THE HYDRATE THEORY OF SOLUTIONS 133 



would be reduced to a series of inclines or intersecting straight 

 lines, and this is the utmost that can be expected in the case of 

 liquid hydrates which do not form separate liquid layers. 



In addition to the question of isomorphism there is, however, 

 an even more important point of contrast between solid and 

 liquid hydrates in reference to the effects produced by dis- 

 sociation in the two cases. Suppose that a hydrate CuS0 4 . 

 4HO, instead of producing a vapour-pressure intermediate 

 between those of CuS0 4 . 3H0O and CuS0 4 . 5H0O, produced a 

 greater vapour-pressure than either. The addition of water 

 vapour to the trihydrate would evidently convert it into the 

 pentahydrate rather than into the tetrahydrate, and even if the 

 tetrahydrate could be prepared it could only exist in a meta- 

 stable condition, and would be liable at any time to decompose 

 or dissociate into a mixture of the tetrahydrate and pentahydrate. 

 As only one vapour-pressure is possible in the system, the 

 dissociation would proceed until the whole of the tetrahydrate 

 had been decomposed. 



In the case of a mixture of liquid hydrates, on the other 

 hand, considerations of mass-action come into play. The rate at 

 which the less stable hydrates decompose decreases as the 

 concentration diminishes, whilst the more stable hydrates 

 decompose with increasing rapidity as they become more and 

 more predominant in the solution. Ultimately, therefore, a 

 point is reached at which even the least stable hydrates are 

 capable of existing, although at greatly reduced concentrations, 

 as permanent constituents of the solution. This implies that a 

 hydrate such as H 2 S0 4 . 2H 2 0, which as a solid could only co-exist 

 with one of the adjacent hydrates, H 2 S0 4 or H 2 S0 4 . 2H2O, may 

 as a liquid exist in contact with a wide range of hydrates, and 

 may even be present in small quantities in solutions containing 

 free water or free sulphur trioxide owing to the reverse action 

 in the processes. 



H 2 S0 4 . H 2 + H,0 J H,S0 4 . 2H 2 

 H„S0 4 .2H. J + S0 3 2 2H -' S0 ^ 



It follows also that whereas in the solid state dissociations such 

 as those represented by the equations 



2(H 2 S0 4 . H 2 0) J H,S0 4 + H 2 SC>4 . 2HX> 

 3HjS0 4 J H.S,0 7 + H 2 SO, . HX> 



must proceed either to completion or not at all, in the liquid state 



