IGCSE Chemistry: Electrochemistry and Electrolysis Practice Questions
Electrolysis is the breakdown of an ionic compound, when molten or in aqueous solution, by the passage of electricity. Positive ions move to the negative cathode and gain electrons, while negative ions move to the positive anode and lose electrons.
Topic 4 of Cambridge IGCSE Chemistry 0620 is worth a lot of marks and is highly predictable, because the same four electrolysis examples recur. Half equations are where marks are gained or lost, so the questions below make you write them out in full each time.
What you need to know for Electrochemistry and Electrolysis
- ElectrolyteA molten or aqueous ionic compound that conducts electricity and is broken down by it. The ions must be free to move, which is why solid ionic compounds cannot be electrolysed.
- Cathode and anodeThe cathode is the negative electrode. Positive ions, cations, move to it and gain electrons, so reduction happens there. The anode is the positive electrode. Negative ions, anions, move to it and lose electrons, so oxidation happens there.
- Molten lead(II) bromideLead is formed at the cathode as a molten metal, and brown bromine vapour is formed at the anode.
- Concentrated aqueous sodium chlorideHydrogen is formed at the cathode, chlorine at the anode, and sodium hydroxide remains in solution.
- Dilute sulfuric acidHydrogen at the cathode and oxygen at the anode, in a volume ratio of two to one, because water is effectively being decomposed.
- ElectroplatingThe object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal. Used to improve appearance and to resist corrosion.
IGCSE Chemistry Electrochemistry and Electrolysis questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Molten lead(II) bromide, PbBr2, is electrolysed using inert electrodes. Name the product at each electrode and write the half equation for each.
Show the worked answer
- Molten lead(II) bromide contains Pb2+ ions and Br- ions that are free to move.
- Pb2+ ions are positive, so they move to the negative cathode and gain electrons. Molten lead metal forms: Pb2+ + 2e- gives Pb.
- Br- ions are negative, so they move to the positive anode and lose electrons. Brown bromine vapour forms: 2Br- gives Br2 + 2e-.
- Reduction happens at the cathode because electrons are gained, and oxidation happens at the anode because electrons are lost.
Concentrated aqueous sodium chloride is electrolysed using inert electrodes. Name the product at each electrode, give one test for the gas formed at the anode, and name the substance left in solution.
Show the worked answer
- The solution contains Na+, H+, Cl- and OH- ions.
- At the cathode, hydrogen ions are discharged in preference to sodium ions because hydrogen is below sodium in the reactivity series, so hydrogen gas is produced.
- At the anode, chloride ions are discharged in preference to hydroxide ions because the solution is concentrated, so chlorine gas is produced. Chlorine bleaches damp blue litmus paper, turning it white.
- Sodium ions and hydroxide ions remain in solution, so the solution left behind is sodium hydroxide.
Describe how a steel spoon could be electroplated with silver, naming the electrodes and the electrolyte.
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- Connect the steel spoon as the cathode, the negative electrode.
- Use a bar of pure silver as the anode, the positive electrode.
- Use an electrolyte containing silver ions, such as silver nitrate solution.
- Silver ions in solution move to the spoon and gain electrons, depositing a layer of silver on it. At the same time the silver anode dissolves, replacing the silver ions used up, so the concentration of the electrolyte stays constant.
Give two advantages of a hydrogen oxygen fuel cell compared with a petrol engine.
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- A hydrogen oxygen fuel cell produces electricity from hydrogen and oxygen, and the only product is water.
- There is therefore no carbon dioxide, carbon monoxide or nitrogen oxide emitted at the point of use, so it does not contribute to local air pollution or to the enhanced greenhouse effect.
- Fuel cells convert a higher proportion of the chemical energy into useful energy than a petrol engine, which loses a large fraction as thermal energy.
- The main drawbacks are that hydrogen is difficult and hazardous to store and transport, and that manufacturing hydrogen often uses energy from fossil fuels.
Common mistakes in this topic
- Mixing up the electrodes. Cathode is negative and attracts cations. Anode is positive and attracts anions.
- Forgetting to balance half equations for diatomic gases such as Cl2, Br2, H2 and O2.
- Putting electrons on the wrong side. At the cathode electrons are added to the left. At the anode they appear on the right.
- Saying a solid ionic compound can be electrolysed. The ions must be free to move.
- Predicting the metal as the cathode product in aqueous solution when the metal is more reactive than hydrogen.
Exam tips
- Learn the mnemonic: reduction at the cathode, oxidation at the anode. Cathode and reduction both involve gain of electrons.
- Write out every ion present in solution before predicting products. In aqueous solutions remember H+ and OH- from water.
- Check every half equation balances for both atoms and charge.
- For electroplating, the object is always the cathode. Fix that one fact and the rest follows.
- In copper purification, the impure copper is the anode and the pure copper is the cathode, and impurities collect as anode sludge.
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Electrochemistry and Electrolysis FAQs
What happens at the cathode and anode during electrolysis?
At the cathode, the negative electrode, positive ions gain electrons and are reduced. At the anode, the positive electrode, negative ions lose electrons and are oxidised. Metals and hydrogen form at the cathode, and non-metals such as chlorine, bromine and oxygen form at the anode.
Why can solid ionic compounds not be electrolysed?
Electrolysis requires ions that are free to move to the electrodes. In a solid ionic lattice the ions are held in fixed positions and cannot move, so no current flows and no reaction occurs. Melting the compound or dissolving it in water releases the ions.
Why is hydrogen produced instead of sodium in aqueous sodium chloride?
The solution contains both sodium ions and hydrogen ions from water. The less reactive of the two is discharged at the cathode. Hydrogen is below sodium in the reactivity series, so hydrogen ions gain electrons and hydrogen gas is released instead of sodium metal being deposited.
How does electroplating work?
The object to be plated is connected as the cathode, the plating metal as the anode, and the electrolyte contains ions of the plating metal. Metal ions move to the cathode and are deposited as a thin layer, while the anode dissolves to replace them, keeping the electrolyte concentration constant.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.