CHEMICAL EQUILIBRIUM 245 



Ifjsomfi-Sfress (e.g., by change of temperature, pressure, or con-^i /_ 

 centration) is brought to heai on a system in equilibrium, a reac- 

 tion occurs, displacing the equilibrium in the direction which tends ^^- 

 to undo the effect of the stress. Thus, raising the temperature 

 furthers the change which absorbs heat and therefore would 

 tend to lower the temperature. Increasing the concentration 

 of the molecules pushes the action in the direction which uses up 

 these very molecules (p. 234). Again pressure causes ice to melt, 

 because the water which is formed occupies a smaller volume, and 

 this change tends to relieve the pressure. But pressure will not 

 cause most substances to melt, because usually the liquid form 

 occupies a greater volume and its production would tend to in- 

 crease pressure. 



The student is cautioned against applying these laws to systems 

 not in equilibrium, for example, to unsaturated or supersaturated 

 solutions. To such cases they do not, necessarily, apply. Thus 

 the addition of a small quantity of cupric chloride to water is 

 attended by evolution of heat. It would be quite wrong to reason 

 from this, however, that the solubility must fall off as the temper- 

 ature is raised. The salt is extremely soluble in water, and if we 

 keep on adding it to the solution until the latter is saturated we 

 find that the last portions dissolve with absorption of heat. Now 

 it is only the saturated solution that is in equilibrium with the solid, 

 hence it is only with respect to this solution that we can apply 

 Van't Hoff's or Le Chatelier's law. In accordance with the behav- 

 ior of this solution, we find that the solubility increases with rising 

 temperature. 



The characteristics of systems in equilibrium (pp. 63, 232) 

 should therefore be kept in mind carefully, in order to avoid mis- 

 takes. 



Summary. In this chapter we have answered three ques- 

 tions: 



