Core practical: electrolysis of copper sulfate

Core practical 3.31 electrolyses copper sulfate solution, first with inert electrodes and then with copper electrodes. The same idea is used to purify copper.

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With inert (graphite) electrodes

Cathode (−): copper ions gain electrons. A pink-brown layer of copper forms.
Cu2+ + 2e− → Cu
Anode (+): bubbles of oxygen form (there is no halide ion). The blue colour fades as copper ions are removed.
Reduction (gain of electrons) happens at the cathode. Oxidation (loss of electrons) happens at the anode.

With copper electrodes

1. Clean two copper electrodes with emery paper, wash, dry and weigh them.
2. Electrolyse copper sulfate solution at a set current (use an ammeter and variable resistor) for a set time.
3. Wash the electrodes, dry them and reweigh.
Anode: copper atoms lose electrons and go into solution, so it loses mass: Cu → Cu2+ + 2e−
Cathode: copper is deposited, so it gains mass: Cu2+ + 2e− → Cu
The mass gained by the cathode equals the mass lost by the anode. A bigger current or longer time moves more copper.

Purifying copper and other half-equations

In industry, the anode is impure copper and the cathode is pure copper. Copper dissolves from the anode and is deposited on the cathode. Impurities fall to the bottom as sludge.
Other half-equations: 2H+ + 2e− → H2 (cathode); 2Cl− → Cl2 + 2e− (anode); 4OH− → O2 + 2H2O + 4e− (anode).
Worked example

A copper cathode goes from 2.50 g to 2.82 g. What mass should the copper anode have lost?

  1. Mass gained by cathode = 2.82 − 2.50 = 0.32 g
  2. Copper lost from the anode is deposited on the cathode.

Answer: 0.32 g

Key idea

Inert electrodes: copper at the cathode, oxygen at the anode. Copper electrodes: the anode loses mass and the cathode gains the same mass. Reduction at the cathode, oxidation at the anode. Purification uses an impure copper anode and a pure copper cathode.

Key words
Electrolysis
Breaking down an ionic compound, molten or in solution, using electricity.
Solution
A mixture formed when a solute dissolves in a solvent.
Cathode
The negative electrode in electrolysis. Cations gain electrons there (reduction).
Anode
The positive electrode in electrolysis. Anions lose electrons there (oxidation).
Reduction
Loss of oxygen, or gain of electrons.
Oxidation
Gain of oxygen, or loss of electrons.

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Extra practice

2 more questions

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  1. A copper cathode goes from 2.84 g to 3.20 g during electrolysis. What mass of copper was deposited?

    Show the answer

    Answer: 0.36 g

    3.20 − 2.84 = 0.36 g

  2. Which half-equation happens at the anode when copper sulfate is electrolysed with copper electrodes?

    1. 2H+ + 2e− → H2
    2. Cu → Cu2+ + 2e−
    3. 2Cl− → Cl2 + 2e−
    4. Cu2+ + 2e− → Cu
    Show the answer

    Answer: Cu → Cu2+ + 2e−

    The copper anode dissolves, losing electrons.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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