JAMB - Chemistry (1978 - No. 16)
During the electrolysis of copper II sulphate between platinum electrodes, if litmus solution is added to the anode compartment,
the litmus turns blue but no gas is evolved
the litmus turns blue and oxygen is evolved
the litmus turns blue and hydrogen is evolved
the litmus turns red and oxygen is evolved
the litmus turns red and then becomes colourless
Explication
Cu\(^+\) is discharged preferentially at the cathode
OH\(^-\) is discharged preferentially at the anode
The presence of SO\(_4\)\(^2-\) and H\(^+\) makes the medium acidic
Which makes the litmus turns red and oxygen is evolved