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IONIZATION 183 



the negative electrode, or cathode. The negative ions are the 

 anions, and move toward the positive electrode, or anode. 



We can usually tell which is the positive radical in a formula 

 because it generally consists of one atom of a metallic element 

 (K + , Cu++, etc.) or of hydrogen. The negative radical may con- 

 tain a metal, like the Mn in K(Mn0 4 ), but always along with a 

 non-metal like oxygen. 



Ions and Electrolysis. Let us first attempt to understand 

 the phenomena of electrolysis in the light of the ionic hypothesis. 

 A solution of HC1 in water contains three kinds of solute; un- 

 dissociated HC1 molecules, posi- 

 tively charged hydrogen ions H + , * - H + 

 and negatively charged chlorine 

 ions Cl~. These are scattered 

 indiscriminately throughout the 

 solution, as indicated in Fig. 60 

 (only the two ions are there 

 shown, the undissociated part is ' ' 

 omitted since it plays no direct 



part in the electrolysis). Equilibrium is kept up by continual 

 dissociation and recombination, according to the equation 

 HC1 <=* H+ + C1-. 



As soon as the circuit is completed, all the ions in the solution 

 begin to migrate towards their proper electrodes. The positively- 

 charged hydrogen ions are attracted towards the negative elec- 

 trode, the negatively-charged chlorine ions are attracted towards 

 the positive electrode. Two orderly processions of ions, moving 

 in opposite directions, proceed therefore through the solution. 

 Ions (Greek, going) derive their name from this fact. 



The rest is easily understood. When a positive ion reaches 

 and touches the negative electrode, its positive charge of elec- 

 tricity is neutralized, and the result is an ordinary atom of hydro- 

 gen. The atoms of free hydrogen unite to give molecules (H 2 ) 



