262 THE REALITIES OF MODERN SCIENCE 



in water as one product and a salt for the other. (The 

 latter, if soluble, is present partly as dissolved mole- 

 cules and partly as ions, namely the anion of the acid 

 and the cation of the base.) 



We have already mentioned the equilibrium be- 

 tween the two phases, liquid and vapor, of a system 

 with only one component, e.g. water. We have seen 

 that the condition is determined by two variables, 

 namely the temperature and the volume. If either of 

 these two conditions is altered the equilibrium is dis- 

 turbed and the proportions of water and vapor are 

 varied. Thus an increase of temperature causes an 

 increased vaporization and a consequent reduction, of 

 the portion which the water is of the entire volume. 

 Similarly if the volume is varied, as for example de- 

 creased, there is a change in the proportions of the two 

 phases, vapor being condensed. 



In both cases we notice that the change is of such a 

 nature as to oppose the cause. Thus in the case of 

 increased temperature there is an increase in the 

 kinetic energy of the molecules of the system, and this 

 results in a separation of more molecules from the 

 liquid. This increased molecular separation means a 

 greater potential energy for the molecules. In so far 

 as the potential energy is increased the kinetic energy is 

 reduced. The change occurring is therefore of such 

 a character as to reduce the molecular kinetic energy 



neutralized by the addition of a base of known concentration and 

 hence known ionization. An indicator, e.g. methyl orange, litmus, 

 or phenolphthalein, is used to show when the amount of the base 

 which is being added is just sufficient to neutralize all the H ions. 

 The amount of acid may then be calculated from the required 

 amount of the base. 



