ELECTROLYSIS is the decomposition of a substance using ELECTRICAL ENERGY.
For electrolysis to occur, the substance must be an ELECTROLYTE — a substance that can conduct electricity AND be decomposed by it.
Electrolytes are ionic compounds that are either:
MOLTEN (melted) — ions are free to move through the liquid.
DISSOLVED in water (aqueous solution) — ions are free to move through the solution.
Solid ionic compounds CANNOT be electrolysed — their ions are fixed in the lattice.
Electrolysis is used to:
Extract reactive metals from their ores (aluminium, sodium).
Electroplate metals (coating objects with a metal layer).
Purify metals (e.g. copper refining).
Manufacture chemicals (e.g. chlorine and sodium hydroxide from brine).
Key Terms and the Apparatus
ELECTRODES: the conductors that dip into the electrolyte.
Connected to a DIRECT CURRENT (DC) power supply.
CATHODE: the NEGATIVE electrode (connected to the negative terminal).
ANODE: the POSITIVE electrode (connected to the positive terminal).
Memory: CATHode = CATs are negative (they scratch!) or: An-ODE is pOsitivE.
How it works:
Positive ions (CATIONS) move TOWARDS the negative cathode.
At the cathode: cations GAIN electrons → REDUCED → solid metal or hydrogen gas deposited.
Negative ions (ANIONS) move TOWARDS the positive anode.
At the anode: anions LOSE electrons → OXIDISED → gas or element released.
OVERALL:
Oxidation at the Anode (OA).
Reduction at the Cathode (RC).
Memory: AN OX, A RED CAT (ANode OXidation; REDuction at CAThode).
Electrodes — Inert vs Reactive
INERT ELECTRODES (e.g. graphite or platinum):
Do not react with the electrolyte or products.
Simply provide a surface for ions to discharge.
Graphite is most commonly used — cheap and conducts electricity.
REACTIVE ELECTRODES (e.g. copper in electroplating):
The anode dissolves into the solution as copper ions.
The cathode grows as copper deposits onto it.
Used in electroplating and copper purification.
WHY INERT ELECTRODES ARE IMPORTANT:
If the anode reacted with the products or electrolyte, it would change the composition of the solution.
Using inert electrodes (graphite or platinum) gives consistent, predictable products.
NB — In aluminium extraction, the CARBON ANODES slowly BURN in the oxygen produced and must be regularly replaced. This is one reason aluminium extraction is expensive.
⚠️ Common Mistake
CATHODE is NEGATIVE — positive ions are attracted to it and gain electrons (reduced). ANODE is POSITIVE — negative ions are attracted to it and lose electrons (oxidised). Students often confuse these. Remember: AN OX (Anode Oxidation) and RED CAT (Reduction at Cathode).
📌 Key Note
Electrolysis: decomposition using electrical energy. Electrolyte: ionic compound that is molten or in solution. Cathode (−): cations attracted, gain electrons, REDUCED. Anode (+): anions attracted, lose electrons, OXIDISED. AN OX, RED CAT. Inert electrodes: graphite or platinum.
🎯 Matching Activity — Electrolysis Key Terms
Match each term to its correct definition. — drag the symbols on the right to match the component names on the left.
Cathode
Drop here
Anode
Drop here
Electrolyte
Drop here
Reduction (at cathode)
Drop here
Oxidation (at anode)
Drop here
Anions lose electrons — non-metal gas produced (O₂, Cl₂)
Positive electrode — negative ions attracted here — oxidation occurs
Negative electrode — positive ions attracted here — reduction occurs
Ionic compound that is molten or dissolved — conducts electricity and is decomposed
Cations gain electrons — metal deposited or H₂ gas produced
⭐ Higher Tier Only
Write ionic half equations for all electrode reactions. Explain using OIL RIG: anode = oxidation (OA), cathode = reduction (RC). Remember AN OX, RED CAT. Understand that current is carried by ions in solution and by electrons in external circuit.
🎯 Test Yourself
Question 1 of 2
1. Why can solid sodium chloride not be electrolysed, but molten sodium chloride can?
2. During electrolysis, where does oxidation take place?
⭐ How Well Do You Understand This Topic?
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