496 PRINCIPLES OF CHEMISTRY 



stability of hydrogen chloride, the easy decomposability of hydrogen 

 iodide, and the intermediate properties of hydrogen bromide. From 

 this it would be expected that chlorine is capable of decomposing water 

 with the evolution of oxygen, whilst iodine has not the energy to 

 produce this disengagement, 69 although it is able to liberate the oxygen 

 from the oxides of potassium and sodium, as the affinity of these metals 

 for the halogens is very considerable. For this reason oxygen, especially 

 in compounds from which it can be evolved (for instance, C1HO, CrO 3 , 

 tic.), easily decomposes hydrogen iodide. A mixture of hydrogen iodide 

 and oxygen burns in the presence of an ignited substance, forming 

 water and iodine. Drops of nitric acid in an atmosphere of hydrogen 

 iodide produce the disengagement of violet fumes of iodine and brown 

 fumes of nitric peroxide. In the presence of alkalis and an excess of 

 water, however, iodine is able to produce oxidation like chlorine that is, 

 it decomposes water ; the action is here aided by the affinity of hydrogen 

 iodide for the alkali and water, just as sulphuric acid helps zinc to decom- 

 pose water. But the relative instability of hydriodic acid is best seen in 

 comparing the acids in a gaseous state. If the halogen acids be dissolved 

 in water, they evolve so much heat that they approach much nearer 

 to each other in properties. This is seen from thermochemical data, 

 because in the formation of HX in solution (in a large excess of water) 

 from the gaseous elements there is evolved for HC1 39000, for HBr 22000, 



therefore H + Cl + Aq = -f 39'3. In taking molecules, all these figures must be doubled. 

 Br + H = +8-4; HBr + Aq = 19'9; H + Br + Aq = + 28'3. According to Berthelot 7' 2 go 

 to the vaporisation of Br.,, hence Bro + Hj = 16'8 + 7'2 = +24, if Br 2 be taken as vapour 

 for comparison with CL. H + I= 6'0, HI + Aq = 19'2 ; H + I + Aq= +13'2, and, accord- 

 ing to Berthelot, the heat of fusion of Io = 3'0, and of vaporisation 6'0 thousand heat units, 

 and therefore Io + H 2 = 2(6'0) + 3 + 6 3'0, if the iodine be taken as vapour. Berthelot, 

 on the basis of his determinations, gives, however, + 0'8 thousand heat units. Similar 

 contradictory results are often met with in thermochemistry with the imperfection of 

 the existing methods, and depend on the necessity of obtaining the fundamental figures 

 by an indirect method. Thus Thomsen decomposed a dilute solution of potassium iodide 

 by gaseous chlorine; the reaction gave + 26'2, whence, having first determined the heat 

 effects of the reactions KHO + HC1, KHO + HI and Cl + H in aqueous solutions, it was 

 possible to find H + I + Aq ; then, knowing HI + Aq, to find I + H. It is evident that the 

 unavoidable errors may multiply themselves. 



69 One would think, however, that on the basis of Berthollet's doctrine, and the obser- 

 vations of Potilitzin (Note 66), a certain trace of slow decomposition of water by iodine 

 may exist. In this sense the observations of Dossios and Weith on the fact that the 

 solubility of iodine in water increases after lapse of several months, will be comprehen- 

 sible. Hydriodic acid is then formed, and it increases the solubility. If the iodine be 

 extracted from such a solution by carbon bisulphide, then, as the authors showed, after 

 the action of nitrous anhydride, iodine may be again found in the solution by means of 

 starch. One can easily imagine that a number of like reactions, requiring much time and 

 taking place in small quantities, have up to now eluded the attention of investigators 

 who even still doubt the universal application of Berthollet's doctrine, or only see the 

 thermochemical side of reactions, or else neglect to pay attention to the element of time 

 and the influence of mass. 



