Question 1 Medium
Name the gas made at the anode when sodium chloride solution is electrolysed.
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Answer: chlorine
- Chloride ions go to the anode.
- They form chlorine.
Electrolysis: GCSE Combined Science Chemistry practice questions
A second set of questions on electrolysis. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
In the extraction of aluminium, which substance is added to lower the melting point?
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Answer: cryolite
- Aluminium oxide has a very high melting point.
- The solvent is cryolite.
Question 3 Medium
Name the product at the anode when molten potassium iodide is electrolysed.
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Answer: iodine
- Iodide ions are negative.
- They go to the anode and lose electrons.
Question 4 Medium
What forms at the cathode when potassium chloride solution is electrolysed?
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Answer: hydrogen
- Potassium is more reactive than hydrogen.
- So hydrogen gas forms instead.
Question 5 Hard
Write a half equation for the formation of aluminium at the cathode.
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Answer: Al³⁺ + 3e⁻ → Al
- Aluminium ions are 3+.
- They gain three electrons.
Question 6 Hard
Write a half equation for the formation of chlorine at the anode.
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Answer: 2Cl⁻ → Cl₂ + 2e⁻
- Two chloride ions lose one electron each.
- 2Cl⁻ → Cl₂ + 2e⁻
Question 7 Hard
Explain why the carbon anodes used to extract aluminium have to be replaced.
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Answer: oxygen reacts with the carbon to make carbon dioxide
- Oxygen forms at the anode.
- It reacts with the carbon to make carbon dioxide.
Question 8 Hard
During the purification of copper, what happens to the mass of the pure copper cathode?
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Answer: it increases
- Copper ions gain electrons at the cathode.
- Copper atoms are deposited, so the mass increases.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What is electrolysis?
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Using an electric current to break down an ionic compound that is molten or in solution.
Check 2
What is an electrolyte?
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The molten or dissolved ionic compound that conducts.
Check 3
What is the cathode?
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The negative electrode.
Check 4
What is the anode?
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The positive electrode.
Check 5
Where do positive ions go?
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To the cathode.
Check 6
Why must an ionic compound be molten or dissolved?
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The ions must be free to move.
Check 7
What forms at the cathode in a solution if the metal is more reactive than hydrogen?
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Hydrogen.
Check 8
What forms at the anode if a halide is present?
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The halogen.
Check 9
What forms at the anode if there is no halide?
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Oxygen.
Check 10
What is cryolite used for in aluminium extraction?
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It lowers the melting point of the aluminium oxide.
Check 11
Why do the carbon anodes need replacing in aluminium extraction?
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The oxygen reacts with them to make carbon dioxide.
Check 12
What is the half equation for copper ions forming copper?
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Cu²⁺ + 2e⁻ → Cu.
Check 13
What is the half equation for chloride ions forming chlorine?
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2Cl⁻ → Cl₂ + 2e⁻.
Check 14
What happens at the cathode: oxidation or reduction?
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Reduction.
Check 15
What are the products of electrolysing brine?
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Hydrogen, chlorine and sodium hydroxide solution.
Check 16
How is copper purified by electrolysis?
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Impure copper is the anode and pure copper builds up on the cathode.
Check 17
What are cations?
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Positive ions, which move to the cathode.
Check 18
What are anions?
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Negative ions, which move to the anode.
Check 19
What does inert mean for an electrode?
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It does not take part in the reaction.
Check 20
What forms at the cathode when molten lead bromide is electrolysed?
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Lead.
Check 21
What is oxidation in terms of electrons?
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Loss of electrons.
Check 22
What is reduction in terms of electrons?
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Gain of electrons.
Check 23
What is electroplating?
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Putting a thin layer of metal on an object using electrolysis.
Check 24
What is the half equation for aluminium ions forming aluminium?
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Al³⁺ + 3e⁻ → Al.
Check 25
Why does hydrogen form at the cathode in sodium chloride solution?
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Sodium is more reactive than hydrogen.
Check 26
What is the test for oxygen gas?
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It relights a glowing splint.
Check 27
What is the test for hydrogen gas?
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A lit splint makes a squeaky pop.
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