Electrolysis questions in IGCSE Chemistry (0620) Paper 4 test a small set of ideas over and over: what electrolysis is, which product forms at each electrode, the ionic half-equation for that product, and how aluminium is extracted. The marks go to precise words. "Inert", "ions", "molten or aqueous" and a balanced half-equation with electrons on the correct side earn marks, while answers that are roughly right but vague do not. This post works through what recent mark schemes credit, then gives you three real Paper 4 questions to try.
What the mark schemes reward
The same marking points come up across papers, so it pays to learn them in the examiners' words.
The definition. In Paper 42, May/June 2025, the definition was a fill-in-the-gaps worth three marks: "ionic", "molten" and "in aqueous solution". When the question just says "state what is meant by the term electrolysis", the scheme splits it into two marks: breakdown by the passage of electricity, and of an ionic compound in molten or aqueous state. Leaving out "ionic" or "molten or aqueous" costs a mark.
Why graphite is used. The two credited reasons are that graphite is inert and that it is a good conductor of electricity. In Paper 41, May/June 2023, a single mark needed both: "graphite is inert AND graphite conducts electricity". Always give both.
What carries the charge. Electrons carry charge in the wires. Ions carry it in the electrolyte. Mixing these up is one of the cheapest marks to lose.
Products at the electrodes. These are the combinations recent schemes have credited:
| Electrolyte (graphite or platinum electrodes) | Cathode | Anode |
|---|---|---|
| Dilute aqueous sodium chloride | hydrogen | oxygen |
| Concentrated aqueous sodium chloride | hydrogen | chlorine |
| Dilute aqueous potassium bromide | hydrogen | oxygen |
| Aqueous copper(II) sulfate | copper | oxygen |
Notice that concentration changes the anode product for a halide solution. A question that says "dilute" where the previous part said "concentrated" is testing exactly that.
Ionic half-equations
Half-equations carry two or three marks each, and the schemes award part marks for the right species.
At the cathode, hydrogen forms from hydrogen ions gaining electrons:
$$2\text{H}^+ + 2e^- \rightarrow \text{H}_2$$The first mark is for \text{H}^+ and electrons on the left-hand side. The second is for the equation being fully correct.
At the anode, oxygen forms from hydroxide ions losing electrons:
$$4\text{OH}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4e^-$$In Paper 42, May/June 2025, this was worth three marks. One mark was for a negatively charged OH species losing electrons, or water and oxygen as the only products. A second needed both of those. The third was for the correct equation. So even an unbalanced attempt with the right species on the right sides can score.
Inert electrodes against copper electrodes
Copper(II) sulfate is a favourite because changing the electrodes changes what you see. The marking points below come from Paper 41, Oct/Nov 2024, Paper 43, Oct/Nov 2024 and Paper 42, May/June 2025.
| Graphite or platinum electrodes | Copper electrodes | |
|---|---|---|
| Cathode | pink solid (copper), mass increases | copper forms, mass increases |
| Anode | bubbles (oxygen) | anode dissolves, mass decreases |
| Electrolyte | becomes paler blue or colourless | no change |
The "no change" in colour with copper electrodes is credited as an answer on its own. The reason is that copper leaves the anode as \text{Cu}^{2+} ions at the same rate as copper is deposited at the cathode. The scheme for the anode losing mass accepted "copper OR anode forms Cu2+ which goes into the solution".
Extracting aluminium
The aluminium parts are mostly one-word recall, and they repeat:
- The ore is bauxite.
- The compound in it is aluminium oxide.
- It is dissolved in molten cryolite. Paper 41, May/June 2023 asked for two reasons for using cryolite and accepted any two of: solvent, lowers the operating temperature, increases conductivity.
- The carbon anodes must be replaced because they react with oxygen and form carbon dioxide. When this is worth two marks, one is for reacting with oxygen and one is for carbon dioxide.
Worked walk-through: Paper 41, May/June 2026, Question 5
This question puts most of the topic into 15 marks. It opens with dilute aqueous sodium chloride electrolysed with graphite electrodes, producing oxygen at the anode and hydrogen at the cathode.
(a)(i) Name the process. The mark is for "electrolysis".
(a)(ii) Two reasons graphite is suitable, two marks: inert, and a good conductor of electricity. "It is cheap" or "it is strong" earns nothing.
(a)(iii) The half-equation for hydrogen at the cathode, two marks. Write 2\text{H}^+ + 2e^- \rightarrow \text{H}_2. The first mark is for \text{H}^+ and electrons on the left, the second for the whole equation.
(a)(iv) Explain why oxygen is produced at the anode, two marks. This is where students lose most. "Because it is the anode" repeats the question. The scheme wants two linked ideas: the hydroxide ion is negative, and it is attracted to the positive anode. Name the ion and say why it moves there.
(a)(v) The particle that carries charge in the external wires: electrons.
(a)(vi) The products from concentrated aqueous sodium chloride, two marks: chlorine at the anode and hydrogen at the cathode. Writing oxygen here, copied from part (a), is the trap.
(b) Aluminium extraction, five marks across four parts: bauxite, aluminium oxide, molten cryolite, then the anodes reacting with oxygen to form carbon dioxide.
Try it yourself before reading the scheme again. Write your answers by hand or type them, and the answer is marked against the official mark scheme point by point.
Common ways marks are lost
- Half-equations with electrons on the wrong side. Reduction happens at the cathode, so electrons are gained on the left. Oxidation happens at the anode, so electrons appear on the right.
- Forgetting the state in the definition. "Breakdown of a compound by electricity" misses both "ionic" and "molten or aqueous".
- One reason for graphite when the question asks for two. Inert and conducts are both needed.
- Describing copper electrodes as if they were inert. With copper electrodes the colour stays the same and no bubbles form at the anode.
- Saying ions carry charge in the wires. Electrons do. Ions carry it in the solution.
- Vague observations. "Bubbles" earns the mark for the anode. "A reaction happens" does not.
Two more real questions to try
Paper 42, May/June 2025, Question 3 is entirely about electrolysis. It starts with the three-gap definition, then runs aqueous copper(II) sulfate first with platinum electrodes and then with copper electrodes, so you have to say how the cathode mass, the electrolyte colour and the anode differ each time. It includes the three-mark oxygen half-equation and ends with aluminium extraction.
Paper 41, May/June 2023, Question 5 uses concentrated aqueous potassium bromide, then asks for the products when the solution is dilute instead. It also covers the hydrogen half-equation, the cryolite reasons and the overall equation for a hydrogen-oxygen fuel cell, which sits next to electrolysis in the syllabus.
You do not need an account to try these: as a guest, each question part can be marked twice for free. For how to read the marking points you get back, see how Cambridge mark schemes work.
Practise more electrolysis questions
The free 0620 electrolysis topical page gathers 118 electrolysis questions from 78 papers, from 2023 to 2026, with each one linked to its paper. Most are multiple choice from Papers 1 and 2, and the structured ones from Paper 4 are the ones to write out in full. To work through a whole paper instead, open the IGCSE Chemistry past papers and pick a Paper 4.
Frequently asked questions
- How do I know which product forms at each electrode in IGCSE Chemistry?
- Start with the electrolyte. A molten binary compound gives the metal at the cathode and the non-metal at the anode. In aqueous solutions water is also present, so hydrogen can form at the cathode and oxygen at the anode. Recent 0620 mark schemes credit hydrogen and oxygen for dilute sodium chloride, but hydrogen and chlorine for concentrated sodium chloride.
- Why are graphite electrodes used in electrolysis?
- In 0620 mark schemes the two creditable reasons are that graphite is inert and that it conducts electricity. One 2023 question gave a single mark only when both reasons were present, so give both every time.
- What is the half-equation at the anode when oxygen is made?
- Hydroxide ions lose electrons: 4OH- gives O2 plus 2H2O plus 4e-. Mark schemes give a first mark for negative hydroxide ions losing electrons and further marks for oxygen and water as the only products and a balanced equation.
- Why do the carbon anodes need replacing in aluminium extraction?
- The carbon anodes react with the oxygen produced at them and form carbon dioxide, so they wear away. Recent mark schemes give one mark for the carbon reacting with oxygen and one for carbon dioxide forming.