What is electrolysis and how is it used in electrochemistry?

Electrolysis is a chemical process that uses an electric current to drive a non-spontaneous chemical reaction. In electrochemistry, it is used to decompose compounds, typically ionic substances, into their constituent elements. This process is fundamental in various applications such as electroplating, extraction of metals, and production of chemicals like chlorine and hydrogen.

How does electrolysis work in terms of ions and electrodes?

During electrolysis, an electric current is passed through an electrolyte, causing ions to move towards electrodes. Positively charged ions (cations) migrate to the cathode (negative electrode) where they gain electrons (reduction), while negatively charged ions (anions) move to the anode (positive electrode) where they lose electrons (oxidation). This movement of ions facilitates the chemical reactions at the electrodes.

What are some common examples of electrolysis in the Cambridge IGCSE Chemistry syllabus?

In the Cambridge IGCSE Chemistry syllabus, common examples of electrolysis include the electrolysis of molten sodium chloride to produce sodium and chlorine gas, and the electrolysis of aqueous copper(II) sulfate solution to purify copper. These examples illustrate the practical applications of electrolysis in industry and laboratory settings.

What factors affect the efficiency of electrolysis?

Several factors can affect the efficiency of electrolysis, including the concentration of the electrolyte, the type of electrodes used, the temperature of the solution, and the applied voltage. Higher concentrations and temperatures generally increase the rate of electrolysis, while the choice of electrode material can influence the products formed and the overall efficiency of the process.

Why is electrolysis important in the extraction of metals?

Electrolysis is crucial in the extraction of metals, especially those that are more reactive and cannot be extracted by reduction with carbon. For example, metals like aluminum are extracted from their ores using electrolysis. This process allows for the separation of metals from their compounds, making it an essential technique in the production of pure metals for industrial use.

Why is "Saying electrons are released at the cathode" a common mistake in Electrolysis?

Why it happens: Confusing direction. How to avoid it: Anode → loses electrons (oxidation). Cathode → gains electrons (reduction). Source: 0620/42 — recurring

Why is "Saying Na is produced at the cathode in NaCl(aq) electrolysis" a common mistake in Electrolysis?

Why it happens: Direct reading of NaCl. How to avoid it: In aqueous solution, the LESS REACTIVE of Na^+ vs H^+ is reduced. H is less reactive → H₂ produced.

Why is "Half-equation not balanced for charge" a common mistake in Electrolysis?

Why it happens: Forgetting electrons. How to avoid it: Add e^- to balance charge. Cathode: e^- on LEFT. Anode: e^- on RIGHT.

Why is "Saying cryolite is a catalyst" a common mistake in Electrolysis?

Why it happens: Both reduce reaction conditions. How to avoid it: Cryolite LOWERS the melting point of Al_2O_3 — it's a solvent, not a catalyst.

ElectrochemistryCambridge IGCSE Chemistry (0620)

Electrolysis

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Electrolysis — Cambridge IGCSE 0620 Chemistry Extended (2026)

Splitting compounds with electricity. Cations to cathode (−), anions to anode (+). The big examples: molten salt, aqueous solutions, and aqueous halides.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

5.1 — Define electrolysis.
5.1 — Predict products of electrolysis of molten and aqueous solutions.
5.1 — Write half-equations for electrode reactions.
5.1 — Describe the use of inert electrodes (graphite, platinum) and reactive electrodes.

What is electrolysis?

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:

Cathode: the negative electrode (attracts positive ions = cations).
Anode: the positive electrode (attracts negative ions = anions).

At each electrode.

Cathode: cations gain electrons (they're reduced).
Anode: anions lose electrons (they're oxidised).

Mnemonic. OIL RIG: Oxidation Is Loss, Reduction Is Gain (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:

Cations (+) drift to the cathode (−), where they're given electrons.
Anions (−) drift to the anode (+), where they're stripped of electrons.

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.

Electrolysis decomposes ionic compounds by electricity.
Cathode (−): reduction (gain e⁻).
Anode (+): oxidation (lose e⁻).
OIL RIG mnemonic.
Solid: no electrolysis (ions locked).

Electrolysis of molten ionic compounds

Metal at cathode, non-metal at anode. Predictable.

Molten lead bromide ( $\text{PbBr}_{2}$ ).

Ions present: Pb²⁺, Br⁻.
Cathode: $\text{Pb}^{2+} + 2\text{e}^{-} \to \text{Pb}$ . Liquid lead forms.
Anode: $2\text{Br}^{-} \to \text{Br}_{2} + 2\text{e}^{-}$ . Brown bromine vapour.

Molten sodium chloride ( $\text{NaCl}$ ).

Cathode: $\text{Na}^{+} + \text{e}^{-} \to \text{Na}$ .
Anode: $2\text{Cl}^{-} \to \text{Cl}_{2} + 2\text{e}^{-}$ .

Industrial use: extracting reactive metals. Aluminium is extracted from molten alumina ( $\text{Al}_{2}\text{O}_{3}$ ) — 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.

Molten ionic compound: metal at cathode.
Non-metal at anode (often gas/vapour).
Half-equations show electron transfer.
Industrial: aluminium extraction.

Electrolysis of aqueous solutions (Extended)

Water competes! At cathode: less reactive metal first, else H₂. At anode: halide first, else O₂.

At the cathode (Extended rules).

If the metal cation is LESS reactive than hydrogen → metal forms (e.g. Cu²⁺ → Cu).
If MORE reactive than hydrogen (e.g. Na⁺, K⁺, Mg²⁺, Ca²⁺) → H₂ gas forms instead.

Reactivity guide: Group 1 and 2 metals + Al are MORE reactive than H. Cu, Ag, Au are LESS reactive.

At the anode (Extended rules).

If the solution contains a CONCENTRATED halide (Cl⁻, Br⁻, I⁻) → halogen gas forms.
Otherwise (e.g. dilute halide, sulfate, nitrate, hydroxide) → O₂ gas forms (from water OH⁻).

Worked: dilute sulfuric acid.

Ions: H⁺, SO₄²⁻ (and H⁺/OH⁻ from water).
Cathode: $2\text{H}^{+} + 2\text{e}^{-} \to \text{H}_{2}(g)$ .
Anode: $4\text{OH}^{-} \to \text{O}_{2}(g) + 2\text{H}_{2}\text{O} + 4\text{e}^{-}$ (no halide present).
Net: water decomposes. $\text{H}_{2}\text{O}$ → $\text{H}_{2} + \frac{1}{2}\text{O}_{2}$ .

Worked: concentrated sodium chloride (brine).

Ions: Na⁺, Cl⁻, plus water.
Cathode: Na⁺ is more reactive than H, so $2\text{H}^{+} + 2\text{e}^{-} \to \text{H}_{2}$ .
Anode: concentrated Cl⁻ → $2\text{Cl}^{-} \to \text{Cl}_{2} + 2\text{e}^{-}$ .
Solution becomes NaOH (Na⁺ and OH⁻ left over).
Industrial process: makes Cl₂ (bleach, PVC), H₂ (margarine, ammonia), NaOH (soap).

Worked: aqueous copper sulfate (with INERT electrodes).

Cathode: Cu²⁺ less reactive than H → $\text{Cu}^{2+} + 2\text{e}^{-} \to \text{Cu}$ .
Anode: SO₄²⁻ doesn't discharge readily; from water: $4\text{OH}^{-} \to \text{O}_{2} + 2\text{H}_{2}\text{O} + 4\text{e}^{-}$ .
Solution becomes more dilute; copper deposits at cathode; bubbles of O₂ at anode.

Worked: aqueous copper sulfate (with copper electrodes — purification).

Cathode: $\text{Cu}^{2+} + 2\text{e}^{-} \to \text{Cu}$ (pure).
Anode: copper itself dissolves: $\text{Cu} \to \text{Cu}^{2+} + 2\text{e}^{-}$ .
Net: copper transfers from impure anode to pure cathode (impurities fall as sludge).
Industrial use: refining copper.

Aqueous: water competes.
Cathode: less reactive metal else H₂.
Anode: concentrated halide else O₂.
Brine: Cl₂ + H₂ + NaOH industrial.
Copper electrodes: refinement.

How it’s examined

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.

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Step-by-step worked examples — Electrolysis

Step-by-step solutions to past-paper-style questions on electrolysis, written exactly the way a tutor would explain them at the board.

1Electrolysis of molten lead bromide
Core• molten
Question
Predict the products at the cathode and anode when molten $\text{PbBr}_{2}$ is electrolysed.
Step-by-step solution
1. Step 1
  Cathode (negative): $\text{Pb}^{2+}$ ions gain electrons → Pb metal.
  $\text{Pb}^{2+} + 2e^{-} \to \text{Pb}$
2. Step 2
  Anode (positive): $\text{Br}^{-}$ ions lose electrons → bromine.
  $2\text{Br}^{-} \to \text{Br}_{2} + 2e^{-}$
Answer
Cathode: lead. Anode: bromine.
2Electrolysis of brine (concentrated $\text{NaCl}$ solution)
Extended• Adapted from 0620/42 May/Jun 2024 Q12• aqueous
Question
Predict the products at each electrode when concentrated NaCl(aq) is electrolysed using inert electrodes.
Step-by-step solution
1. Step 1
  Cathode: $\text{Na}^{+}$ vs $\text{H}^{+}$ . Hydrogen is below sodium in the reactivity series → H₂ produced.
  $2\text{H}^{+} + 2e^{-} \to \text{H}_{2}$
2. Step 2
  Anode: $\text{Cl}^{-}$ vs $\text{OH}^{-}$ . Concentrated halide → halogen produced.
  $2\text{Cl}^{-} \to \text{Cl}_{2} + 2e^{-}$
3. Step 3
  Solution becomes more alkaline (NaOH remains in solution).
Answer
Cathode: hydrogen. Anode: chlorine. Solution: sodium hydroxide.
3Industrial extraction of aluminium
Extended• industry
Question
State the half-equations for the extraction of aluminium from molten $\text{Al}_{2}\text{O}_{3}$ in cryolite.
Step-by-step solution
1. Step 1
  Cathode (carbon).
  $\text{Al}^{3+} + 3e^{-} \to \text{Al}$
2. Step 2
  Anode (carbon).
  $2\text{O}^{2-} \to \text{O}_{2} + 4e^{-}$
3. Step 3
  Anodes burn away over time as $\text{O}_{2}$ reacts with hot carbon to form $\text{CO}_{2}$ .
Answer
Cathode: $\text{Al}^{3+} + 3e^{-} \to \text{Al}$ . Anode: $2\text{O}^{2-} \to \text{O}_{2} + 4e^{-}$ .
Examiner tip
Cryolite is used to LOWER the melting point of $\text{Al}_{2}\text{O}_{3}$ — it does not change the products.
4Purification of copper
Extended• copper
Question
Describe how copper is purified by electrolysis.
Step-by-step solution
1. Step 1
  Anode: impure copper. It DISSOLVES into solution as $\text{Cu}^{2+}$ .
2. Step 2
  Cathode: pure copper rod. $\text{Cu}^{2+}$ ions deposit as pure metal.
3. Step 3
  Impurities fall off the anode as 'anode sludge' (containing precious metals).
Answer
Impure copper anode dissolves; pure copper deposits on a pure copper cathode.

Key Formulae — Electrolysis

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.

Cathode rule (aqueous)
$\text{Less reactive of the available cation OR } \text{H}^{+}\text{ is reduced}$
When to use
Predicting cathode product in aqueous electrolysis.
Anode rule (aqueous)
$\text{Halide if concentrated; otherwise OH}^{-} \to \text{O}_{2}$
When to use
Predicting anode product in aqueous electrolysis.

Key Definitions and Keywords — Electrolysis

Definitions to memorise and the exact keywords mark schemes credit for electrolysis answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Electrolysis
Examiner keyword
Decomposition of a molten or aqueous electrolyte by passing electricity through it.
Electrolyte
Examiner keyword
A liquid or solution that can conduct electricity (contains free-moving ions).
Cathode / Anode
Examiner keyword
Cathode = negative electrode (cations migrate here, gain electrons). Anode = positive electrode (anions migrate here, lose electrons).
Inert electrode
Examiner keyword
An electrode that does not take part in the reaction (e.g. graphite, platinum).

Common Mistakes and Misconceptions — Electrolysis

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.

✕Saying electrons are released at the cathode
0620/42 — recurring
Why it happens
Confusing direction.
How to avoid it
Anode → loses electrons (oxidation). Cathode → gains electrons (reduction).
✕Saying Na is produced at the cathode in NaCl(aq) electrolysis
Why it happens
Direct reading of NaCl.
How to avoid it
In aqueous solution, the LESS REACTIVE of $\text{Na}^{+}$ vs $\text{H}^{+}$ is reduced. H is less reactive → H₂ produced.
✕Half-equation not balanced for charge
Why it happens
Forgetting electrons.
How to avoid it
Add $e^{-}$ to balance charge. Cathode: $e^{-}$ on LEFT. Anode: $e^{-}$ on RIGHT.
✕Saying cryolite is a catalyst
Why it happens
Both reduce reaction conditions.
How to avoid it
Cryolite LOWERS the melting point of $\text{Al}_{2}\text{O}_{3}$ — it's a solvent, not a catalyst.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Electrolysis — frequently asked questions

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Electrolysis

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Electrolysis of molten lead bromide

2Electrolysis of brine (concentrated NaCl\text{NaCl}NaCl solution)

3Industrial extraction of aluminium

4Purification of copper

Cathode rule (aqueous)

Anode rule (aqueous)

Electrolysis

Electrolyte

Cathode / Anode

Inert electrode

✕Saying electrons are released at the cathode

✕Saying Na is produced at the cathode in NaCl(aq) electrolysis

✕Half-equation not balanced for charge

✕Saying cryolite is a catalyst

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Electrolysis — frequently asked questions

2Electrolysis of brine (concentrated $\text{NaCl}$ solution)