506 JOHN JOHNSTON AND PAUL NIGGLI 
of the solvent) and thus produces a greater apparent solubility of 
the CaCO,;. Moreover, the solubility of solids in fluids at tempera- 
tures above the critical point of the solution will be dependent on 
the pressure, since the effect of the latter is then to change the con- 
centration of the solvent, just as pressure affects the solubility of 
gases, by changing the concentration of the solute itself, and hence 
the concentration of the solute in the solution." 
One point remains to be noticed, namely, that in making use of 
solubility data we must remember that solubility, and the heat effect 
and volume change accompanying dissolution of a substance, are 
affected by the presence of other substances? in the solvent. Now 
from a complex solution from which any one of a number of sub- 
stances can conceivably separate out first, the product which actu- 
ally separates out is that which under the particular actual condi- 
tions is least soluble, or has reached its limit of saturation.’ 
Solubility relations are therefore of the highest importance; 
but they must always be determined under the proper conditions, 
and cannot be inferred from the relative solubilities of the various 
substances in pure water. 
Importance of solubility relations in determining the course of a 
reaction in solution.—The importance of this fact—that solubility 
(or perhaps we should rather say, lack of solubility) is the pre- 
dominating factor in determining the order of separation of the 
products of reaction of any system—is not so universally appre- 
ciated as it ought to be. The appearance of any one product is 
very often ascribed to the existence of a greater affinity between its 
component parts than exists between the components of the other 
compounds which might conceivably be formed by the reaction. 
In a certain sense this is true, but in the sense in which the term 
« This question is discussed later. 
2 The following are two extreme examples, which illustrate the two main modes 
of action of added substances. Ag(CN). is very sparingly soluble in water, but quite 
soluble in water containing KCN, owing to the formation in this case of the complex 
KAg(CN)2. The solubility of AgCl, on the other hand, is much smaller in water 
containing KCl (or AgNO;), owing to the effect of the common ion Cl’ (or Agt). 
3 This is the basic principle underlying van’t Hoff’s great series of investigations 
of the oceanic salt deposits. 
