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What Is Electrolysis?

Understand why electrolytes need mobile ions, distinguish charge movement in wires and liquids, and identify oxidation and reduction at the electrodes.

  • SEC G3 Pure Chemistry 2027

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Electrolysis uses electrical energy to cause chemical changes. Begin by following the charge carriers: electrons move in the wires and electrodes, while ions move through the electrolyte. Later lessons use this foundation to predict products and write electrode equations.

1. Definition

A. Electrolysis

Electrolysis is the conduction of electricity through an electrolyte, leading to chemical changes at the electrodes. The electrolyte must have mobile ions, as in a molten ionic compound or an aqueous ionic solution.

B. Electrolyte

An electrolyte is a substance that contains mobile ions when molten or in aqueous solution, so it conducts electricity. Chemical changes occur at the electrodes; the dissolved electrolyte is not necessarily used up overall. For example, copper can transfer between copper electrodes while the copper(II) ion concentration remains roughly constant.

C. Electrodes

Electrodes are conductors, usually metals or graphite, at whose surfaces electrons are exchanged with reacting particles. Electrons move through the electrode; ions carry charge through the liquid.

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

Keep the charge pathways separate
  • The DC power supply provides electrical energy.
  • The anode is positive and the cathode is negative in electrolysis.
  • Positive ions move towards the cathode; negative ions move towards the anode.
  • Electrons carry charge through wires and electrodes; ions carry charge through the liquid.

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 DC supply delivers electrons to the negative cathode and removes them from the positive anode. Ions carry charge through the electrolyte. The ion colours and sizes are symbolic, not actual appearances or relative sizes; the liquid shading does not identify a particular chemical. Apparatus is schematic and not to scale.
  • Power source (DC) supplies electrical energy.
  • Electrodes conduct electrons to or from the reacting particles at their surfaces.
  • 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 an electrode surface. Moving towards an electrode and being discharged are different: in an aqueous electrolyte, several ions carry charge but only selected species react. A reactive metal anode can itself lose electrons instead of an anion being discharged.

  • At the cathode, reduction occurs. In a molten salt, the cation gains electrons. In aqueous examples, a selected cation or water is reduced.
  • At the anode, oxidation occurs. With an inert anode, selected anions or water are oxidised. With a reactive anode, the electrode metal may be oxidised.
Recall: OILRIG

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

Redox Reactions

D. Charge carriers in each part of the circuit

  • 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

Explain movement and reaction separately

“Positive ions move towards the negative cathode, where reduction occurs. Negative ions move towards the positive anode, where oxidation occurs.” Then identify the particles actually reacting for the stated electrolyte and electrodes.

  • Use discharged to describe an ion gaining or losing electrons, rather than merely moving.
  • 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

Why melting changes conductivity

Core

Problem

Sodium chloride is tested in the solid and molten states. Which state conducts, and does melting create the ions?
View solution step by step

  1. Identify the particles

    Method

    Recognise sodium and chloride ions in both states.

    Reason

    The solid already has ions; melting changes their arrangement and mobility.

    Working

    Both states contain Na⁺ and Cl⁻.

  2. Check their mobility

    Method

    Distinguish fixed lattice positions from mobile liquid ions.

    Reason

    Charge carriers must move through the substance to sustain current.

    Working

    Solid: ions cannot travel through the lattice. Molten: ions can move.

  3. Explain the result

    Method

    State that the melt conducts and the solid does not.

    Reason

    Melting frees existing ions to carry charge; it does not make ions from neutral atoms.

    Working

    Conduction changes because mobility changes.

Continue this worked example in Writing and Checking Electrode Half-Equations.

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.

Continue this worked example in Writing and Checking Electrode Half-Equations.

Continue this worked example in Writing and Checking Electrode Half-Equations.

7. Mind Stretchers

The extension question is now in Writing and Checking Electrode Half-Equations.

The extension question is now in Writing and Checking Electrode Half-Equations.

Try independently: A clear, colourless sugar solution and a clear, colourless sodium chloride solution are tested with the same suitable DC apparatus. Only the salt solution conducts appreciably. Explain why transparency and colour cannot decide whether a substance is an electrolyte.

Show answer and reasoning

The appearances do not identify the charge carriers. Dissolved sugar remains as neutral molecules, while dissolved sodium chloride supplies mobile ions. The distinction is mobile ions, not colour or whether the solution is clear.

Try independently: In an electrolytic cell, a sulfate ion moves through the solution but is not discharged at the inert anode. Has it failed to carry charge? Explain.

Show answer and reasoning

No. A moving ion carries charge through the electrolyte whether or not it is the species discharged. Discharge is electron transfer at an electrode, a different event. Other species can produce the anode product.

Practise and check

Practise and check

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

Open the Redox Chemistry topic check
Syllabus and review details

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