Electrolysis of Molten and Aqueous Compounds

• New lesson editor
• Chemistry
• Upper Secondary (Key Stage 4)
• GCSE

Electrolysis: Splitting with Electricity

This GCSE chemistry lesson explores Electrolysis of Molten and Aqueous Compounds. Students explain the movement of ions, predict products at electrodes and apply half-equations to electrolysis of molten salts and aqueous solutions. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.

Electrolysis, KS4 Chemistry, GCSE Chemistry, Electrolyte, Anode, Cathode, Ions, Cations, Anions, Oxidation, Reduction, Redox, Molten Compounds, Aqueous Solutions, Extraction of Aluminium, Half Equations, Electroplating, MCQs, Open Questions, Polls, Emoji Polls, Exit Tickets

Learning goal

I can explain how electrolysis breaks down ionic compounds and predict the products at each electrode in simple molten and aqueous examples.

What are some words associated with the process of electrolysis?

Use this to surface prior knowledge and misconceptions such as 'electricity', 'ions', 'splitting', 'electrodes', 'battery' or 'melting'. Pick out any useful terms to revisit during the lesson.

What electrolysis means

Electrolysis is the breakdown of an ionic compound by electricity.

The electrical energy makes ions move and allows a chemical change to happen at the electrodes.

It only works when the ions are free to move.

Students often think the electricity 'melts' the compound. Keep the focus on electricity causing decomposition, while molten or aqueous conditions allow ion movement.

Why ionic compounds must be molten or aqueous

• In a solid ionic compound, ions are fixed in a lattice

• Fixed ions cannot move to carry charge

• When molten, the lattice breaks apart and ions can move

• When dissolved in water, ions are free to move in solution

Stress the particle model: conduction in ionic substances depends on mobile charged particles, not free electrons as in metals.

Why these substances conduct

Ionic compounds conduct electricity only when their ions can move. In the molten state or in aqueous solution, positive and negative ions move through the liquid and carry charge. In the solid state, the ions are still charged, but they cannot move, so the substance does not conduct.

The electrolyte

The electrolyte is the molten or aqueous ionic substance that contains moving ions.

It conducts electricity and is the substance that is decomposed during electrolysis.

Make clear that the electrolyte is not just 'any liquid'. It must contain ions that can move.

Electrodes, anode and cathode

• Electrodes are conductors placed in the electrolyte

• The anode is the positive electrode

• The cathode is the negative electrode

• Reactions happen at the electrode surfaces

• Ions gain or lose electrons there

For GCSE, keep the sign convention clear for electrolysis: anode positive, cathode negative.

How ions move in electrolysis

• Positive ions are attracted to the negative cathode

• Negative ions are attracted to the positive anode

• Opposite charges attract

• This movement brings ions to the electrodes, where products form

Useful language: cations go to cathode, anions go to anode. If helpful, point out the matching first letters.

What happens at each electrode

At the cathode, positive ions gain electrons.

At the anode, negative ions lose electrons.

This is why new substances form at the electrodes during electrolysis.

You may mention reduction at the cathode and oxidation at the anode if your class already knows these terms, but they are not essential here.

Why does an ionic compound conduct when molten?

A

Ions can move

B

Atoms break apart

C

Electrons are free

D

Water is present

Tests the key idea from slides 2 to 4. 'Electrons are free' is a common confusion with metals, and 'water is present' checks that students do not think aqueous is the only case.

Complete the summary

In electrolysis, the ionic compound must be molten or aqueous so that ions can move. Positive ions go to the cathode, and negative ions go to the anode.

molten

aqueous

cathode

anode

solid

electrolyte

electrode

bromine

This anchors the load-bearing vocabulary from the first subtopic: molten, aqueous, cathode and anode.

Molten lead bromide

Molten lead bromide contains:

• Pb²⁺ ions

• Br⁻ ions

Because it is molten, both ions can move to the electrodes.

Use this as the first clean example because only the compound's own ions are present, with no water to compete.

Products from molten lead bromide

Pb²⁺ → cathode → lead

Br⁻ → anode → bromine

So electrolysis of molten lead bromide produces lead at the cathode and bromine at the anode.

If using half-equations: Pb²⁺ + 2e⁻ → Pb and 2Br⁻ → Br₂ + 2e⁻. Bromine is usually seen as brown fumes.

Molten compounds: the simple rule

For a molten ionic compound, the products come from the ions in that compound.

• Metal ions form the metal at the cathode

• Non-metal ions form the non-metal at the anode

There is no water present to change the products.

This rule is a key contrast with aqueous electrolysis and should be made secure before moving on.

What forms at the anode in molten lead bromide?

A

Lead

B

Hydrogen

C

Bromine

D

Oxygen

Checks application of the molten rule. Hydrogen and oxygen are plausible distractors from aqueous electrolysis.

Why are molten products easier to predict?

Expected answer elements: molten compounds contain only the compound's own ions, so products come directly from those ions. Water is not present, so there is no competition from hydrogen or oxygen.

Why aqueous solutions are different

In an aqueous solution, the ionic compound is dissolved in water.

This means there are ions from the compound and particles from water in the mixture. Water can affect which products form, so predicting products is less direct than in molten electrolysis.

Keep it simple: students do not need the full self-ionisation story of water here. The main idea is that water introduces competition at the electrodes.

At the cathode in aqueous solutions

• If the metal is more reactive than hydrogen, hydrogen is produced

• If the metal is less reactive than hydrogen, the metal is produced

At the cathode, either:

• hydrogen forms, or

• a metal forms

Rule

Hydrogen or a metal?

For simple GCSE cases, students mainly need to apply the hydrogen-versus-metal rule rather than justify it in detail.

At the anode in aqueous solutions

Halogen or oxygen?

• If chloride, bromide or iodide ions are present, the halogen forms

• Otherwise, oxygen usually forms

Rule

At the anode, either:

• a halogen forms, or

• oxygen forms

This is the standard school rule for common aqueous examples. Emphasise chloride, bromide and iodide as the halide ions to recognise.

A quick comparison

Molten compound

• Only the compound's ions are present

• Products are easier to predict

Aqueous solution

• The compound is dissolved in water

• Hydrogen or oxygen may form instead of the ions you first expect

This contrast often clears up confusion before students try examples.

Aqueous sodium chloride

Aqueous sodium chloride contains sodium ions and chloride ions in water.

When predicting the products, each electrode must be considered using the aqueous rules.

You can refer to this as brine if appropriate, but keep the main term as aqueous sodium chloride for consistency.

Predicting products: aqueous sodium chloride

• Chloride ions are present

• So chlorine forms at the anode

• Sodium is more reactive than hydrogen

• So hydrogen forms at the cathode

Anode

Cathode

Students often guess sodium metal because they see Na⁺ in the formula. Use this example to reinforce the aqueous cathode rule.

Why sodium chloride gives different products

Compare the two cases:

Molten sodium chloride would produce sodium and chlorine.

Aqueous sodium chloride produces hydrogen and chlorine because water changes what happens at the cathode.

This is the key example for the distinction between molten and aqueous electrolysis.

How to predict products step by step

1. Decide whether the electrolyte is molten or aqueous

2. Identify the positive and negative ions present

3. At the cathode, decide between hydrogen and a metal

4. At the anode, decide between a halogen and oxygen

5. Name the products at each electrode

Encourage students to always start by spotting molten versus aqueous before naming ions.

Product rules summary

• Molten compounds: metal at the cathode, non-metal at the anode

• Aqueous cathode: hydrogen forms unless the metal is less reactive than hydrogen

• Aqueous anode: halogen forms if chloride, bromide or iodide is present

• Otherwise, oxygen forms at the anode

This slide is the lesson landing point: students should now be able to apply these rules to simple examples.

In aqueous sodium chloride, what forms at the cathode?

A

Sodium

B

Hydrogen

C

Chlorine

D

Oxygen

Targets the common misconception noted on slide 17: students often choose sodium because they notice Na+ and forget the aqueous cathode rule.

Fill in the product rules

In an aqueous solution, water can affect the products. At the cathode, hydrogen forms if the metal is more reactive than hydrogen. At the anode, a halogen forms if chloride, bromide or iodide is present; otherwise oxygen usually forms.

aqueous

hydrogen

halogen

oxygen

molten

metal

electrolyte

bromine

Reinforces the two main aqueous rules from slides 12 to 20 without introducing a new example.

Do you feel more confident in your understanding of electrolysis?

Glossary

electrolysis - the breakdown of an ionic compound using electricity

electrolyte - the molten or aqueous ionic substance that conducts electricity and is decomposed

electrode - a conductor through which electricity enters or leaves the electrolyte

anode - the positive electrode in electrolysis

cathode - the negative electrode in electrolysis

ion - a charged particle formed when an atom or group of atoms gains or loses electrons

aqueous - dissolved in water

Glossary

molten - melted so that ions are free to move

cation - a positive ion attracted to the cathode

anion - a negative ion attracted to the anode

halogen - an element in Group 7, such as chlorine or bromine

reactivity - how easily an element takes part in a chemical reaction

product - a substance formed in a chemical reaction