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What Is Electrolysis?

Electrolysis basics: definition, electrolyte/anode/cathode terms, electrode signs (PANIC), redox at electrodes, and simple worked examples.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe electrolysis as the conduction of electricity through an ionic compound (an electrolyte), when molten or dissolved in water, leading to chemical changes (including decomposition) at the electrodes
  • describe electrolysis as evidence for the existence of ions which are held in a lattice when solid but which are free to move when molten or in solution

Electrolysis questions connect three representations: the visible products at the electrodes, the movement and discharge of ions, and balanced half-equations.

1. Definition

A. Electrolysis

Electrolysis is the conduction of electricity through an ionic compound (the electrolyte), leading to chemical changes at the electrodes.

B. Electrolyte

An electrolyte is a substance that contains mobile ions when molten or in aqueous solution, so it conducts electricity and is decomposed at the electrodes.

C. Electrodes

Electrodes are conductors (usually metals or graphite) that allow electrons to enter or leave the electrolyte.

2. Key Ideas

  • Electrolysis uses direct current (DC), not alternating current (AC).
  • Cations (positive ions) move to the cathode.
  • Anions (negative ions) move to the anode.
  • Reduction happens at the cathode (gain of electrons).
  • Oxidation happens at the anode (loss of electrons).
  • In the wires, current is carried by electrons; in the electrolyte, current is carried by ions.
  • Electrolysis is evidence for ions: products form because ions are discharged at electrodes.
Keep the electrode signs distinct

Electrolysis needs an external DC power supply (electrical energy is supplied). A simple cell produces electricity from a spontaneous redox reaction.

In electrolysis: anode is positive, cathode is negative. In a simple cell the signs are reversed.

Oxidation is at the anode and reduction is at the cathode in both cases — it is the signs that swap. Simple Electric Cells

3. Detailed Explanations

Quick Recall (before Section 3)
  • Electrolysis uses DC and needs mobile ions (molten or aqueous).
  • PANIC (electrolysis): anode is positive, cathode is negative.
  • Cations → cathode (reduction). Anions → anode (oxidation).
  • Electrons move in wires; ions move in the electrolyte.

A. Main Components

Labelled electrolytic cell and charge movementA direct-current power supply is connected to two inert electrodes in an electrolyte. The positive anode attracts anions and is where oxidation occurs. The negative cathode attracts cations and is where reduction occurs. Electrons travel in the external wires while ions carry charge through the liquid.DC power supply+−electrons move in the external circuitelectrolyte: mobile ions carry chargeANODE (+)CATHODE (−)oxidation: loses e−reduction: gains e−−anion+cationions move through the liquid, not through the wiresLabelled electrolytic cell and charge movementMobile layout of a direct-current supply connected to a positive anode and negative cathode in an electrolyte. Anions move to the anode, cations move to the cathode, and electrons move through the wires.DC power supply+−electrons move through the wireselectrolyte: mobile ionsANODE (+)CATHODE (−)−anion+cationoxidationloses e−reductiongains e−
A direct-current supply pushes electrons to the cathode and removes them from the anode; cations and anions move through the electrolyte in opposite directions.
  • Power source (DC) supplies electrical energy.
  • Electrodes are conductors through which electrons enter or leave the electrolyte.
  • Electrolyte is the molten substance or aqueous solution that contains mobile ions.

B. Polarity and Definitions

TermMeaning (electrolysis)
AnodePositive electrode
CathodeNegative electrode
CationPositive ion (moves to cathode)
AnionNegative ion (moves to anode)

C. Discharge and Redox

Ions are discharged when they gain or lose electrons at the electrodes.

  • At the cathode, cations gain electrons (reduction).
  • At the anode, anions lose electrons (oxidation).
Recall: OILRIG

Oxidation Is Loss (of electrons). Reduction Is Gain (of electrons). Redox Reactions

D. Charge Carriers (Common Exam Trap)

  • In the external circuit (wires), current is carried by electrons.
  • In the electrolyte, current is carried by ions (cations and anions).
Do not write 'electrons flow through the solution'

Ions carry charge through the electrolyte. Electrons move through the wires and electrodes.

Because ions move to opposite electrodes and are discharged to form substances (metals/gases), electrolysis is strong evidence that ions exist in molten and aqueous electrolytes.

E. When Ionic Substances Conduct

Substance or stateAre charged particles mobile?Electrical conductivity
Solid ionic compoundNo. Its ions are held in fixed positions in a lattice.Does not conduct
Molten ionic compoundYes. Its ions can move through the liquid.Conducts
Aqueous electrolyteYes. Its dissolved ions can move through the solution.Conducts
Sugar or ethanol solutionNo mobile ions are formed.Does not conduct

Pure water contains very few ions, so it is a very poor conductor. Adding an electrolyte supplies more mobile ions.

4. Common Mistakes

  • Swapping the signs: in electrolysis, the anode is positive and the cathode is negative.
  • Writing oxidation at the cathode or reduction at the anode.
  • Saying “electrons move through the electrolyte”; ions carry charge there.
  • Treating pure water as a good conductor. It is not.
  • Forgetting to state tests for gases (pop test for hydrogen, relights for oxygen, bleaches litmus for chlorine).

5. Exam Tips

Two sentences that score

“Cations move to the cathode (negative) and are reduced (gain electrons). Anions move to the anode (positive) and are oxidised (lose electrons).”

  • Use the word discharged in your explanation (it is mark-scheme language).
  • In aqueous electrolysis, do not assume the ions from the salt are discharged. Water ions (H⁺ and OH⁻) can compete.

6. Worked Examples

Modelled example 1

Molten Magnesium Chloride MgCl₂(l)

Core

Problem

Molten magnesium chloride is electrolysed using carbon electrodes. Identify each electrode product and write both half-equations.
Study the worked solution

  1. List mobile ions

    Method

    Use Mg²⁺ and Cl⁻ only.

    Reason

    A molten ionic compound contains its own mobile ions but no water-derived ions.

    Working

    Ions: Mg²⁺(l), Cl⁻(l).

  2. Reduce the cation

    Method

    Send Mg²⁺ to the negative cathode and add electrons.

    Reason

    Cations gain electrons by reduction at the cathode.

    Working

    Mg²⁺(l) + 2e⁻ → Mg(l).

  3. Oxidise the anion

    Method

    Send chloride ions to the positive anode and remove electrons.

    Reason

    Anions lose electrons by oxidation at the anode; chlorine is diatomic.

    Working

    2Cl-(l) → Cl₂(g) + 2e⁻.

  4. State products and observation

    Method

    Name magnesium at the cathode and chlorine at the anode.

    Reason

    The half-equations identify the discharged products.

    Working

    Cathode: Mg; anode: greenish-yellow toxic Cl₂ gas.

Guided practice 2

Electrode Names and Direction of Ion Movement

About 5 min

Problem

A student writes: “Na⁺ moves to the anode because sodium is a metal.” Identify the mistake and correct the ion-movement rule.

Use ion charge, not element type

Na+ moves to
Anions move to

Hints

Hint 1: opposite charges attract
The cathode is negative and the anode is positive in an electrolytic cell.
Hint 2: name the ion classes
Cations are positive; anions are negative.
View solution step by step

  1. Locate the wrong criterion

    Method

    Reject metal/non-metal identity as the direction rule.

    Reason

    Movement is determined by ionic charge and electrode polarity.

    Working

    Na⁺ is a cation.

  2. Apply the charge rule

    Method

    Send cations to the cathode and anions to the anode.

    Reason

    Opposite charges attract.

    Working

    Na⁺ → cathode; anions → anode.

Common misconception 3

Identify Electrolytes (No Ions, No Electrolysis)

Find and correct the mistake

Learner claim

A student says every substance dissolved in water is an electrolyte. Classify aqueous sodium chloride, sugar, ethanol and dilute sulfuric acid.

Ask whether mobile ions form

NaCl(aq)
Sugar solution
Ethanol solution
Dilute H2SO4

View solution step by step

  1. Classify ionic and acidic solutions

    Method

    Classify NaCl(aq) and dilute H₂SO₄(aq) as electrolytes.

    Reason

    They contain mobile ions that carry current.

    Working

    Na⁺/Cl⁻ and acid ions are present.

  2. Classify molecular solutions

    Method

    Classify sugar and ethanol solutions as non-electrolytes.

    Reason

    Dissolving those molecular substances does not create ions.

    Working

    No mobile ions means no electrolysis.

Examiner practice 4

Aqueous Sodium Chloride NaCl(aq): Dilute vs Concentrated

6 marks

Examination question

Aqueous sodium chloride is electrolysed with carbon electrodes. Predict the cathode product for both concentrations and the anode product for dilute solution and concentrated brine; include gas tests. [6 marks]

Separate cathode rule from anode concentration rule

View solution step by step

  1. Cathode product and equation

    2 marks

    Method

    Use hydrogen for dilute and concentrated solutions.

    Reason

    Water is preferentially reduced instead of sodium ions.

    Working

    2H₂O(l) + 2e⁻ → H₂(g) + 2OH-(aq); lighted splint gives a pop.

  2. Dilute anode product

    2 marks

    Method

    Use oxygen for dilute sodium chloride.

    Reason

    Hydroxide ions are discharged in the dilute sodium chloride case.

    Working

    4OH-(aq) → O₂(g) + 2H₂O(l) + 4e⁻; glowing splint relights.

  3. Concentrated anode product

    2 marks

    Method

    Use chlorine for concentrated brine.

    Reason

    High chloride concentration favours chloride discharge.

    Working

    2Cl-(aq) → Cl₂(g) + 2e⁻; damp blue litmus turns red then bleaches white.

Challenge 5

Aqueous Copper(II) Sulfate CuSO₄(aq) (Inert Electrodes)

Minimal support

Aqueous-selection transfer

Copper(II) sulfate solution is electrolysed using carbon electrodes. State both products, write both half-equations and give the main observations.

Include water-derived ions

Cathode product
Anode product

Hints

Hint 1: four ions
Include Cu²⁺, SO₄²⁻, H⁺ and OH⁻.
Hint 2: selective discharge
Copper is deposited at the cathode; sulfate is not discharged at the inert anode.
View solution step by step

  1. List aqueous ions

    Method

    Include solute ions and water-derived ions.

    Reason

    Aqueous electrolysis has more discharge candidates than molten electrolysis.

    Working

    Cu²⁺, SO₄²⁻, H⁺, OH⁻.

  2. Reduce copper ions

    Method

    Deposit copper at the cathode.

    Reason

    Cu²⁺ is preferentially reduced.

    Working

    Cu²⁺(aq) + 2e⁻ → Cu(s); reddish-brown coating forms.

  3. Oxidise hydroxide ions

    Method

    Produce oxygen at the inert anode.

    Reason

    Hydroxide is discharged rather than sulfate.

    Working

    4OH-(aq) → O₂(g) + 2H₂O(l) + 4e⁻; bubbles form and the blue solution becomes paler.

7. Mind Stretchers

Mind stretcher 1: Inert vs Reactive Electrodes (Copper Electrodes)Extension

Copper(II) sulfate solution is electrolysed using copper electrodes instead of carbon. Predict what happens at each electrode and state one observation.

Show Answer

Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s) (copper deposited).

Anode: copper dissolves: Cu(s) → Cu²⁺(aq) + 2e⁻

Observation: anode gets smaller (mass decreases). The blue colour stays roughly the same because Cu²⁺ removed at the cathode is replaced at the anode.

Mind stretcher 2: Electrolysis of Acidified Water (Gas Ratio)Extension

Dilute sulfuric acid is electrolysed using inert electrodes. State the gases formed at each electrode and the volume ratio of gases collected (at the same conditions).

Show Answer

Cathode: hydrogen gas.

Anode: oxygen gas.

Overall equation: 2H₂O(l) → 2H₂(g) + O₂(g)

So the volume ratio is H₂:O₂ (2:1).

Electrolysis of water: gas volume ratioRelative gas volumes produced when electrolysing water, showing hydrogen collected in twice the volume of oxygen (2:1).Electrolysis of water: gas volume ratioGasRelative volume
At the same conditions, hydrogen is collected in twice the volume of oxygen (2:1).
Data table
GasVolume
H22
O21

8. Quiz

Quiz Time!

Ready to test your knowledge? Start with the basics: electrode signs, ion movement, and redox at the electrodes.

Go to Quiz Page