Electrolysis | GCSE Chemistry

Chemical changes

Lesson overview

This lesson introduces the core chemistry idea, the useful equipment and the calculation or data skills used on this page.

Focuselectrolysis of molten and aqueous ionic compounds

Time45-60 minutes

EquipmentPeriodic table, calculator and notebook.

Practical linkelectrolysis practical observations and product tests

Maths tagsion movement, charge and half equations

What you will learn

Describe the key chemistry ideas behind electrolysis of molten and aqueous substances.
Use precise GCSE command-word language in explanations.
Apply the idea to unfamiliar substances, data or practical contexts.
Check answers using units, state symbols, observations or balanced equations where relevant.

Core knowledge

Electrolysis uses electricity to break down an ionic compound called an electrolyte.
The electrolyte must be molten or dissolved so ions can move.
Positive ions move to the negative cathode and gain electrons; this is reduction.
Negative ions move to the positive anode and lose electrons; this is oxidation.
In aqueous solutions, hydrogen or oxygen may form because water also supplies ions.

Electrolysis of ionic compounds infographic

Infographic explaining electrolysis of ionic compounds, including molten and aqueous electrolysis, electrodes, ions and product prediction. — Use this visual to trace ion movement and product formation during electrolysis.Download visual

Practical link

Use inert electrodes unless told otherwise.
Record the product at each electrode.
Test hydrogen with a lit splint.
Test oxygen with a glowing splint and chlorine with damp litmus.

Electrolysis practice set

Use the worked examples and practice questions on this page as a complete study task: learn the electrolyte and anode meanings, summarise the infographic in your own words, then answer the questions using the data, equations and observations given here. Check every answer for ion movement, charge and half equations.

Clear explanation

Electrolysis of molten and aqueous substances is built around electrolysis of molten and aqueous ionic compounds. Start by learning what electrolyte and anode mean, then connect them to the sequence in the infographic.

The key GCSE skill is to move from evidence to explanation: name the observation, data trend, particle model, structure or reaction, then explain why it supports the conclusion.

For calculation or data work, show ion movement, charge and half equations. For written explanations, include the specific chemistry reason rather than a memorised topic sentence.

Worked examples

Core example: molten lead bromide

Molten lead bromide contains Pb2+ and Br- ions.

Pb2+ moves to the cathode and forms lead.

Br- moves to the anode and forms bromine.

Answer: Lead forms at the cathode and bromine forms at the anode.

Extension example: copper half equation

Copper ions gain electrons at the cathode.

Cu2+ needs two electrons.

Electrons go on the left for reduction.

Answer: Cu2+ + 2e- -> Cu.

Quick checks

Choose an answer, then check your thinking.

1. Why must the ionic substance be molten or dissolved?

Atoms must get larger Electrodes must dissolve Ions must move

2. Which electrode attracts positive ions?

Cathode Anode Filter

Practice questions

Question 1

Define electrolyte.

Reveal answer and marking guidance

Answer: A molten or dissolved ionic substance that conducts electricity and is decomposed during electrolysis.

Marking: Credit ionic, mobile/conducts and decomposed.

Question 2

Predict products from molten sodium chloride.

Reveal answer and marking guidance

Answer: Sodium at the cathode and chlorine at the anode.

Marking: Credit products and electrodes.

Question 3

Write the half equation for chloride ions forming chlorine.

Reveal answer and marking guidance

Answer: 2Cl- -> Cl2 + 2e-.

Marking: Credit balanced atoms and charge.

Question 4

Stretch: Why does hydrogen form instead of sodium in aqueous sodium chloride?

Reveal answer and marking guidance

Answer: Sodium is more reactive than hydrogen, so hydrogen is discharged at the cathode instead.

Marking: Credit reactivity comparison and cathode product.

Practice ladder

FluencyRecall the key definition, symbol, structure, equation or observation.

ApplicationApply electrolysis of molten and aqueous substances to unfamiliar substances, practicals or data.

Practical interpretationUse evidence, method quality, uncertainty or conclusion wording where asked to evaluate.

Maths skillUse units, ratios, graphs and significant figures accurately.

Answers and marking guidance

The exact practice answers are hidden under each question so you can try first. Marks come from using the correct chemistry model, choosing the right equation where needed, keeping units with values, and explaining changes with precise words such as particles, ions, bonds, energy, rate, evidence and uncertainty.

Common mistakes

Mixing up cathode and anode charges.
Saying electrons flow through the electrolyte.
Ignoring water ions in aqueous electrolysis.
Writing half equations without electrons.

Extension challenge

Create a six-line revision card for electrolysis of molten and aqueous substances: two definitions, one infographic detail or equation line, one equation or data check, one common mistake, and one complete explanation sentence using electrolyte, anode, cathode.

Reveal answer

Example answer: A complete response names the chemistry model, uses accurate units or observations, and explains why the evidence supports the conclusion.