The electrolysis of copper(ii) sulfate solution using inert electrodes
3 marks · NormalDescribe how the appearance and composition of copper(II) sulfate solution change after it is electrolysed for a long period using inert electrodes.
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The blue colour of the solution fades to colourless as copper(II) ions are removed. The solution becomes dilute sulfuric acid, containing mostly H^+ and SO_4^{2-} ions.
Also accept: Solution fades to colourless and contains dilute sulfuric acid
Common mistakes: Saying the colour remains blue; Saying copper(II) ions remain after electrolysis
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Marking points
M1 · 1 mark · States the blue colour fades or disappears
M2 · 1 mark · Mentions copper(II) ions are removed from solution
A1 · 1 mark · States the solution is left as dilute sulfuric acid or contains H^+ and SO_4^{2-} ions
Worked solution
During electrolysis, copper ions are removed from the solution, so the blue colour (caused by Cu^{2+} ions) fades and eventually disappears.
After long electrolysis, the solution is mainly left with H^+ and SO_4^{2-} ions because Cu^{2+} and OH^- are removed at the electrodes.
This means the resulting solution is dilute sulfuric acid, which is colourless.