IONIZATION 197 



for crystals of common salt to behave like a mixture of sodium and 

 of chlorine. Metallic sodium Na reacts with water to form a 

 solution of sodium hydroxide. In sodium chloride solution, 

 however, the ionic sodium Na + is already in the same state as it 

 is in sodium hydroxide solution, and is in no need of trying to 

 enter that state. 



2. Salt is a very stable substance. The union of sodium and 

 chlorine evolves a great deal of heat. A great deal of work will 

 be required, therefore, to decompose sodium chloride. How can 

 mere addition of water break it up? We have here the same 

 misunderstanding in another form. It is true that it would be 

 very difficult to decompose sodium chloride into free sodium and 

 free chlorine, but its dissociation into sodium ion and chloride ion 

 is an entirely different question. As a matter of fact, the heat 

 of ionization is extremely small. Sodium chloride is stable only 

 in the solid state. In solution, it reacts with very great facility 

 with many other electrolytes. 



3. Why do not the ions Na+ and Cl~ recombine at once, in re- 

 sponse to the attractions of their charges? The answer is that 

 they do combine. The tendency towards combination is, how- 

 ever, opposed by the tendency of undissociated NaCl (the at- 

 tractive forces between the bound radicals of which are weakened 

 in solution) to decompose into free ions. An equilibrium be- 

 tween the two tendencies is, therefore, set up, which we may 

 express by the reversible reaction NaCl ^ Na + -j- Cl~. 



4. Why can we not separate sodium ions from chloride ions 

 in a solution of sodium chloride before we pass a current through 

 the solution? Does not this show that it is the electric current 

 which breaks up the sodium chloride? The charges on the ions 

 are not derived from the electric current. Free sodium ions and 

 free chloride ions are present in the solution the instant the salt 

 is dissolved, whether a current is passing or not. Before we pass 

 a current through the solution, however, any portion of it of 

 sensible magnitude contains just as many sodium ions as chlo- 



