Electrolysis: Principles, Electrodes and Electrolytes

Electrolysis uses electricity to break down an ionic compound. Nearly every mark lost here comes from one thing: mixing up the anode and the cathode.

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1. The key terms

Electrolysis — the breakdown of an ionic compound, when molten or in aqueous solution, by the passage of electricity.

Term	Meaning
Electrolyte	the molten or aqueous ionic compound that conducts
Electrodes	the rods dipped in, usually graphite or platinum (inert)
Anode	the POSITIVE electrode
Cathode	the NEGATIVE electrode
Anion	a negative ion — attracted to the anode
Cation	a positive ion — attracted to the cathode

PANIC — Positive Anode, Negative Is Cathode. This mnemonic was given directly in lessons, and it is worth writing at the top of the page. Confusing the two electrodes was recorded several times, and it inverts every product.

Opposites attract: negative anions go to the anode; positive cations go to the cathode.

Inert electrodes (graphite or platinum) are used so the electrodes do not react and take no part in the chemistry.

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2. Why electrolytes conduct

An ionic compound conducts only when its IONS are FREE TO MOVE — that is, when it is MOLTEN or AQUEOUS.

Solid ionic compounds do NOT conduct, because the ions are fixed in the lattice and cannot move.

The charge is carried by MOVING IONS, not by electrons. Electrons flow in the external wires; ions carry the charge through the electrolyte.

A recorded error claimed that “fixed lattice structure prevents conduction” was incorrect reasoning — in fact that is exactly right for the solid. The point to be precise about is that melting or dissolving frees the ions.

Pure water is a very poor electrolyte — it barely ionises. Uncertainty about this was recorded; water needs a dissolved ionic compound (or acid) to conduct appreciably.

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3. What happens at each electrode

At the CATHODE (negative): positive ions GAIN electrons → REDUCTION. At the ANODE (positive): negative ions LOSE electrons → OXIDATION.

Two mnemonics that work together:

RED CAT — REDuction at the CAThode AN OX — ANode is OXidation

Reduction is GAIN of electrons. Confusing reduction with loss was recorded — go back to OIL RIG.

Electron flow: electrons travel from the power supply to the cathode, where positive ions collect them; and from the anode back to the supply.

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4. Half-equations

Cathode (reduction) — electrons on the LEFT: Na⁺ + e⁻ → Na Pb²⁺ + 2e⁻ → Pb 2H⁺ + 2e⁻ → H₂

Anode (oxidation) — electrons on the RIGHT: 2Cl⁻ → Cl₂ + 2e⁻ 2Br⁻ → Br₂ + 2e⁻ 4OH⁻ → O₂ + 2H₂O + 4e⁻

Electrons go on the LEFT for reduction (gained) and the RIGHT for oxidation (lost). Confusion about electron placement was recorded twice.

Balance both the atoms AND the charges. The number of electrons must make the charges balance on each side.

Note the 2 in 2Cl⁻ → Cl₂ + 2e⁻ — chlorine is diatomic, so two chloride ions are needed. Confusion caused by coefficients in half-equations was recorded.

When combining half-equations, cancel the electrons — tutors flagged this. Multiply so the electron numbers match, then add.

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5. Naming the products

A chloride ion (Cl⁻) becomes chlorine gas (Cl₂). A bromide (Br⁻) becomes bromine (Br₂).

Confusing “bromide” with “bromine” was recorded. The -ide is the ion in the compound; the element released is the plain name. Say “bromide ions are oxidised to bromine”.

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6. Predicting products — the rules

Molten compounds (simple)

Only two ions are present, so:

The metal forms at the CATHODE. The non-metal forms at the ANODE.

Molten lead(II) bromide: lead at the cathode, bromine at the anode.

Aqueous solutions (harder)

Water also provides H⁺ and OH⁻ ions, so there is competition.

Remember that H⁺ and OH⁻ ions are ALWAYS present in aqueous solutions. Forgetting this was a specific recorded error, and it is why aqueous electrolysis differs from molten.

At the CATHODE — use the reactivity series:

If the metal is MORE reactive than hydrogen → HYDROGEN is produced. If the metal is LESS reactive than hydrogen (e.g. copper, silver) → the METAL is produced.

So for aqueous sodium chloride, HYDROGEN forms at the cathode — not sodium, because sodium is more reactive than hydrogen.

This was recorded as a misconception — a student expected sodium at the cathode. Compare it with molten sodium chloride, where sodium is produced because there is no water to compete.

Use the reactivity series to decide. Tutors flagged this, and forgetting the series was itself a recorded error.

At the ANODE:

If a HALIDE (Cl⁻, Br⁻, I⁻) is present and reasonably CONCENTRATED → the HALOGEN is produced. Otherwise (or if very dilute) → OXYGEN is produced from OH⁻ ions.

Concentration matters at the anode. Misunderstanding its role was recorded — concentrated sodium chloride gives chlorine, while very dilute gives oxygen.

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7. Setting up the apparatus

A typical diagram shows: a beaker of electrolyte, two electrodes dipping in, connected by wires to a d.c. power supply (a battery, not a.c.).

Label the anode as positive and the cathode as negative, and show the electrodes not touching. Difficulty drawing and labelling the setup was recorded.

Collecting the products: invert a test tube over each electrode to collect the gases.

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8. Mistakes that cost marks

Swapping anode and cathode.

Saying reduction happens at the anode.

Putting electrons on the wrong side of a half-equation.

Saying electrons carry the charge through the electrolyte.

Saying solid ionic compounds conduct.

Forgetting H⁺ and OH⁻ in aqueous solutions.

Predicting the metal at the cathode for a reactive metal in solution.

Confusing “bromide” with “bromine”.

Ignoring concentration at the anode.

Unbalanced charges in half-equations.

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Frequently asked questions

What is electrolysis? The breakdown of an ionic compound, when molten or aqueous, by electricity.

Which electrode is positive? The anode — remember PANIC.

Where do negative ions go? To the anode, where they are oxidised.

What happens at the cathode? Positive ions gain electrons — reduction.

Why must the compound be molten or aqueous? So the ions are free to move.

What carries the charge in the electrolyte? Moving ions — not electrons.

Which side do electrons go in a cathode half-equation? The left — they are gained.

What forms at the cathode in aqueous sodium chloride? Hydrogen, because sodium is more reactive than hydrogen.

What forms at the cathode in MOLTEN sodium chloride? Sodium — there is no water to compete.

Why are graphite electrodes used? They are inert and do not react.

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Quick revision checklist

• I know PANIC and can identify each electrode
• I know anions go to the anode and cations to the cathode
• I know why molten and aqueous compounds conduct but solids don’t
• I know ions carry the charge in the electrolyte
• I know RED CAT and AN OX
• I can write cathode half-equations with electrons on the left
• I can write anode half-equations with electrons on the right
• I balance atoms and charges
• I remember diatomic products need two ions
• I can name products correctly (bromide → bromine)
• I can predict products for molten compounds
• I remember H⁺ and OH⁻ are present in solution
• I use the reactivity series at the cathode
• I consider concentration at the anode
• I can draw and label the apparatus

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These notes cover the principles of electrolysis in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and data booklet for your own exam series.