xx DISSOCIATION 523 



that the value rose to 4/0 at 60 C. The values required by 

 Avogadro's hypothesis are: for C. 2 H 4 O 2 2*073, for C 4 H 8 O 4 

 4-146. The density at 60 C. approximates to that of an acid 

 of the formula C 4 H 8 O 4 ; at 23oC. it approximates to the 

 value for an acid C 2 H 4 O 2 , and is not changed by further 

 heating. Playfair and Wanklyn concluded (loc. tit. p. 143) 

 that "in the few cases where any great difference is found 

 between a vapour-density taken at a point near the point 

 of liquefaction, and one taken at a point far removed from 

 the point of liquefaction, there is a chemical and not a 

 merely physical change." The vapour " has sometimes one 

 and sometimes another chemical formula, according as it 

 is more or less heated." In the case of acetic acid the 

 dissociation is expressed by the scheme : 



High temp. 



C 4 H 8 4 - 2 C 2 H 4 2 , 



Low temp. 



the action proceeding from left to right with increasing 

 temperature and conversely. 



The dissociation of phosphorus pentachloride. For 

 phosphorus pentachloride vapour, Cahours found (Ann. 

 Chim. Phys., 1847, [3], 20, 373) : 



Temp 182 200 250 300 336 C. 



Density 5-078 4-851 3-991 3-654 3-656 



By diluting with air, Wurtz was able to measure the 

 density of the vapour at 129 C. under a partial pressure of 

 175 mm., and found that the value increased to 6-4 (Comptes 

 rendus, 1873, 76, 603), thus approaching the value 7*217 

 required by the formula PC1 5 . The lower values observed 

 at temperatures above 129 C. were obviously due to dissocia- 

 tion, as shown by the equation : 



PC1 5 ^ PC1 3 + C1 2 , 

 the dissociation being substantially complete above 30oC 



