. The haloid acids also readily enter into reaction with, or neutralize, the hydroxides, forming water and the cor responding fluoride, chloride, bromide, or iodide H 2 0; Water 2H 2 O. Water. NaOH + HC1 = NaCl + Sodium hydroxide. Hydrogen chloride. Sodium chloride. Ba(OH) 2 + 2HBr = BaBr 2 + Barium hydroxide. Hydrogen bromide. Barium bromide. The thermochemical behaviour of hydrochloric acid with hydroxides has already been discussed. Chlorine enters into reaction with many of the elements at ordinary atmospheric temperatures, and with all of them, with few exceptions, at more or less elevated tem peratures ; its union with phosphorus, and with finely divided arsenic, antimony, tin, and copper, is attended with combustion. Bromine and iodine closely resemble chlorine in their behaviour with other elements ; the union of bromine with other elements, however, appears always to take place less readily, and to be accompanied by the development of less heat than is the case with chlorine, and the affinity of iodine for most other elements is still weaker than that of bromine. The iodides also are usually less stable than the corresponding bromides, which are usually less stable than the corresponding chlorides. The com pounds of the halogens with metals and with positive elements generally are mostly stable in presence of water, but their compounds with negative elements are mostly decomposed by water, and the halogen separated in the form of haloid acid. The same element does not always enter into combina tion with chlorine, bromine, and iodine in the same proportions ; thus, phosphorus forms a stable trichloride, PC1 3 , and an unstable pentachloride, PC1 5 ; a stable tri- bromide, PBr 3 , and a pentabromide, PBr 5 , less stable than the pentachloride; but with iodine it forms the com pounds PI 3 and P 2 I 4 . Many elements furnish compounds with fluorine containing a higher proportion of fluorine than corresponds to the amount of chlorine in their highest chlorides, or they form stable fluorides but unstable chlorides, bromides, and iodides ; for example, phosphorus pentafluoride, PF 5 , is not decomposed by heat, whereas the pentachloride, PC1 5 , is readily resolved into the tri chloride and chlorine by heating. The halogens do not only combine with hydrogen and other elements, however, but they are capable of forming compounds with each other. Thus, by the action of iodine on silver fluoride, Gore has obtained iodine pentafluoride, IF 5 , as a colourless highly volatile liquid ; iodine at once enters into reaction with chlorine, and is first converted into liquid iodine chloride, IC1, but ultimately yields iodine trichloride, IC1 3 , which is a crystalline solid; with bromine iodine appears to furnish only a liquid mono- bromide, IBr . In like manner, bromine forms with chlorine a liquid bromine chloride, Bid . On account of its great affinity for hydrogen, chlorine readily enters into reaction with a large number of hydrogenized carbon compounds, and displaces the hydro gen more or less completely. Thus, methane, CH 4 , when submitted to the action of chlorine, is successively con verted into monochloromethane, CH 3 C1, dichloromethane, CH 2 C1 2 , trichloromethane, CHC1 3 , and tetrachloromethane, CCf 4 , the displaced hydrogen being evolved as hydrogen chloride. Bromine behaves similarly, but enters into such reactions much less readily ; iodine seldom, if ever, behaves
in this way.