Supersolubilittj from the Osmotic Standpoint. 265 



(20) The foregoing discussion indicates how the properties 

 o£ a binary liquid solution depend on the temperature and 

 pressure under which it is observed. If we exclude irreversible 

 chemical changes and metastable states, we may accordingly 

 place any solution in one of the following categories : — 



State 1. — If the temperature and pressure are such that no 

 alteration in the concentration of the solution w T ill cause it to 

 throw out another phase, it is in State l,and the components 

 are completely miscible. This state is characterized by the 

 solutions having BP/dc positive throughout the whole range 

 from c = to c=l. 



State 2. — If the temperature and pressure are such that 

 the only phase which comes out when the concentration is 

 altered is a liquid or gaseous one, then the solution is in 

 State 2, and the components are partially miscible *. It is 

 characterized by the fact that on throwing out the liquid or 

 the gaseous phase, equation (3) has been operative on the 

 globules. 



State o. — If the temperature and pressure are such that an 

 alteration of the concentration in one direction or the other 

 (or both) causes a solid phase to come out, the solution is in 

 State 3. This state includes, at ordinary temperatures and 

 pressures, most solutions of solids in liquids, and is char- 

 acterized by the operation of equation (1) on the contents of 

 the globules. 



This category embraces all cases where an alteration in 

 the concentration in one direction causes the appearance of 

 a solid, while in the other direction a liquid phase, or even 

 a gaseous phase, may be thrown out. 



(21) It follows from the above that (assuming no irre- 

 versible change takes place), as the solubility of sugar in 

 water increases both with rise of temperature and increase of 

 pressure, we should get a solution of these two substances in 

 State 1 — probably somewhere above the melting-point of 

 sugar — and presumably at some slightly lower temperature 

 we could obtain partially miscible liquids, where liquid 

 sugar saturated with water would be thrown out. It must 

 not be forgotten that as there is a limit to the negative 

 pressure f which a solution (or, for that matter, a pure 

 liquid) will support, there may be cases (e. g. those in which 

 the solubility decreases with increase of pressure) where the 

 three states above enumerated may not all be realizable. 



* In a solution of a gas in a liquid, as there is partial solution, the 

 components come under this designation. 



+ If the solution were able to support an infinite negative pressure 

 (apart from a defect of adhesion to the containing- avails), it would imply 

 an infinite attraction between the molecules, which is scarcely possible. 



