xin CHEMICAL CHANGE BY ELECTRIC CURRENT 203 



Electrolysis of Copper Sulphate. EXPT. 195. Make a 

 strong solution of copper sulphate (blue vitriol) in water. Pour 

 some into a beaker. Dip two platinum plates connected by 

 copper wires, as previously described (Expt. 194), into the 

 solution and notice that after a few minutes a deposit of copper 

 is found on the kathode, and that bubbles of gas (which if col- 

 lected and tested are found to be oxygen) are seen to rise 

 from the anode. 



The passage of the electric current through copper sulphate is 

 most easily expressed by means of chemical symbols, and we 

 shall consequently suppose the reader has studied the section 

 dealing with such symbols before attempting to understand this 

 description. Copper sulphate is constituted as shown in the 

 formula CuS0 4 . The effect of the passage of the electric current 

 is to cause it to decompose according to the equation into copper 

 and the group of elements, SO 4 , which is, however, immediately 

 acted upon by the water in which the blue vitriol is dissolved as 

 shown in the second equation : 



CuSO 4 = Cu + SO 4 

 SO 4 + H 2 O=H 2 SO 4 + O. 



Hence we see that there is a continual withdrawal of metallic 

 copper from the solution by the passage of the electric current, 

 and the liquid is gradually converted into sulphuric acid. This 

 can be demonstrated by its effect on a piece of blue litmus 

 paper. 



If instead of having two platinum plates as electrodes we 

 substitute copper plates, we have a slightly different state of 

 affairs. 



EXPT. 196. Arrange the apparatus as described in Experi- 

 ment 195, substituting copper plates for the platinum ones 

 there used, and weigh the copper electrodes before passing 

 the current. After the current has passed for, say, ten minutes, 

 break the circuit and weigh the electrodes again. Notice the 

 anode has lost in weight by a certain amount, and that the 

 kathode has increased in weight by the same amount. There is 

 no evolution of oxygen. 



Secondary Results. The evolution of oxygen which is 

 noticed in Expt. 195 is a result of what is known as a secondary 

 kind. The liberated group of elements, SO 4 , is the first effect of 



