AQA GCSE Chemistry Chemistry Paper 1 (Foundation), November 2020: Question 4
8 marks · Low Demand difficulty · Short Answer
Identify observations, analyze experimental results, and explain electrode properties and ion migration during the electrolysis of aqueous salt solutions.
Practise this questionQuestion
Question text
04 This question is about electrolysis.
A student investigated the hypothesis:
‘The electrolysis of a salt solution produces a metal at the negative electrode and a
gas at the positive electrode.’
Figure 4 shows the apparatus used.
Figure 4
04.1 What observation would be made at each electrode if the hypothesis is correct?
[2 marks]
Observation if metal produced at the negative electrode
Observation if gas produced at the positive electrode
Table 3 shows the student’s results.
Table 3
Product at the Product at the
Salt solution
negative electrode positive electrode
Copper chloride Copper Chlorine
Potassium nitrate Hydrogen Oxygen
Silver nitrate Silver Oxygen
04.2 Which salt solution in Table 3 does not match the student’s hypothesis?
Give one reason why.
[2 marks]
Salt solution
Reason
04.3 Give two reasons why graphite is used for the electrodes.
[2 marks]
A different student investigated what happens during electrolysis.
Figure 5 shows the apparatus.
Figure 5
The purple crystal contained:
• colourless positive ions
• purple coloured negative ions.
The purple crystal dissolved in the electrolyte solution.
04.4 What happens to the purple coloured ions?
Give one reason for your answer.
[2 marks]
Tick ( ) one box.
The ions do not move.
The ions move towards the negative electrode.
The ions move towards the positive electrode.
Reason
Mark scheme
Show the mark scheme
Question 4
AO /
Question Answers Extra information Mark
Spec. Ref.
04.1 (negative electrode) solid allow the electrode changes 1 AO3
produced colour 4.4.3.4
RPA 3
ignore metal produced
(positive electrode) bubbles / ignore gas produced 1
fizzing / effervescence
04.2 potassium nitrate 1 AO3
4.4.3.4
hydrogen is not a metal allow hydrogen is a gas 1 RPA 3
allow hydrogen is not a solid
allow the products at both
electrodes are gases
allow the product at the negative
electrode is not potassium
allow potassium is more reactive
than hydrogen
04.3 (graphite) conducts (electricity) allow (graphite) has delocalised 1 AO1
/ free electrons 4.2.3.2
4.4.3.4
RPA 3
(graphite) is inert allow (graphite) is unreactive 1
04.4 the ions move towards the 1 AO3
positive electrode
the electrode attracts ions of the allow opposite charges attract 1 AO1
opposite charge
4.4.3.1
Total 8
How to answer it
Electrolysis of Aqueous Solutions & Ion Migration
This question assesses practical and theoretical understanding of Electrolysis (AQA Topic 4.4.3 / Required Practical 3):
- Linking microscopic chemical products to macroscopic experimental observations.
- Evaluating experimental data against a stated hypothesis.
- Recalling why graphite is suitable as an inert electrode material (structure and bonding).
- Applying fundamental principles of electrostatic attraction and ionic movement during electrolysis.
Electrolysis Observations at Each Electrode
Linking chemical products to visible observations
✅ Mark Scheme Answers
- Negative electrode (metal produced): A solid is produced (or: electrode changes colour / coated in solid). [1 mark]
- Positive electrode (gas produced): Bubbles / fizzing / effervescence. [1 mark]
🧠 Exam Technique: Observations vs Inferences
An observation is something you can see, hear, or smell with your senses.
- Never repeat the deduction from the question (e.g. saying "a metal is formed" or "gas is given off"). The question specifically asked what you see.
- For any gas: write "bubbles" or "fizzing".
- For any metal: write "solid forms" or "electrode changes colour" (e.g., pink-brown coating for copper).
❌ Common Errors
- Repeating the prompt: Writing "metal forms" or "gas is made" scores 0 marks. The examiner explicitly notes: ignore metal produced / ignore gas produced.
- Naming specific substances: Writing "copper" or "chlorine" instead of an observable change.
Evaluating the Hypothesis with Results Table
Identifying anomalies against a proposed rule
✅ Mark Scheme Answers
- Salt solution: Potassium nitrate [1 mark]
- Reason: Hydrogen is not a metal (or: hydrogen is a gas / both products are gases / negative electrode product is not potassium) [1 mark]
💡 Key Knowledge: Electrolysis Rules of Aqueous Solutions
The student's hypothesis stated: "produces a metal at the negative electrode and a gas at the positive electrode."
- Negative electrode (cathode): Hydrogen gas (H₂) is discharged if the metal in solution is more reactive than hydrogen (e.g. potassium, sodium). A metal is only discharged if it is less reactive than hydrogen (e.g. copper, silver).
- In potassium nitrate, the negative electrode produces hydrogen, which is a non-metal / gas, disproving the student's rule!
❌ Common Errors
- Selecting copper chloride because chlorine is not oxygen (chlorine is still a gas, so it fits the hypothesis).
- Giving an incomplete reason such as "because it makes hydrogen" without explaining why that violates the hypothesis (you must state that hydrogen is not a metal or is a gas).
Why Graphite is Used as Electrodes
Linking structure and properties of giant covalent carbon
✅ Mark Scheme Answers
Any two from:
- Conducts electricity (or: has delocalised / free electrons) [1 mark]
- It is inert (or: unreactive) [1 mark]
💡 Key Knowledge: Why Graphite?
- Conductivity: Each carbon atom forms only 3 covalent bonds, leaving one delocalised electron per atom that is free to move throughout the structure and carry charge.
- Inertness: Graphite is chemically unreactive, meaning it does not react with the electrolyte or products (like reactive chlorine gas or oxygen) during the experiment.
❌ Common Errors
- Stating that graphite has "ions that are free to move". Graphite is a giant covalent non-metal structure; it conducts via delocalised electrons, not ions!
- Mentioning properties irrelevant to electrodes, like "it is slippery / soft / has layers that slide" (those apply to lubricants or pencils, not electrode suitability).
Movement of Ions During Electrolysis
Electrostatic attraction in electrolysis
✅ Mark Scheme Answers
- Box ticked: The ions move towards the positive electrode. [1 mark]
- Reason: The electrode attracts ions of the opposite charge (or: opposite charges attract / negative ions are attracted to the positive electrode). [1 mark]
🧠 Exam Technique: Unpack the Prompt
The question provides two crucial facts:
- Colourless ions are positive.
- Purple coloured ions are negative.
Since the purple ions carry a negative charge, they must migrate towards the positive electrode (anode) because opposite charges attract.
❌ Common Errors
- Confusing charge signs: Forgetting that purple = negative ions, leading to selecting the negative electrode.
- Vague reasons: Stating simply "charges attract" without specifying that opposite charges attract, or failing to mention that negative ions are drawn to the positive electrode.
Topics
Chemistry · Required Practicals · C4: Chemical Changes · C2: Bonding, Structure and the Properties of Matter · Required Practicals
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Foundation), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.