]96 NON-METALS AND THEIR COMBINATIONS. 



is designed to collect gases. The electrodes are made of materials which are 

 not affected by the products that collect on them. Platinum plates are often 

 used. 



The result of passing a current through a solution of an electrolyte is a 

 chemical decomposition. The products that appear at the electrodes are always 

 different. For illustration, one of the simplest cases of electrolysis may be 

 considered, namely, the decomposition of hydrochloric acid. When the appa- 

 ratus (Fig. 34) is filled with the concentrated acid and a current is turned on, 

 hydrogen gas is found to collect and rise into the tube from the cathode or 

 ( ) electrode, and chlorine gas collects in the other tube from the anode or 

 (+) electrode. The mechanism of the process is conceived to be as follows : 

 The solution of the acid contains some ions of hydrogen (H') and of chlorine 

 (Cl'), the presence of which is entirely independent of the electric current. 

 These ions, before the current is turned on, are attracted no more in one direc- 

 tion than in any other. When the current is turned on, one electrode receives 

 from the battery or dynamo, or whatever the source of electricity, a positive 

 charge, and the other receives a negative charge, and a constant difference of 

 voltage or electromotive force is maintained between the electrodes. If the 

 latter should be connected by a continuous conductor, as a piece of copper wire, 

 a current would flow from the positive to the negative electrode by their dis- 

 charge through the wire. But the source of electricity would constantly renew 

 the charges on the electrodes, and thus a continuous current would be kept up. 

 When the connecting wire is replaced by hydrochloric acid, the positive elec- 

 trode attracts the negatively charged chlorine ions and repels the positive 

 hydrogen ions, while the negative electrode attracts the positive hydrogen ions 

 and repels the negative chlorine ions. In this way there is a general move- 

 ment of all positive ions to one electrode, and of all negative ions to the other. 

 When the ions come in contact with the electrodes, they lose their charges by 

 neutralizing the charges of opposite kind on the electrodes. The discharged 

 ions then unite and pass off as molecules of hydrogen and chlorine respect- 

 ively. The discharged electrodes receive new charges as before, and the pro- 

 cess is repeated until all of the electrolyte is removed from the solution. The 

 effect as far as the conduction of the current is concerned is the same as if a 

 wire connected the electrodes, and the current flowed through a circuit entirely 

 metallic. In the light of this ionic explanation of electrolysis, we can under- 

 stand why solutions of substances which do not dissociate into ions like sujrar 

 do not conduct electricity, and why substances which conduct in aqueous solu- 

 tion do not conduct in solvents in which they are not dissociated. 



Secondary changes in electrolysis. In the electrolysis of hydrochloric 



id the products liberated consist of the same elements of which the ions are 



constituted, but the majority of cases are not so simple as this one. Many ions 



are atomic groups which are not known in the free state, but exist only as ions 



solution. When these lose their charges, secondary chemical changes occur 



the resulting products accumulate around the electrodes. Thus in the 



case^of sulphuric acid, the H' ions become molecules of hydrogen gas, but the 



>0 4 ion when discharged becomes a group not known in the free state It 



reacts with water thus : H 2 O + SO 4 = H 2 SO 4 + O. Sulphuric acid accumu- 



ound the positive electrode, but is gradually disseminated again through 



