"486 PRINCIPLES OF CHEMISTRY 



which fumes in the air and has a specific gravity 1\7 8 at 15 (some- 

 times, after being kept for some time, it decomposes with a violent ex- 

 plosion). It explodes violently when brought into contact with charcoal, 

 paper, wood, and other organic substances. If a small quantity ot 

 water be added to this hydrate, and it be cooled, a.crystallo-hydrate, 

 C1HO 4 ,H 2 O, separates out. This is much more stable, but the liquid 

 hydrate HC1O 4 ,2H 2 O is still mofe so. The acid dissolves in water in 

 all proportions, and its solutions are distinguished for their stability. 48 

 When ignited both, the acid and its salts are decomposed, with the 

 evolution of oxygen. 47 



also be obtained by adding one-fojirth part of strong sulphuric acid to potassium chlorate, 

 carefully distilling and subjecting the crystals of the hydrate HC1O 4 ,H 2 O obtained in 

 the distillate Jto a fresh distillation. Perchloric aci<i, HCIO^ itself does not distil, and 

 is decomposed on distillation until the more stable hydrate HC1O 4 ,H 2 O is formed ; this 

 decomposes into HC1O 4 and HC1O 4 ,2H 2 0, which latter hydrate distils without decom- 

 position. This forms an excellent example of the influence of water on stability, and of 

 the property of chlorine of giving compounds of the type C1X 7 , of which all the above 

 hydrates, C1O 3 (OH), C1O 2 (OH) 3 , and C10(OH) 5 , are members. Probably further re- 

 search will lead to the discovery of a hydrate C1(OH) 7 . 



M According to Eoscoe the specific gravity of perchloric acid = 1'782 and of the 

 hydrate HClO^H-jO in a liquid state (50) 1-811 ; hence a considerable contraction takes 

 place in the combination of HC1O 4 with H 3 O. 



47 The decomposition of salts analogous to potassium chlorate has been more fully 

 studied in recent years by Potilitzin and P. Frankland. Professor Potilitzin, by de- 

 composing, for example, lithium chlorate LiCIO 5 , found (from the quantity of lithium 

 chloride and oxygen) that at first the decomposition of the fused salt (868) takes place 

 according to the equation, 8LiClO 3 =2LiCl + LiClO 4 -f 6O, and that towards the end the 

 remaining salt is decomposed thus: 5LiClO3=4liiCl.-fc.LiClO4-l-10O. The phenomena 

 observed by Potilitzin obliged him to admit that lithium perchlorate is capablaof decom- 

 posing simultaneously with lithium chlorate, with the formation of the latter salt and 

 oxygen ; and thia was confirmed by direct experiment, which showed that lithium chlorate 

 is always formed in the decomposition of the perchlorate. Potilitzin drew particular 

 attention to the fact that the decomposition of potassium chlorate and of salts analogous 

 to it, although exothermal (Chapter III., Note 12), not only does not proceed spon- 

 taneously, but requires time" and a rise of temperature in order to attain completion, 

 which again shows that chemical equilibria are not determined by the heat effects of re- 

 actions only. 



P. 'Frankland and J. Dingwall (1887) showed that at 448 (in the vapour of sulphur) 

 a mixture of potassium chlorate and powdered glass is decomposed almost in accordance 

 with the equation 2KC1O 5 = KC1O 4 + KC1 + O 2 , whilst the salt by itself evolves about half 

 as much oxygen, in accordance with the equation, 8KC1O 3 =5KC1O 4 + 8KC1-1-2O 2 . The 

 decomposition of potassium perchlorate in "admixture with manganese peroxide proceeds 

 to completion, KC1O 4 =KC1 + 2O 2 . But in decomposing by itself the salt at first gives 

 potassium chlorate, approximately according to the equation 7KC1O 4 = 2KC1O 3 + 5KC1 

 + HO 2 . Thus there is now no doubt that when potassium chlorate is heated, the per- 

 chlorate is formed, and that this salt, in decomposing with evolution of oxygen, again 

 gives the former salt. 



In the decomposition of barium hypochlorite, 50 per cent, of the whole amount passes 

 into chlorate, in the decomposition of strontium hypochlorite {Potilitzin, .1890) 12'5 pe 

 cent., and of calcium hypochlorite about 2 ; 5 per cent. Besides which Potilitzin showed 

 that the decomposition of the hypochlorites and also of the chlorates is always accom- 

 panied by the formation of a certain quantity of the oxides and by the evolution of 



