192 NON-METALS AND THEIR COMBINATIONS. 



Ionic equilibrium. The dissociation of molecules into ions must 

 be considered as a species of chemical change, and, like many others, 

 it is a reversible action (see page 114). In ordinary dilute solutions 

 there is always a certain proportion of undissociated molecules which 

 are in equilibrium with the ions. The degree of dissociation varies 

 with the concentration of the solution, and of course with the nature 

 of the dissolved substance, since for the same concentration different 

 substances vary widely in the amount of dissociation. If the solution 

 is made more concentrated, as by evaporation, more and more of the 

 ions unite to form molecules, until finally, when all the solvent is 

 removed, the dry substance is left entirely in the molecular state. On 

 the other hand, diluting a solution results in more molecules being 

 dissociated. Many substances in highly diluted solutions are almost 

 completely dissociated. The reversible character of dissociation is 

 represented by reversible ionic equations thus : 



HC1 ^ H- -f Cl' ; NaCl ; Na' + Cl' ; NaOH ; Na' -f (OH)'. 



The first equation means that in any given solution of hydrochloric 

 acid there is a certain proportion of molecules and a certain propor- 

 tion of ions. The molecules tend to form ions, but only as fast as 

 the ions tend to revert to the molecular state, so that an equilibrium 

 is maintained. By changing the conditions, ions may be forced to 

 unite to form molecules, or vice versa. In other words, the equilib- 

 rium may be displaced forward or backward. Likewise for the other 

 two equations dealing with sodium chloride and sodium hydroxide 

 respectively. 



Theoretical deductions, as well as experimental results, show that 

 in varying solutions of the same substance there is a constant rela- 

 tionship, which is expressed thus : The product of the concentration of 

 the ions divided by the concentration of the undissociated molecules is a 

 constant quantity (or nearly so in some cases), expressed by a numeral, 

 and called the IONIZATION CONSTANT. The concentration is ex- 

 pressed in terms of the number of molecular weights or ion weights 

 in grammes in a liter of solution. A solution containing one molec- 

 ular weight or ion weight per liter is taken as unit concentration. 



Effect of ionic equilibrium in chemical reactions. Some fa- 

 miliar results. If in any manner, as by precipitation, one kind of 

 the ions of a substance is removed from solution, some molecules of 

 the substance are dissociated to replace the kind of ions removed, 

 until the balance between ions and molecules, as indicated in the 

 lonization constant, is restored. This process may go on until all the 



