THE HALOGENS 511 



200) or acting as an oxidiser, periodic acid first gives iodic acid, but it 

 may also be ultimately decomposed. 



Compounds formed between chlorine and iodine must be classed 

 among the most interesting halogen bodies. 86 These elements com- 

 bine together directly with evolution of heat, and form iodine 

 monochloride, 1C1, or iodine trichloride, IC1 3 . 87 As water reacts on 

 these substances, forming iodic acid and iodine, they have to be pre- 

 pared from dry iodine and chlorine. 88 Both substances are formed in a 

 number of reactions ; for example, by the action of aqua regia on iodine, 

 of chlorine on hydriodic acid, of hydrochloric acid on periodic acid, of 

 iodine on potassium chlorate (with the aid of heat, &c.) Trapp obtained 

 iodine monochloride, in beautiful red crystals, by passing a rapid 

 current of chlorine into molten iodine. The monochloride then distils 

 over. and solidifies, melting at 27 By passing chlorine over the 



88 With respect to hydrogen, oxygen, chlorine, and other elements, bromine occupies 

 an intermediate position between chlorine and iodine, and therefore there is no particular 

 need for considering at length the compounds of bromine This is the great advantage 

 of a natural grouping of the elements. 



87 They were both obtained by Gay-Lussao and many others. Recent data respect- 

 ing iodine monochloride, IC1, entirely confirm the numerous observations of Trapp 

 (1854), and even confirm his statement as to the existence of two isomeric (liquid and 

 crystalline) forms (Stortenbeker). With a small excess of iodine, iodine monochloride 

 remains liquid, but -in the presence of traces of iodine trichloride it easily crystallises. 

 Tanatar (1893) showed that of the two modifications of IC1, one is stable, and melts at 

 27 ; while the other, which easily passes into the first, and is formed in the absence of IC1 3 , 

 melts at 14. Schiitzenberger amplified the data concerning the action of water on the 

 chlorides (Note 88), and Christomanos gave the fullest data regarding the trichloride. 



After being kept for some time, the liquid monochloride of iodine yields red deliques- 

 cent octahedra, having the composition IC1 4 , which are therefore formed from the mono- 

 chloride with the liberation of free iodine, which dissolves in the remaining quantity of the 

 monochloride. This substance, however, judging by certain observations, is impure iodine 

 trichloride. If 1 part of iodine be stirred up in 20 parts of water, and chlorine be passed 

 through the liquid, then all the iodine is dissolved, and a colourless liquid is ultimately 

 obtained which contains a certain proportion .of chlorine, because this compound gives a 

 metallic chloride and iodate with alkalis without evolving any free iodine : IC1 5 + 6KHO 

 = 5KC1 + KIO 3 + 3H 2 O The existence -of a pentachloride IC1 5 is, however, denied, 

 because this substance has not been obtained in a free state. 



Stortenbeker (1888) investigated the equilibrium of the system containing the mole- 

 cules I 2 , IC1, Ids, and C1 2 , in the same way that Roozeboom (Chapter X. Note 38) examined 

 the equilibrium of the molecules HC1, HC1,2H 2 O, and H 2 O. He found that iodine 

 monochloride appears in two states, one (the ordinary) is stable and melts at 27 0- 2, whilst 

 the other is obtained by rapid cooling, and melts at 13'9, and easily passes into the 

 first form. Iodine trichloride melts at 101 only in a closed tube under a pressure of 16 

 atmospheres. 



88 By the action of water on iodine monochloride and trichloride a compound IHC1 8 

 is obtained, which does not seem to be altered by water. Besides this compound, iodine 

 and iodic acid are always formed, 10IC1 4- 3H 2 O = HIO 5 + 5IHC1 2 + 2I 2 ; and in this respect 

 iodine trichloride may be regarded as a mixture, IC1 + IC1 5 = 2IC1 3 , but ICl 5 -f 3HoO 

 = IHOs + 5HC1 ; hence iodic acid, iodine, the compound IHC1 2 , and hydrochloric acid 

 are also formed by the action of water. 



*10 



