THE HALOGENS 503 



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, 

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

 from the gaseous elements there is evolved for HC1 39,000, for HBr 32,000, 

 and for HI 18,000 heat units. 70 But it is especially evident from 

 the fact that solutions of hydrogen bromide and iodide in water have 

 many points in common with solutions of hydrogen chloride, both in 

 their capacity to form hydrates and fuming solutions of constant boiling 

 point, and in their capacity to form haloid salts, &c. by reacting on 



In consequence of what has been said above, it follows that hydro- 

 bromic and hydriodic acids, being substances which are but slightly 

 stable, cannot be evolved in a gaseous state under many of those condi- 

 tions under which hydrochloric acid is formed. Thus if sulphuric acid 

 in solution acts on sodium iodide, all the same phenomena take place 

 as with sodium chloride (a portion of the sodium iodide gives hydri- 

 odic acid, and all remains in solution), but if sodium iodide be mixed 

 with strong sulphuric acid, then the oxygen of the latter decomposes 

 the hydriodic acid set free, with liberation of iodine, H 2 SO 4 + 2HI 

 = 2H 2 O + S0 2 + la- This reaction takes place in the reverse direction 

 in the presence of a large quantity ^of water (2,000 parts of water per 

 1 part of SO 2 ), in which case not only the affinity of hydriodic acid for 

 water is brought to light but also the action of water in directing chemi- 

 cal reactions in which it participates. 71 Therefore, with a halogen salt, 

 it is easy to obtain gaseous hydrochloric acid by the action of sulphuric 

 acid, but neither hydrobromic nor hydriodic acid can be so obtained in 

 the free state (as gases). 72 Other methods have to be resorted to for their 

 preparation, and recourse must not be had to compounds of oxygen, which 

 are so easily able to destroy these acids. Therefore hydrogen sulphide, 

 phosphorus, &c., which themselves easily take up oxygen, are introduced 

 as means for the conversion of bromine and iodine into hydrobromic and 

 hydriodic acids in the presence of water. For example, in the action of 

 phosphorus the essence of the matter is that the oxygen of the water goes 



7 On the basis of the data in Note 68. 



71 A number of similar cases confirm what has been said in Chapter X. 



72 This is prevented by the reducibility of sulphuric acid. If volatile acids be taken 

 they pass over, together with the hydrobromic and hydriodic acids, when distilled; 

 whilst many non-volatile acids which are not reduced by hydrobromic and hydriodio 

 acids only act feebly (like phosphoric acid), or do not act at all (like boric acid). 



