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Detailed notes on Electrochemistry for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.
Splitting compounds with electricity. Cations to cathode (−), anions to anode (+). The big examples: molten salt, aqueous solutions, and aqueous halides.
Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).
Pass DC through molten or aqueous ionic compound. Ions migrate to opposite electrodes; products form there.
Setup. A power source pushes current through an electrolyte (molten ionic compound or aqueous solution) via two electrodes:
At each electrode.
Mnemonic. OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
| Electrode | Charge | Attracts | Process |
|---|---|---|---|
| Cathode | Negative (−) | Cations (positive ions) | Reduction — gain of electrons |
| Anode | Positive (+) | Anions (negative ions) | Oxidation — loss of electrons |
Why does this happen? The power source pumps electrons OUT of the anode and IN to the cathode. Ions migrate to balance the charge:
Why solid ionic compound DOESN'T electrolyse: ions are locked in the lattice, can't move.
Inert electrodes (graphite, platinum) don't take part in the reactions; the products form purely from the electrolyte. Reactive electrodes (e.g. copper) participate themselves — used in copper purification.
Metal at cathode, non-metal at anode. Predictable.
Molten lead bromide (PbBr2).
Molten sodium chloride (NaCl).
Industrial use: extracting reactive metals. Aluminium is extracted from molten alumina (Al2O3) — covered in detail in the Extraction of Metals study note.
Tip. "Molten" = no water → only the ionic compound's own ions are present → straightforward predictions.
Water competes! At cathode: less reactive metal first, else H₂. At anode: halide first, else O₂.
At the cathode (Extended rules).
Reactivity guide: Group 1 and 2 metals + Al are MORE reactive than H. Cu, Ag, Au are LESS reactive.
At the anode (Extended rules).
Worked: dilute sulfuric acid.
Worked: concentrated sodium chloride (brine).
Worked: aqueous copper sulfate (with INERT electrodes).
Worked: aqueous copper sulfate (with copper electrodes — purification).
| Electrolyte | At the cathode (−) | At the anode (+) |
|---|---|---|
| Dilute sulfuric acid | H₂ | O₂ |
| Concentrated NaCl (brine) | H₂ | Cl₂ |
| Aqueous CuSO₄, inert electrodes | Cu | O₂ |
| Aqueous CuSO₄, copper electrodes | Cu (pure) | Anode dissolves: Cu → Cu²⁺ + 2e⁻ |
Verbatim phrases and definitions Cambridge mark schemes credit.
Electrolysis is examined every Paper 4 (8-10 marks) — typically a half-equation, a prediction of products, and an industrial application. Examiner reports flag wrong direction of electron flow (electrons go FROM anode → external circuit → cathode, while ions go opposite ways inside the electrolyte) and the cathode/anode polarity confusion.
Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (5.1); 0620/42 Oct/Nov 2024 — Q9 (electrolysis of brine); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.
Step-by-step solutions to past-paper-style questions on electrolysis, written exactly the way a tutor would explain them at the board.
Question
Predict the products at the cathode and anode when molten PbBr2 is electrolysed with inert electrodes, and write a half-equation for each.
Step-by-step solution
Step 1
Molten PbBr2 contains only Pb2+ and Br− ions (no water, so no other ions to consider).
Step 2
Cathode (negative): cations migrate here and are REDUCED. Pb2+ gains 2 electrons to form molten lead.
Pb2++2e−→Pb
Step 3
Anode (positive): anions migrate here and are OXIDISED. Two Br− each lose 1 electron to form bromine.
2Br−→Br2+2e−
Answer
Cathode: lead metal (Pb2++2e−→Pb). Anode: bromine (2Br−→Br2+2e−).
Examiner tip
The compound must be MOLTEN (or aqueous) so the ions are free to move. A solid ionic compound does not conduct.
Question
Define electrolysis and state which way cations move and what happens to them when they reach that electrode.
Step-by-step solution
Step 1
Electrolysis is the breakdown (decomposition) of an ionic compound, when molten or in aqueous solution, by the passage of electricity.
Step 2
Cations are POSITIVE ions, so they are attracted to the CATHODE (the negative electrode).
Step 3
At the cathode the cations GAIN electrons (reduction). The anode is positive, where anions lose electrons (oxidation).
Answer
Electrolysis is the breakdown of an ionic compound (molten or aqueous) by electricity. Cations move to the cathode (negative electrode) and are reduced — they gain electrons.
Examiner tip
Remember 'OIL RIG': Oxidation Is Loss, Reduction Is Gain (of electrons). Reduction happens at the cathode.
Question
Predict the products at each electrode when concentrated NaCl(aq) is electrolysed using inert electrodes, and explain how the solution changes.
Step-by-step solution
Step 1
The solution contains Na+, Cl− and, from water, H+ and OH−.
Step 2
Cathode: choice is Na+ vs H+. Hydrogen is LESS reactive than sodium, so H+ is discharged → hydrogen gas.
2H++2e−→H2
Step 3
Anode: choice is Cl− vs OH−. The halide is CONCENTRATED, so chloride is discharged in preference → chlorine gas.
2Cl−→Cl2+2e−
Step 4
H+ and Cl− are removed, leaving Na+ and OH− — so the solution becomes sodium hydroxide and turns alkaline.
Answer
Cathode: hydrogen. Anode: chlorine. The remaining solution is sodium hydroxide (alkaline).
Examiner tip
If the brine were DILUTE, oxygen would form at the anode instead of chlorine — the halide rule needs the solution to be concentrated.
Question
Predict and explain the products at each electrode when dilute CuSO4(aq) is electrolysed using carbon electrodes.
Step-by-step solution
Step 1
Ions present: Cu2+, SO42−, and H+, OH− from water.
Step 2
Cathode: Cu2+ vs H+. Copper is LESS reactive than hydrogen, so copper is discharged → a red-brown copper layer forms.
Cu2++2e−→Cu
Step 3
Anode: SO42− vs OH−. Sulfate is not discharged; OH− is oxidised → oxygen gas (bubbles).
4OH−→O2+2H2O+4e−
Step 4
As Cu2+ is removed at the cathode and H+ builds up, the blue colour fades and the solution turns more acidic (sulfuric acid forms).
Answer
Cathode: copper (red-brown deposit). Anode: oxygen. The blue solution fades and becomes acidic.
Examiner tip
Because the electrodes are INERT (carbon), copper from the solution is not replaced, so the blue colour fades. Compare this with copper electrodes, where the colour stays the same.
Question
Explain, with half-equations, what happens at each electrode when CuSO4(aq) is electrolysed using copper electrodes, and why this is used to purify copper.
Step-by-step solution
Step 1
Anode (impure copper): instead of OH− being discharged, the copper electrode itself dissolves — copper atoms lose electrons and go into solution as Cu2+.
Cu→Cu2++2e−
Step 2
Cathode (pure copper): Cu2+ ions are reduced and deposited as pure copper, so the cathode gains mass.
Cu2++2e−→Cu
Step 3
Cu2+ enters solution at the anode at the same rate as it is deposited at the cathode, so the blue colour and concentration stay roughly constant.
Step 4
Impurities (e.g. silver, gold) do not dissolve and drop off as 'anode sludge'; less reactive metals stay in solution. So pure copper builds up on the cathode.
Answer
Anode (impure Cu) dissolves: Cu→Cu2++2e−. Cathode (pure Cu) gains pure metal: Cu2++2e−→Cu. Impurities fall as anode sludge — this purifies the copper.
Examiner tip
Active copper electrodes break the normal anode discharge rule: the electrode reacts instead of OH− or sulfate. The cathode half-equation is the same as for carbon electrodes.
Question
Describe how you would electroplate a steel spoon with silver, identifying the electrolyte and the electrode reactions.
Step-by-step solution
Step 1
Make the object to be plated (the steel spoon) the CATHODE, and use a bar of pure silver as the ANODE.
Step 2
Use an electrolyte that contains the plating-metal ions, e.g. a solution of a soluble silver salt (Ag+ ions).
Step 3
At the anode, silver dissolves to replace the ions used up:
Ag→Ag++e−
Step 4
At the cathode (the spoon), Ag+ ions are reduced and deposited as a thin, even layer of silver:
Ag++e−→Ag
Answer
Spoon = cathode, silver bar = anode, electrolyte = a silver salt solution. Cathode: Ag++e−→Ag (silver coats the spoon). Anode: Ag→Ag++e− (replenishes the ions).
Examiner tip
Two reasons for electroplating: to improve appearance and to protect the metal underneath from corrosion. The OBJECT is always the cathode.
High-scoring sample answers for electrolysis on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State what is meant by the term electrode. (1 mark)
Model answer
An electrode is an electrical conductor (rod or plate) that carries the current into and out of the electrolyte.
Why this scores
One mark for 'conductor that carries the current into/out of the electrolyte'. The anode is positive and the cathode is negative.
Molten potassium iodide, KI, is electrolysed using inert electrodes. State the product at the cathode and the product at the anode. (2 marks)
Model answer
At the cathode, potassium metal is produced (K+ ions gain electrons). At the anode, iodine is produced (I− ions lose electrons).
Why this scores
Two marks: potassium at the cathode; iodine at the anode. In a MOLTEN compound there is no water, so the metal is always discharged — even a reactive metal like potassium.
Concentrated hydrochloric acid is electrolysed. Write the half-equation for the reaction at the cathode and explain why this product is formed. (3 marks)
Model answer
The half-equation at the cathode is 2H++2e−→H2. Hydrogen ions move to the negative cathode and are reduced (they gain electrons). Hydrogen is discharged because it is the only suitable cation present, forming hydrogen gas.
Why this scores
Three marks: correct balanced half-equation (1); idea that H+ is reduced / gains electrons (1); product is hydrogen gas (1). Electrons must be on the LEFT for a cathode (reduction).
Dilute sulfuric acid is electrolysed using inert electrodes. Name the products at each electrode and explain why these products form. (4 marks)
Model answer
At the cathode, hydrogen is produced because H+ ions are the only cations and are reduced (2H++2e−→H2). At the anode, oxygen is produced: the solution is not a concentrated halide, so OH− ions are discharged instead of sulfate (4OH−→O2+2H2O+4e−). Overall this is effectively the electrolysis of water, so the volume of hydrogen collected is twice the volume of oxygen.
Why this scores
Four marks: hydrogen at cathode; oxygen at anode; reason (OH− discharged because not a concentrated halide / sulfate stays in solution); and the 2:1 volume ratio is a strong final mark.
Aqueous copper(II) sulfate is electrolysed using copper electrodes. Describe what you would observe at each electrode and what happens to the colour of the solution. Explain your answers. (5 marks)
Model answer
At the cathode, a layer of red-brown copper is deposited, so the cathode gains mass, as Cu2+ ions are reduced (Cu2++2e−→Cu). At the anode, the copper electrode dissolves and loses mass, because copper atoms are oxidised and go into solution (Cu→Cu2++2e−). The blue colour of the solution stays the same, because Cu2+ ions are added to the solution at the anode at the same rate as they are removed at the cathode, so their concentration does not change.
Why this scores
Five marks: copper deposited / cathode gains mass; cathode half-equation; anode dissolves / loses mass; anode half-equation; blue colour unchanged WITH the reason (ions replaced as fast as removed).
Describe how impure copper can be purified by electrolysis, and explain why this method gives very pure copper. (6 marks)
Model answer
Use a piece of impure copper as the anode and a piece of pure copper as the cathode, dipped in a solution of a copper(II) salt (such as copper(II) sulfate) as the electrolyte. When current flows, the anode dissolves as copper atoms are oxidised: Cu→Cu2++2e−. These Cu2+ ions move through the solution to the cathode, where they are reduced and deposited as pure copper: Cu2++2e−→Cu, so the cathode grows. The method is very pure because less reactive impurities such as silver and gold do not dissolve and instead fall to the bottom as anode sludge, while more reactive metal impurities stay dissolved in the solution and are not discharged. Only copper is transferred from anode to cathode, so the copper deposited is almost 100% pure.
Why this scores
Six marks across: impure Cu = anode and pure Cu = cathode; copper(II) salt electrolyte; anode half-equation (anode dissolves); cathode half-equation (pure Cu deposited); impurities fall as anode sludge; more reactive impurities remain in solution. Naming the half-equations earns the technical marks.
The formulae you need to memorise for electrolysis on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.
Metal discharged if it is LESS reactive than hydrogen (e.g. Cu, Ag); otherwise H2
When to use
Predicting the cathode product in the electrolysis of an aqueous solution.
Halogen if halide is concentrated; otherwise 4OH−→O2+2H2O+4e−
When to use
Predicting the anode product in aqueous electrolysis with inert electrodes.
Definitions to memorise and the exact keywords mark schemes credit for electrolysis answers — sharpened from recent examiner reports for the 2026 0620 sitting.
The breakdown (decomposition) of an ionic compound, when molten or in aqueous solution, by the passage of electricity.
A molten ionic compound or an aqueous solution of ions that conducts electricity and is decomposed by it (contains free-moving ions).
An electrical conductor that carries current into the electrolyte. The anode is the positive electrode (oxidation occurs); the cathode is the negative electrode (reduction occurs).
An electrode that does not react during electrolysis (e.g. graphite/carbon or platinum), unlike an active electrode such as copper.
Coating an object (made the cathode) with a thin layer of metal by electrolysis, to improve its appearance or protect it from corrosion.
The traps other students keep falling into on electrolysis questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.
0620/42 — recurring
Why it happens
Confusing the direction of electron transfer.
How to avoid it
Anode → anions LOSE electrons (oxidation, electrons on the RIGHT). Cathode → cations GAIN electrons (reduction, electrons on the LEFT). Use 'OIL RIG'.
Why it happens
Reading the formula directly and ignoring the water.
How to avoid it
In AQUEOUS solution, hydrogen is discharged unless the metal is LESS reactive than hydrogen (e.g. Cu, Ag). Reactive metals are only discharged from MOLTEN compounds.
Why it happens
Applying the halide rule without checking concentration.
How to avoid it
A halogen forms only if the halide is CONCENTRATED. From dilute solutions (or with sulfate/nitrate), oxygen forms from OH− instead.
Why it happens
Forgetting to count electrons or balancing only atoms.
How to avoid it
Check that the total charge is equal on both sides. The number of electrons must make the charges balance, e.g. 2Br−→Br2+2e− (each side overall neutral).
The things students keep getting wrong in this sub-topic, answered.