238 SMITH'S INTERMEDIATE CHEMISTRY 



veniently, by modifying the thickness of the arrows used to express 

 reversibility. Thus : 



HC1 ^ H+ + Cl- H 2 ^ H+ + OH- 



In the case of phosphorus pentachloride vapor, 80 per cent of 

 the whole weight of material in the equilibrium mixture at 250 

 and 760 mm. pressure is dissociated into PCls and Cb 



Homogeneous and Inhomogeneous Systems. While 

 there are all degrees of speed in chemical actions, yet in practice 

 we quickly distinguish two different classes. There is a class of 

 actions of which most examples are almost instantaneously accom- 

 plished, and a class in which, frequently, the operation takes 

 minutes or even hours. The classes overlap, but, in a general 

 way, the following distinction may be made. 



To the former, speedy class belong the explosion of hydrogen 

 and oxygen or other gaseous mixtures, and the interactions when 

 solutions are mixed, as in precipitations. In view of the foregoing 

 explanations, we perceive that the rapid accomplishment of such 

 actions is due, not so much to any especially great intrinsic affin- 

 ity, as to the homogeneous state of mixture of the interacting 

 materials. This, of course, is a purely physical, and not a chem- 

 ical motive for speedy interaction. In intimate mixtures, every 

 molecule has an equal opportunity freely to encounter every other 

 molecule and there is therefore no mechanical impediment to 

 the operation of the affinities of the substances. Hence the ap- 

 parent activity is great. 



To the second class, comprising the slower actions, belong cases 

 like the interaction of a piece of zinc with hydrochloric acid, or of 

 manganese dioxide (p. 142) with the same acid, whereby hydrogen 

 and chlorine, respectively, are slowly evolved, and the solid is grad- 

 ually consumed. Here the hindrance is evidently the fact that 

 the interacting substances are not intimately mixed. In the slow 

 actions, the system is inhomogeneous. Pulverizing the solid 

 before use will increase the speed, indeed, by providing more 



