Electrolysis
Cambridge IGCSE Chemistry (0620) · Unit 4: Electrochemistry · 17 flashcards
Electrolysis is topic 4.1 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 4 — Electrochemistry , alongside Electrolysis of specific substances and Hydrogen-oxygen fuel cells. In one line: Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical). Past papers from 2022 to 2025 confirm this is a high-yield topic: Cambridge has set undefined questions worth 440 marks here (about 7.0% of all Chemistry marks across those years).
The deck below contains 17 flashcards — 3 definitions, 9 key concepts and 5 identification cards — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.
Electrolysis
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
What the Cambridge 0620 syllabus says
Official 2026-2028 specThese are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.
- Define Define electrolysis as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current
- Identify Identify in simple electrolytic cells: (a) the anode as the positive electrode (b) the cathode as the negative electrode (c) the electrolyte as the molten or aqueous substance that undergoes electrolysis
- Describe Describe the transfer of charge during electrolysis to include: (a) the movement of electrons in the external circuit (b) the loss or gain of electrons at the electrodes (c) the movement of ions in the electrolyte Supplement
- Identify Identify the products formed at the electrodes and describe the observations made during the electrolysis of: (a) molten lead(II) bromide (b) concentrated aqueous sodium chloride (c) dilute sulfuric acid using inert electrodes made of platinum or carbon/graphite
- State State that metals or hydrogen are formed at the cathode and that non-metals (other than hydrogen) are formed at the anode
- Predict Predict the identity of the products at each electrode for the electrolysis of a binary compound in the molten state
- State State that metal objects are electroplated to improve their appearance and resistance to corrosion
- Describe Describe how metals are electroplated
- Identify Identify the products formed at the electrodes and describe the observations made during the electrolysis of aqueous copper(II) sulfate using inert carbon/graphite electrodes and when using copper electrodes Supplement
- Predict Predict the identity of the products at each electrode for the electrolysis of a halide compound in dilute or concentrated aqueous solution Supplement
- Construct Construct ionic half-equations for reactions at the anode (to show oxidation) and at the cathode (to show reduction) Supplement
Define electrolysis.
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
In an electrolytic cell, what is the anode and what is its charge?
The anode is the positive electrode in an electrolytic cell. It's where oxidation (loss of electrons) occurs.
In an electrolytic cell, what is the cathode and what is its charge?
The cathode is the negative electrode in an electrolytic cell. It's where reduction (gain of electrons) occurs.
What is the electrolyte in electrolysis?
The electrolyte is the molten or aqueous substance that undergoes electrolysis. It contains ions that are free to move and conduct electricity. An example is molten lead(II) bromide or aqueous sodium chloride.
Describe the movement of charge during electrolysis in the external circuit.
Electrons move in the external circuit from the power supply to the cathode and from the anode back to the power supply. This flow of electrons constitutes the electric current. These electrons are gained or lost by ions at the electrodes.
Describe the movement of charge during electrolysis within the electrolyte.
Ions move in the electrolyte. Positive ions (cations) move towards the cathode, and negative ions (anions) move towards the anode. This movement of ions carries the charge through the electrolyte.
What are the products of electrolysis of molten lead(II) bromide using inert electrodes?
At the cathode (negative electrode), molten lead is formed. At the anode (positive electrode), bromine gas is formed. Observation: Brown fumes are produced at the anode.
What are the products of electrolysis of concentrated aqueous sodium chloride using inert electrodes?
At the cathode (negative electrode), hydrogen gas is formed. At the anode (positive electrode), chlorine gas is formed. Observation: Bubbles of gas produced at both electrodes, pungent smell at the anode. The remaining solution is sodium hydroxide.
What are the products of electrolysis of dilute sulfuric acid using inert electrodes?
At the cathode (negative electrode), hydrogen gas is formed. At the anode (positive electrode), oxygen gas is formed. Observation: Bubbles of gas are produced at both electrodes. More hydrogen is produced than oxygen.
What type of substance is formed at the cathode during electrolysis?
Metals or hydrogen are formed at the cathode during electrolysis. This is because positive metal ions or H+ ions are attracted to the negative electrode and gain electrons (are reduced).
What type of substance is formed at the anode during electrolysis?
Non-metals (other than hydrogen) are formed at the anode during electrolysis. This is because negative non-metal ions are attracted to the positive electrode and lose electrons (are oxidized).
What is electroplating, and why is it done?
Electroplating is the process of coating a metal object with a thin layer of another metal using electrolysis. It is done to improve the appearance of the object and to increase its resistance to corrosion.
Write the ionic half-equations for the electrolysis of molten lead bromide.
At the cathode (reduction): Pb²⁺ + 2e⁻ → Pb
At the anode (oxidation): 2Br⁻ → Br₂ + 2e⁻
Lead is deposited as a silvery metal at the cathode. Bromine is released as a brown/orange gas at the anode.
Remember: cathode = reduction (gain electrons), anode = oxidation (lose electrons). The mnemonic is OILRIG (Oxidation Is Loss, Reduction Is Gain).
Why must an ionic compound be molten or dissolved in water for electrolysis to occur?
In a solid ionic compound, the ions are held in fixed positions in a lattice and cannot move. For electrolysis to work, the ions must be free to move towards the electrodes. Melting or dissolving in water breaks the lattice, allowing ions to move: cations migrate to the cathode and anions migrate to the anode.
What products form at each electrode during the electrolysis of concentrated sodium chloride solution (brine)?
At the cathode: hydrogen gas (H₂) — from the water, not sodium, because sodium is too reactive
At the anode: chlorine gas (Cl₂) — from the chloride ions
Sodium hydroxide (NaOH) remains in solution.
Test: hydrogen gives a squeaky pop with a lit splint; chlorine bleaches damp litmus paper.
Describe how to electroplate a steel spoon with silver.
1. The steel spoon is the cathode (negative electrode)
2. A piece of pure silver is the anode (positive electrode)
3. The electrolyte is silver nitrate solution (contains Ag⁺ ions)
4. When current flows, Ag⁺ ions move to the cathode and are deposited on the spoon
5. At the anode, silver atoms dissolve to replace the Ag⁺ ions in solution
This gives the spoon a thin, even coating of silver for improved appearance and corrosion resistance.
During the electrolysis of aqueous copper sulfate with copper electrodes, why does the mass of the anode decrease?
At the anode (copper electrode), copper atoms lose electrons and dissolve into the solution as Cu²⁺ ions:
Cu → Cu²⁺ + 2e⁻
This is oxidation, which removes copper from the anode, decreasing its mass. At the same time, Cu²⁺ ions from the solution gain electrons at the cathode and are deposited as copper, increasing its mass. The concentration of the solution stays the same.
Key Questions: Electrolysis
Define electrolysis.
Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
What is the electrolyte in electrolysis?
The electrolyte is the molten or aqueous substance that undergoes electrolysis. It contains ions that are free to move and conduct electricity. An example is molten lead(II) bromide or aqueous sodium chloride.
What is electroplating, and why is it done?
Electroplating is the process of coating a metal object with a thin layer of another metal using electrolysis. It is done to improve the appearance of the object and to increase its resistance to corrosion.
Tips to avoid common mistakes in Electrolysis
- ● Name the charge carriers in each case: metals (electrons), ionic solutions (ions), covalent compounds (usually none).
- ● Master electrolysis of salts by drawing a diagram, labeling the ions present, and writing the half-equations at each electrode.
- ● When labelling electrolysis diagrams, double-check that your labels point to the correct part (anode, cathode, electrolyte). Review the process.
- ● When answering an electrolysis question, read carefully and be sure you're giving the product formed (e.g., hydrogen at the cathode) and not just naming the electrodes.
- ● During electrolysis, double-check the substance's state AND whether the product would react with water in aqueous solution.
More topics in Unit 4 — Electrochemistry
Electrolysis sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Key terms covered in this Electrolysis deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
Related Chemistry guides
Long-read articles that go beyond the deck — cover the whole subject's common mistakes, high-yield content and revision pacing.
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