492 CHEMISTRY [THE HALOGENS. their general behaviour, show that the affinity of chlorine for hydrogen is much greater than that of bromine, whilst that of bromine is greater than that of iodine ; and as already pointed out (p. 475), a considerable amount of heat is developed in the formation of hydrogen chloride, but a less amount in the formation of hydrogen bromide, and the production of hydrogen iodide from its elements involves the absorption of heat. We have seen, how ever, that the heat developed or absorbed in the forma tion of these compounds cannot be regarded as an abso lute measure of the affinity of chlorine, bromine, and iodine for hydrogen, since there is an expenditure of energy due to the change of state, the three compounds being gases, but one being formed from two gases, the second from a gas and a liquid, and the third from a gas and a solid. Moreover, as we believe that the mole cules which enter into reaction consist each of two atoms, the combination of the dissimilar atoms to form mole cules of hydrogen chloride, bromide, and iodide must be preceded by the separation of the similar atoms which constitute the molecules of hydrogen, and of chlorine, bromine, and iodine, and there must be an expenditure of energy to effect this separation ; it appears probable that different amounts of energy are expended in separating the atoms of chlorine, of bromine, and of iodine from each other, and we may further assume as probable that the amount of energy expended in the separation of the atoms is greater in the case of chlorine than of bromine, and greater in the case of bromine than of iodine, although at present there is scarcely any experimental evidence which enables us to draw conclusions of any value in proof of this. Hydrogen fluoride is miscible with water in all proportions, and much heat is developed on mixing the two substances ; hydrogen chloride, bromide, and iodide are also extremely soluble in water, forming colourless strongly acid solutions. Water at C. absorbs 825 of its own weight, or about 492 times its bulk, of hydrogen chloride gas, increasing in volume about one-third, and acquiring a density of about 1 23. This saturated solution consists of hydrogen chloride and water in about the proportions indicated by the formula HC1 + 3H 2 O ; it fumes strongly, and when heated it gives off hydrogen chloride until at 1 12 C. a more dilute solution dis tils over, corresponding approximately in composition with the formula HC1 + 8H 2 0. A weaker solution when heated parts with water until it acquires this composition, and then distils unchanged. In like manner, a solution con taining about 48 per cent, of hydrogen bromide distils un changed at 125 C., and a solution containing about 57 per cent, of hydrogen iodide boils constantly at 127 C. ; these solutions correspond approximately in composition with the formulae HBr + 5H 2 O and 2HI + 1 1 H 2 O. A. solution of hydrogen fluoride approximately of the composition IIF + 2H 2 O distils unchanged at 120 C. Roscoe s experi ments have shown, however, that the composition of these solutions varies with the pressure, and that there exists for each pressure a solution of corresponding strength which undergoes no change in composition when distilled under this pressure, and therefore has a constant boiling point. Hence, the apparent constancy of composition cannot be regarded as proof of the existence of a definite hydrate corresponding in composition to that of the solutions ob tained on distilling under a given pressure. There is no evidence, however, to invalidate, but much to support the conclusion that the haloid acids do not dissolve in water as such, but that they form definite hydrates ; and if it be admitted that their solutions contain definite hydrates, their behaviour is precisely similar to that of other com pounds which decompose when heated, but furnish pro ducts which can reunite to form the parent compound. According to Thomsen s determinations, the amounts of heat developed when gaseous hydrochloric, hydrobromic, and hydriodic acids are dissolved in water, in the propor tion of 1 molecule of the acid to 400 molecules of water, are as follows In the dissolution of hydrogen chloride, 17,314 heat-units. bromide, 19,207 iodide, 19,207 The development of so large an amount of heat, al though, doubtless, in a great measure arising from the change from the gaseous to the liquid state, may be regarded as evidence that the acids really enter into combination with water, and are not merely dissolved; it may be noticed also that the same amounts of heat are developed in dissolving hydrogen bromide and iodide, and that their solutions, which under the ordinary atmo spheric pressure distil unchanged, have nearly the same boiling point, and correspond approximately in composition. Solutions of hydrogen chloride and bromide may be preserved unchanged, but the solution of hydrogen iodide gradually becomes brown, especially on exposure to light, owing to the separation of iodine, the hydrogen being oxidized by the oxygen of the air 4HI + O = 2I + 20H 2 = 2 2 . Hydrogen iodide. Oxygen. Iodine. Water. = 2I 2 Iodine. On the other hand, solutions of chlorine or bromine in water may be preserved unchanged in the dark, but when they are exposed to light oxygen is evolved, and hydrogen chloride or bromide produced 2C1 + 2H 2 O = O 2 + 4IIC1. Chlorine. Water. Oxygen. Hydrogen chloride. A solution of iodine does not undergo alteration. In the presence of substances which have a tendency to enter into reaction with oxygen, water is often readily decom posed in this manner by chlorine and bromine without the aid of light, but iodine seldom effects this change ; thus, on passing chlorine into a solution of sulphurous acid, the latter is oxidized to sulphuric acid H 2 S0 3 + H. 2 O + C1 2 = H 2 S0 4 + 2HC1. Sulphurous acid. Water. Chlorine. Sulphuric acid. Hydrogen chlnride. The bleaching action which is powerfully exerted by chlorine in presence of water, and less powerfully by bromine, is in most cases the result of a similar change, the substance bleached being usually one which has a tendency to enter into reaction with oxygen. Chlorine readily displaces bromine and iodine from their compounds with hydrogen and other elements, and, similarly, bromine displaces iodine ; indeed the affinity of chlorine for other elements, with few exceptions, is superior to that of bromine, the affinity of bromine being in like manner superior to that of iodine. The decom position of many chlorine compounds by the corresponding bromine and iodine compounds, and of bromine com pounds by the corresponding iodine compounds, would appear at first sight to be in contradiction to this state ment, but on examination of all such reactions it is found that they involve a development of heat, and therefore are perfectly normal in character. For example, silver chloride is converted into silver iodide by digestion with an aqueous solution of hydriodic acid AgCl + HI = Agl + HC1 . It will be obvious that, in a molecule of silver iodide and a molecule of hydrogen chloride, the same quantity of the same elements is present as in a molecule of silver chloride and of a molecule of hydrogen iodide. But in the formation of the latter system from its elements, less heat is developed than in the formation of the former ; hence reaction in the direction indicated by the above equation is attended with
the development of heat. Thus, in the formation of a mole-