Electrolysis of Aqueous Solutions
Summary: In aqueous electrolysis, the ions discharged depend on the reactivity series and concentration. Water can be oxidized/reduced in competition with dissolved ions. The ion that is discharged at each electrode is the one that is most easily gained/lost. Tags: igcse chemistry electrolysis Created: 2026-07-14 Last Updated: 2026-07-16
Learning Objectives
By the end of this topic, you should be able to:
- Predict the products at the cathode and anode for any given aqueous solution
- Apply the rules for ion competition at electrodes (reactivity series for cations, halide/OH- for anions)
- Write half-equations for reactions at both electrodes
- Explain how concentration affects which ion is discharged (concentrated vs dilute)
- Describe observations for common electrolysis reactions (gas production, colour changes, metal deposition)
Content
1. How Aqueous Electrolysis Differs from Molten Electrolysis
In molten electrolysis, there is only one cation and one anion present — the products are predictable. In aqueous electrolysis, water dissociates slightly:
H2O(l) ⇌ H+(aq) + OH-(aq)
This means four ions are present in the electrolyte:
- From the solute: the cation (e.g., Na+) and anion (e.g., Cl-)
- From water: H+ and OH-
2. Rules for Predicting Electrode Products
At the Cathode (Negative Electrode) — Reduction Occurs
Cations compete to gain electrons. The rule depends on the reactivity series:
| Cation Position | Discharged at Cathode? | Product |
|---|---|---|
| K+, Na+, Ca2+, Mg2+, Al3+ (highly reactive metals) | NO — H+ from water is discharged instead | Hydrogen gas (H2) |
| Zn2+, Fe2+, Sn2+, Pb2+ (moderately reactive) | Sometimes (depends on conditions) | Metal or H2 |
| Cu2+, Ag+, Au+ (low reactivity) | YES — the metal ion is discharged | Metal deposited |
Key rule: The less reactive the metal, the more easily its ions are reduced. H+ is easier to discharge than reactive metal ions (K+, Na+, Ca2+, Mg2+, Al3+), so hydrogen is produced at the cathode when these reactive metals are in solution.
At the Anode (Positive Electrode) — Oxidation Occurs
Anions compete to lose electrons. The rule depends on the type of anion:
| Anion Present | Discharged at Anode? | Product |
|---|---|---|
| Cl-, Br-, I- (halides) | YES in concentrated solution | Halogen gas (Cl2, Br2, I2) |
| Cl-, Br-, I- (halides) | Maybe (competes with OH-) in dilute solution | Halogen gas or O2 |
| SO4 2-, NO3 -, CO3 2-, F- (all others) | NO — OH- from water is discharged instead | Oxygen gas (O2) |
Key rule: If a halide ion is present, it tends to be discharged. If not, OH- is discharged producing oxygen. In dilute halide solutions, OH- can compete and produce oxygen instead.
3. The Effect of Concentration
Concentrated solutions of halides favour the discharge of the halide ion at the anode:
- Concentrated NaCl(aq): Cl- discharged → chlorine gas at anode
- Dilute NaCl(aq): OH- may be discharged instead → oxygen gas at anode
This is because concentration affects the position of ion competition — a higher concentration of a particular ion makes it more likely to be discharged.
4. Half-Equations for Common Electrode Reactions
Cathode Half-Equations (Reduction — gain of electrons)
| Product | Half-Equation |
|---|---|
| Hydrogen gas | 2H+ + 2e- → H2(g) |
| Copper metal | Cu2+ + 2e- → Cu(s) |
| Sodium metal (molten only) | Na+ + e- → Na(s) |
Anode Half-Equations (Oxidation — loss of electrons)
| Product | Half-Equation |
|---|---|
| Chlorine gas | 2Cl- → Cl2(g) + 2e- |
| Bromine liquid | 2Br- → Br2(l) + 2e- |
| Iodine solid | 2I- → I2(s) + 2e- |
| Oxygen gas | 4OH- → O2(g) + 2H2O(l) + 4e- |
Exam tip: The oxygen half-equation at the anode (4OH- → O2 + 2H2O + 4e-) is frequently tested. Memorise it exactly.
5. Specific Examples
Example 1: Electrolysis of Concentrated NaCl(aq) — Brine
| Feature | Detail |
|---|---|
| Ions present | Na+, Cl-, H+, OH- |
| At Cathode (-) | H+ discharged (Na+ too reactive) → H2 gas (bubbles, ‘pop’ with lit splint) |
| At Anode (+) | Cl- discharged (concentrated) → Cl2 gas (yellow-green, bleaches litmus) |
| Ions left in solution | Na+ and OH- → sodium hydroxide (NaOH) formed |
Overall: 2NaCl(aq) + 2H2O(l) → H2(g) + Cl2(g) + 2NaOH(aq)
Example 2: Electrolysis of CuSO4(aq) with Inert Electrodes
| Feature | Detail |
|---|---|
| Ions present | Cu2+, SO4 2-, H+, OH- |
| At Cathode (-) | Cu2+ discharged (low reactivity) → Copper metal deposited (pink/brown solid) |
| At Anode (+) | OH- discharged (SO4 2- is not discharged) → O2 gas (relights glowing splint) |
| Observation | Blue colour of solution fades as Cu2+ ions are removed; cathode gains brown deposit |
Cathode: Cu2+ + 2e- → Cu(s) Anode: 4OH- → O2(g) + 2H2O(l) + 4e-
Example 3: Electrolysis of CuSO4(aq) with Copper Electrodes
This is copper refining (not a typical aqueous electrolysis question but related):
- Cathode: Cu2+ + 2e- → Cu(s) (pure copper plates onto cathode)
- Anode: Cu(s) → Cu2+ + 2e- (impure copper anode dissolves)
- Impurities fall to the bottom as anode sludge
Example 4: Electrolysis of Dilute H2SO4(aq)
| Feature | Detail |
|---|---|
| Ions present | H+, SO4 2-, H+, OH- (essentially H+ and OH- / SO4 2-) |
| At Cathode (-) | H+ discharged → H2 gas |
| At Anode (+) | OH- discharged (SO4 2- not discharged) → O2 gas |
| Net result | Electrolysis of water: 2H2O(l) → 2H2(g) + O2(g) |
| Volume ratio | 2 volumes H2 : 1 volume O2 (H2 collected at cathode, O2 at anode) |
Example 5: Electrolysis of NaOH(aq)
| Feature | Detail |
|---|---|
| Ions present | Na+, OH-, H+, OH- |
| At Cathode (-) | H+ discharged (Na+ too reactive) → H2 gas |
| At Anode (+) | OH- discharged → O2 gas |
| Net result | Electrolysis of water — NaOH concentration increases, water is decomposed |
Example 6: Electrolysis of KI(aq)
| Feature | Detail |
|---|---|
| Ions present | K+, I-, H+, OH- |
| At Cathode (-) | H+ discharged (K+ too reactive) → H2 gas |
| At Anode (+) | I- discharged (halide) → I2 solid (brown/black solid or brown solution) |
6. Observations Summary Table
| Solution | Cathode Product | Cathode Observation | Anode Product | Anode Observation |
|---|---|---|---|---|
| NaCl(aq) conc. | H2 | Colourless gas, ‘pop’ | Cl2 | Yellow-green gas, bleaches litmus |
| CuSO4(aq) | Cu | Pink/brown solid deposited | O2 | Colourless gas, relights splint |
| H2SO4(aq) dilute | H2 | Colourless gas, ‘pop’ | O2 | Colourless gas, relights splint |
| NaOH(aq) | H2 | Colourless gas, ‘pop’ | O2 | Colourless gas, relights splint |
| KI(aq) | H2 | Colourless gas, ‘pop’ | I2 | Brown/black solid formed |
Worked Examples (Exam-Style)
Example 1: Predicting Products
Question: What are the products at each electrode during the electrolysis of concentrated aqueous sodium chloride using inert electrodes? Explain your answer. [4 marks]
Solution:
- At the cathode: Hydrogen gas (H2) ✓ — Na+ is above hydrogen in the reactivity series, so H+ from water is preferentially discharged ✓
- At the anode: Chlorine gas (Cl2) ✓ — Cl- is a halide ion (present in concentrated solution), so it is preferentially discharged over OH- ✓
Example 2: Half-Equations
Question: Write the half-equations for the electrolysis of aqueous copper(II) sulfate using inert electrodes. [2 marks]
Solution:
- Cathode: Cu2+ + 2e- → Cu ✓
- Anode: 4OH- → O2 + 2H2O + 4e- ✓
Practice Questions
-
Name the products formed at the cathode and anode when concentrated aqueous sodium chloride is electrolysed using inert electrodes. [2 marks]
-
Explain why hydrogen, and not sodium, is produced at the cathode during the electrolysis of aqueous sodium chloride. [2 marks]
-
Write the half-equation for the reaction at the anode when aqueous copper(II) sulfate is electrolysed using inert electrodes. [1 mark]
-
A student electrolyses dilute sulfuric acid. Name the gas produced at each electrode and state the volume ratio in which they are collected. [3 marks]
-
Describe what would be observed at each electrode when aqueous potassium iodide is electrolysed using inert electrodes. [4 marks]
Key Facts to Memorise
- Cathode (negative): reduction (gain of electrons). Anode (positive): oxidation (loss of electrons)
- Reactive metal ions (K+, Na+, Ca2+, Mg2+, Al3+) are NOT discharged in aqueous solution — H+ is discharged instead producing H2
- Less reactive metal ions (Cu2+, Ag+) ARE discharged — metal deposited
- Halide ions (Cl-, Br-, I-) are discharged at anode → halogen produced (especially in concentrated solutions)
- All other anions (SO4 2-, NO3 -, CO3 2-) are NOT discharged — OH- is discharged instead producing O2
- Oxygen half-equation: 4OH- → O2 + 2H2O + 4e-
- Electrolysis of dilute H2SO4 = electrolysis of water: 2H2 : 1O2 volume ratio
Common Misconceptions
| Students often think… | But the correct understanding is… |
|---|---|
| ”Sodium metal is produced at the cathode when NaCl(aq) is electrolysed” | H2 is produced because H+ is easier to reduce than Na+ in aqueous solution. Sodium is only produced from MOLTEN NaCl |
| ”SO4 2- is discharged at the anode producing SO2 or SO3” | SO4 2- is NOT discharged. OH- from water is oxidised instead, producing O2 gas |
| ”All halides are always discharged at the anode” | In very dilute solutions, OH- can compete and be discharged instead, producing O2 |
| ”The same products are always obtained regardless of concentration” | Concentration matters for halide solutions — concentrated favours halogen, dilute may give oxygen |
Key Concepts from Past Papers
Definitions You MUST Know (Exact Mark Scheme Wording)
- Electrolysis: the decomposition of an ionic compound using electricity
- Cathode: the negative electrode where reduction (gain of electrons) occurs
- Anode: the positive electrode where oxidation (loss of electrons) occurs
- Electrolyte: a substance that conducts electricity when molten or dissolved in water
Recurring Mark Scheme Answers
- negative electrode: potassium positive electrode: chlorine observations: (yellow-) green vapour
- anode (left) AND cathode (right) correctly labelled leads connected correctly to electrodes test-tubes over both electrodes and dipping into electrolyte
- positive electrode: chlorine negative electrode: magnesium
- positive electrode: iodine negative electrode: sodium
- left hand electrode labelled anode electrolyte labelled
- negative electrode: lead positive electrode: bromine
- anode (on right) and cathode (on left) power supply added AND connecting wire from each end of power supply to separate electrodes
- positive electrode: bromine / Br2 negative electrode: magnesium / Mg
Common Question Types
- Extended writing (describe/explain/suggest): “Explain why copper is produced at the cathode when aqueous copper sulfate is electrolysed”
- Diagram/labelling task: “Label the anode, cathode, and electrolyte on the diagram”
- Short recall (state/give/name): “State the product at the anode when concentrated NaCl(aq) is electrolysed”
- Half-equation writing: “Write the half-equation for the anode reaction”
Exam Tips
- This topic appears in 26 papers in the database
- Total Q+A entries: 41
- Average marks per question: 1.7
- The most common error is confusing molten and aqueous electrolysis — read the question carefully for “molten” vs “aqueous”
- Always write half-equations with the correct number of electrons — check the charge balances
- For diagrams, anode is on the LEFT (connected to positive terminal) and cathode on the RIGHT (connected to negative terminal) in standard IGCSE diagrams — check past paper conventions
- The O2 half-equation (4OH- → O2 + 2H2O + 4e-) is the most frequently tested half-equation in this topic
Keywords from Past Papers
electrode, labelled, negative, positive, anode, cathode, correctly, electrolyte, chlorine, bromine, magnesium, observations, left, electrodes, power
Related Notes
- Electrolysis — Molten electrolysis basics
- Electrolysis Applications — Industrial uses (aluminium extraction, copper refining, electroplating)
- IGCSE-Chem-Index — Full IGCSE Chemistry index
Sources
- OpenStax Chemistry 2e — Chapter 17: Electrochemistry, Rice University (free, CC BY 4.0)
- BBC Bitesize GCSE Chemistry — Electrolysis of Solutions, BBC (free educational resource)
- Cambridge IGCSE Chemistry 0620 — Syllabus topic 4 (Electrochemistry), Cambridge Assessment International Education
- CK-12 Chemistry for High School — Electrolysis of Aqueous Solutions, CK-12 Foundation (free, CC BY-NC 3.0)
Past Paper Sources
- 0620/32 Feb/March 2017: Q22(a)(ii) (1m)
- 0620/32 Feb/March 2018: Q55(a)(i) (1m)
- 0620/32 Feb/March 2019: Q66(a)(ii) (2m), Q66(d)(i) (2m)
- 0620/32 Feb/March 2020: Q77(a)(i) (3m), Q77(a)(i) (2m)
- 0620/32 Feb/March 2021: Q77(b)(ii) (1m)
- 0620/32 Feb/March 2022: Q66(c)(iii) (2m)
- 0620/32 Feb/March 2023: Q55(b)(i) (2m), Q55(b)(ii) (1m)
- 0620/32 May/June 2018: Q55(a)(i) (1m), Q55(a)(ii) (1m), Q55(a)(iii) (1m)
- 0620/32 May/June 2020: Q55(a)(i) (2m), Q55(a)(ii) (2m)
- 0620/33 May/June 2016: Q22(c)(ii) (1m)
- 0620/33 May/June 2017: Q22(d)(i) (1m)
- 0620/33 May/June 2022: Q33(b)(iv) (2m), Q77(b)(i) (2m), Q77(b)(ii) (2m)