Electrolysis
Summary: Electrolysis is the decomposition of an ionic compound by passing electricity through it when molten or in aqueous solution. Cations migrate to the cathode (reduction) and anions migrate to the anode (oxidation). Tags: igcse chemistry electrolysis Created: 2026-07-14 Last Updated: 2026-07-16
What is Electrolysis?
Electrolysis is the decomposition of an ionic compound by passing electricity through it when it is molten or in aqueous solution.
For electrolysis to work, the ionic compound must be in a state where its ions are free to move. In a solid ionic lattice, ions are fixed in position and cannot carry charge — so solid ionic compounds do NOT conduct electricity and cannot undergo electrolysis. However:
- When molten (melted by heating), the ions break free from the lattice and can move
- When dissolved in water (aqueous solution), the ions dissociate and can move freely
An electric current is passed through the electrolyte using two electrodes connected to a DC power supply. The current causes a chemical change — the electrolyte is decomposed into its elements.
Key Terminology
| Term | Definition |
|---|---|
| Electrolyte | The ionic compound (molten or aqueous) that is decomposed during electrolysis |
| Electrode | A solid conductor (usually graphite/platinum) that makes contact with the electrolyte and carries current into/out of it |
| Anode | The positive electrode — attracts anions. Oxidation occurs here |
| Cathode | The negative electrode — attracts cations. Reduction occurs here |
| Anion | A negatively charged ion (e.g., Cl⁻, Br⁻, O²⁻, OH⁻). Moves to the anode |
| Cation | A positively charged ion (e.g., Na⁺, Mg²⁺, Cu²⁺, H⁺). Moves to the cathode |
Mnemonic — PANIC: Positive Anode, Negative Is Cathode.
In IGCSE diagrams, the anode is connected to the positive terminal of the power supply (long line in the cell symbol) and the cathode to the negative terminal (short line).
How Electrolysis Works
When a DC current is passed through the electrolyte:
- Cations (positive ions) are attracted to the cathode (negative electrode). At the cathode, cations gain electrons — this is reduction.
- Anions (negative ions) are attracted to the anode (positive electrode). At the anode, anions lose electrons — this is oxidation.
Mnemonic — OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
Mnemonic — RED CAT: REDuction occurs at the CAThode. (And therefore oxidation occurs at the anode.)
The overall process converts electrical energy into chemical energy, breaking down the ionic compound into its constituent elements.
Electrolysis of Molten Ionic Compounds
Molten electrolysis is simpler than aqueous electrolysis because the electrolyte contains only one cation and one anion — there are no competing species.
Example 1: Electrolysis of molten lead(II) bromide (PbBr₂)
- Ions present: Pb²⁺ and Br⁻
- At the cathode (−): Pb²⁺ + 2e⁻ → Pb (grey lead metal forms; reduction)
- At the anode (+): 2Br⁻ → Br₂ + 2e⁻ (brown bromine gas bubbles off; oxidation)
- Observation: Silvery-grey lead deposits at the cathode; reddish-brown bromine vapour at the anode
Example 2: Electrolysis of molten sodium chloride (NaCl)
- Ions present: Na⁺ and Cl⁻
- At the cathode (−): Na⁺ + e⁻ → Na (sodium metal; reduction)
- At the anode (+): 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas; oxidation)
- Observation: Silvery sodium metal at the cathode; yellow-green chlorine gas at the anode
Example 3: Electrolysis of molten aluminium oxide (Al₂O₃)
- Ions present: Al³⁺ and O²⁻
- At the cathode (−): Al³⁺ + 3e⁻ → Al (molten aluminium; reduction)
- At the anode (+): 2O²⁻ → O₂ + 4e⁻ (oxygen gas; oxidation)
- Observation: Molten aluminium collects at the cathode; oxygen gas at the anode
- This is how aluminium is extracted industrially from bauxite ore (dissolved in cryolite to lower the melting point from ~2050 °C to ~950 °C).
Predicting Products for Molten Compounds
For any molten ionic compound, there is no competition — products are straightforward:
- Identify the cation and anion from the formula
- The cation is discharged at the cathode to form the metal element (reduction)
- The anion is discharged at the anode to form the non-metal element (oxidation)
Electrolysis of Aqueous Solutions
Aqueous solutions are more complex because water (H₂O) introduces additional ions — H⁺ and OH⁻ — which compete with the solute ions at the electrodes.
At the cathode: H⁺ (from water) competes with the metal cation. The ion that is less reactive (more easily reduced) is discharged.
At the anode: OH⁻ (from water) competes with the anion from the solute. If the anion is a halide (Cl⁻, Br⁻, I⁻), it is more easily oxidised than OH⁻ — so the halogen is produced. Otherwise, OH⁻ is discharged and O₂ is produced.
Rules for Predicting Products in Aqueous Electrolysis
At the Cathode (−)
| Position of metal in reactivity series | Product at cathode | Reason |
|---|---|---|
| Metal is more reactive than hydrogen (K, Na, Ca, Mg, Al, Zn, Fe, etc.) | Hydrogen gas (H₂) | H⁺ is more easily reduced than reactive metal ions |
| Metal is less reactive than hydrogen (Cu, Ag, Au) | The metal itself | The metal ion is more easily reduced than H⁺ |
At the Anode (+)
| Anion present | Product at anode | Reason |
|---|---|---|
| Halide ion (Cl⁻, Br⁻, I⁻) present | The halogen (Cl₂, Br₂, I₂) | Halides are more easily oxidised than OH⁻ |
| No halide (e.g., SO₄²⁻, NO₃⁻, CO₃²⁻) | Oxygen gas (O₂) | OH⁻ is discharged instead; the sulfate/nitrate/carbonate ion stays in solution |
Half Equations
Half equations show what happens at each electrode. They must be balanced for atoms and charge.
At the cathode (reduction) — cations gain electrons:
- Na⁺ + e⁻ → Na
- Cu²⁺ + 2e⁻ → Cu
- 2H⁺ + 2e⁻ → H₂
- Al³⁺ + 3e⁻ → Al
- Pb²⁺ + 2e⁻ → Pb
At the anode (oxidation) — anions lose electrons:
- 2Cl⁻ → Cl₂ + 2e⁻
- 2Br⁻ → Br₂ + 2e⁻
- 2I⁻ → I₂ + 2e⁻
- 4OH⁻ → O₂ + 2H₂O + 4e⁻
Electrons lost at the anode = electrons gained at the cathode. The total half equations should balance when combined.
Observations During Electrolysis
| Electrode | Product | Observation |
|---|---|---|
| Cathode (molten NaCl) | Sodium (Na) | Silvery metal |
| Anode (molten NaCl) | Chlorine (Cl₂) | Yellow-green gas/vapour |
| Cathode (molten PbBr₂) | Lead (Pb) | Grey/silvery metal deposit |
| Anode (molten PbBr₂) | Bromine (Br₂) | Red-brown gas/vapour |
| Cathode (aqueous CuSO₄ with Cu electrodes) | Copper (Cu) | Pink-brown metal deposit on cathode |
| Anode (aqueous CuSO₄ with Cu electrodes) | — | Anode dissolves (Cu → Cu²⁺ + 2e⁻) |
| Cathode (aqueous NaCl) | Hydrogen (H₂) | Colourless gas, “pop” with lighted splint |
| Anode (aqueous NaCl) | Chlorine (Cl₂) | Yellow-green gas, bleaches damp litmus paper |
| Cathode (aqueous CuSO₄ with inert electrodes) | Copper (Cu) | Pink-brown deposit on cathode; blue colour of solution fades |
| Anode (aqueous CuSO₄ with inert electrodes) | Oxygen (O₂) | Colourless gas, relights a glowing splint |
Key Points
- Electrolysis requires ions to be free to move — the compound must be molten or aqueous (dissolved in water)
- PANIC: Positive Anode, Negative Is Cathode
- OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
- RED CAT: REDuction occurs at the CATthode
- Anions (negative ions) → Anode (positive electrode) → Oxidation (lose electrons)
- Cations (positive ions) → Cathode (negative electrode) → Reduction (gain electrons)
- Cathode rule: If metal is more reactive than H → H₂ produced; if less reactive → metal produced
- Anode rule: If halide present → halogen produced; if not → O₂ produced
- Electrons flow through the external circuit from anode to cathode; ions carry charge through the electrolyte
- Inert electrodes (graphite/platinum) do not take part in the reaction
Key Concepts from Past Papers
- Electrolysis: the decomposition of an ionic compound by passing electricity through it when molten or in aqueous solution
- Electrolyte: a molten or aqueous ionic compound that conducts electricity and is decomposed during electrolysis
- Anode: the positive electrode, where oxidation occurs
- Cathode: the negative electrode, where reduction occurs
- Ions must be free to move (so the compound must be molten or in solution)
- Cations move to the cathode; anions move to the anode
- At the cathode, cations gain electrons (reduction); at the anode, anions lose electrons (oxidation)
- In aqueous solutions, H⁺ and OH⁻ from water compete with ions from the solute
- If the metal is more reactive than hydrogen, hydrogen gas is produced at the cathode
- If a halide ion is present, the halogen is produced at the anode; otherwise oxygen is produced
Keywords from Past Papers
electrode, negative, labelled, positive, anode, cathode, correctly, electrolyte, chlorine, bromine, observations, left, electrodes, magnesium, power
Related Notes
- Electrolysis of Aqueous Solutions
- Electrolysis Applications
- Ions and Ionic Bonds
- Redox Reactions
- Metal Extraction
Sources
- OpenStax Chemistry 2e — Chapter 17: Electrochemistry, Rice University (free, CC BY 4.0)
- BBC Bitesize GCSE Chemistry — Electrolysis, BBC (free educational resource)
- Cambridge IGCSE Chemistry 0620 — Syllabus topic 4 (Electrochemistry), Cambridge Assessment International Education
- CK-12 Chemistry for High School — Electrolysis, 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), Q66(a)(ii) (2m) (+1 more)
- 0620/32 Feb/March 2020: Q77(a)(i) (3m), Q77(a)(i) (2m), Q77(a)(i) (3m) (+1 more)
- 0620/32 Feb/March 2021: Q77(b)(ii) (1m)
- 0620/32 Feb/March 2022: Q66(c)(iii) (2m), Q66(c)(iii) (2m)
- 0620/32 Feb/March 2023: Q55(b)(i) (2m), Q55(b)(ii) (1m), Q55(b)(i) (2m)
- 0620/32 May/June 2018: Q55(a)(i) (1m), Q55(a)(ii) (1m), Q55(a)(iii) (1m) (+1 more)
- 0620/32 May/June 2020: Q55(a)(i) (2m), Q55(a)(ii) (2m), Q55(a)(i) (2m) (+1 more)
- 0620/33 May/June 2016: Q22(c)(ii) (1m), Q22(c)(ii) (1m)
- 0620/33 May/June 2017: Q22(d)(i) (1m), Q22(d)(i) (1m)
- 0620/33 May/June 2022: Q33(b)(iv) (2m), Q77(b)(i) (2m), Q77(b)(ii) (2m) (+2 more)
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”The anode is negative and the cathode is positive” | In electrolysis, the anode is positive and the cathode is negative (opposite to electrochemical cells). Remember PANIC. |
| ”Solid ionic compounds can be electrolysed” | Solid ionic compounds do not conduct electricity — ions are fixed in the lattice. The compound must be molten or aqueous. |
| ”Water doesn’t affect electrolysis of aqueous solutions” | Water introduces H⁺ and OH⁻ ions that compete at the electrodes, changing the products compared to molten electrolysis. |
| ”Oxidation happens at the cathode” | Oxidation happens at the anode — anions lose electrons there. Remember: AN OX (ANode — OXidation) and RED CAT (REDuction — CAThode). |
| ”Any metal can be extracted by electrolysis of its aqueous salt” | Metals more reactive than hydrogen (K to Fe) form H₂, not the metal, when their aqueous salts are electrolysed. |
| ”Electrons flow through the electrolyte” | Electrons flow through the external wire/circuit. Ions carry charge through the electrolyte. |
| ”The anode and cathode are always graphite” | Graphite and platinum are common inert electrodes, but active electrodes like copper can also be used — and will take part in the reaction. |