AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), June 2025: Question 1
9 marks · Low Demand difficulty · Short Answer
Analyze an electrolysis experiment involving copper chloride and potassium chloride solutions, including safety precautions, graph plotting, data interpretation, concentration calculation, and predicting products based on reactivity.
Practise this questionQuestion
Question text
01 A teacher demonstrated the electrolysis of copper chloride solution.
Figure 1 shows the apparatus.
Figure 1
The teacher measured the mass of the negative electrode every 60 seconds.
01.1 Chlorine gas is given off during the electrolysis of copper chloride solution.
Chlorine gas is poisonous.
Give one way of reducing the risk of harm.
[1 mark]
Table 1 shows the results.
Table 1
Time Mass of negative
in seconds electrode in grams
0 20.2
60 22.0
120 23.4
180 24.2
240 24.8
*03* 300 25.2
360 25.2
01.2 Plot the data from Table 1 on Figure 2.
Draw a line of best fit.
The first point has been plotted for you.
[3 marks]
Figure 2
01.3 A student made the hypothesis:
‘The negative electrode will increase by a constant mass every 60 seconds.’
How do the results in Table 1 show that the hypothesis is not correct?
[1 mark]
01.4 25 cm3 of the solution of copper chloride contained 5.5 g of copper chloride.
Calculate the concentration of the solution in g/dm3.
1 dm3 = 1000 cm3
[3 marks]
Concentration =6 g/dm3
01.5 The teacher also electrolysed potassium chloride solution.
Hydrogen gas was given off at the negative electrode.
Why was hydrogen gas given off?
[1 mark]
Tick ( ) one box.
Hydrogen is less reactive than potassium.
Hydrogen has the same reactivity as potassium.
Hydrogen is more reactive than potassium.
Mark scheme
Show the mark scheme
Question 1
AO /
Question Answers Extra information Mark
Spec. Ref.
01.1 ignore references to safety AO3
glasses or gloves 5.4.3.4
RPA9
any one from: 1
• do experiment in a fume allow use an extractor fan
cupboard
• do experiment in a allow descriptions of well-
well-ventilated laboratory / ventilated such as open
room windows
allow use a gas mask
AO /
Spec. Ref.
01.2 all 6 points plotted correctly allow a tolerance of ± ½ a small 2 AO2
square 5.4.3.4
RPA9
allow 1 mark for 4 or 5 points
plotted correctly
line of best fit 1
AO /
Spec. Ref.
01.3 any one from: 1 AO3
5.4.3.1
• the increase in mass (for allow evidence of two correctly 5.4.3.4
successive 60 second determined changes in mass RPA9
intervals) decreases (for 60 second intervals) from
Table 1
allow the difference in mass
(every 60 seconds) is not the
same
• after 300 seconds the mass
remains constant – – 8464/C/1H –
AO /
Spec. Ref.
01.4 (volume conversion 25 cm3 =) AO2 7
0.025 (dm3) 1 5.3.2.5
5.5 allow correct use of an incorrect 1
(concentration =) / no conversion of volume
0.025
= 220 (g/dm3) 1
OR
5.5
(concentration =) (1)
= 0.22 (g/cm3) (1)
(conversion = 0.22 × 1000) allow correct use of an
= 220 (g/dm3) (1) incorrectly determined
concentration in g/cm3
using the values in the question
OR
1000
(ratio of volume =) 40 (1)
(so ratio of mass =) 40 × 5.5 (1)
= 220 (g/dm3) (1)
– TRILOGY – 8464/C/1H –
AO /
Spec. Ref.
01.5 hydrogen is less reactive than 1 AO2
potassium 5.4.3.4
RPA9
Total Question 1 9
How to answer it
Electrolysis & Solution Concentrations
This question assesses practical and theoretical skills from AQA GCSE Combined Science (Trilogy) - Topic 4.4: Chemical Changes and Required Practical 9 (Electrolysis):
- Laboratory Safety & Risk Management: Safe handling of toxic gases (chlorine).
- Data Handling & Graph Skills: Plotting Cartesian coordinates accurately and drawing a smooth line/curve of best fit.
- Evaluating Hypotheses: Using quantitative experimental data to disprove a statement.
- Concentration Calculations: Converting volumes from cm³ to dm³ and calculating concentration in g/dm³.
- Rules of Electrolysis in Aqueous Solutions: Predicting cathode products using the reactivity series.
Part 01.1: Laboratory Safety with Toxic Gases
Reducing the risk of harm from chlorine gas (1 mark)
✅ Correct Answers (Accept any one)
- Carry out the experiment in a fume cupboard.
- Ensure the room is well-ventilated (e.g. open windows).
- Use an extractor fan.
- Wear a gas mask / respirator.
❌ Common Errors (Zero Marks)
- "Wear safety goggles / glasses" – Goggles protect eyes from splashes, not from inhaling toxic gases.
- "Wear gloves / lab coat" – Protects skin, not respiratory inhalation.
- Vague answers like "be careful" or "stand back".
Part 01.2: Graph Plotting & Line of Best Fit
Plotting experimental data from Table 1 onto Figure 2 (3 marks)
📊 Data to Plot
| Time (s) | 0 | 60 | 120 | 180 | 240 | 300 | 360 |
|---|---|---|---|---|---|---|---|
| Mass (g) | 20.2 (given) | 22.0 | 23.4 | 24.2 | 24.8 | 25.2 | 25.2 |
💡 Mark Breakdown
- 2 marks: All 6 remaining points plotted correctly within ± ½ a small square.
- (1 mark allowed if 4 or 5 points are plotted correctly).
- 1 mark: A smooth line of best fit drawn through the points.
🧠 Exam Technique: Drawing the Best Fit Line
- Identify the pattern: The mass increases quickly at first, slows down, and flattens out (levels off) at 25.2 g from 300 s onwards.
- Do NOT join dot-to-dot: You will lose the mark if you connect the points using a ruler in a jagged zig-zag line.
- Draw a single, continuous, smooth curve that levels off horizontally at 25.2 g.
Part 01.3: Evaluating a Hypothesis
Why the results show the mass does not increase by a constant amount (1 mark)
✅ Acceptable Explanations (Any one)
- The increase in mass each 60 seconds decreases (slows down) over time.
- After 300 seconds, the mass stops increasing / remains constant.
- Quoting evidence from the table: e.g. mass increases by 1.8 g (0–60 s), but then by 1.4 g (60–120 s), which are not equal.
❌ Common Errors
- Simply stating "the mass increases" without evaluating the rate of increase.
- Stating that "it is not directly proportional" without explaining how the numbers from Table 1 prove this.
Part 01.4: Concentration Calculation
Calculate the concentration in g/dm³ from 25 cm³ and 5.5 g (3 marks)
📐 Step-by-Step Calculation
- Step 1: Convert volume to dm³ (1 mark)
Since 1 dm³ = 1000 cm³:
Volume = 25 cm³ ÷ 1000 = 0.025 dm³ - Step 2: Apply the concentration formula (1 mark)
Concentration (g/dm³) = Mass (g) ÷ Volume (dm³)
Concentration = 5.5 ÷ 0.025 - Step 3: Calculate the final answer (1 mark)
Concentration = 220 g/dm³
💡 Alternative Valid Method (Scaling)
You can find how many 25 cm³ portions fit in 1000 cm³:
- Scale factor: 1000 ÷ 25 = 40
- Total mass in 1 dm³: 40 × 5.5 g = 220 g/dm³
❌ Calculation Traps
- Forgetting to convert cm³: Calculating 5.5 ÷ 25 = 0.22 g/cm³. If you do this, you can still get up to 2 marks if your working is clear, but you lose the conversion mark.
- Inverting the division: Doing volume ÷ mass (0.025 ÷ 5.5). Remember: units are g/dm³ (grams divided by decimetres cubed).
Part 01.5: Electrolysis Rules at the Cathode
Why hydrogen gas is produced instead of potassium (1 mark)
✅ Correct Selection
☑ Hydrogen is less reactive than potassium.
(Box 1 should be ticked)
💡 Aqueous Electrolysis: Negative Electrode (Cathode) Rule
- In an aqueous solution, water dissociates slightly into H⁺ and OH⁻ ions.
- Both the metal cation (K⁺) and H⁺ travel to the negative electrode.
- Rule: The less reactive element is discharged.
- Since potassium is higher than hydrogen in the reactivity series, hydrogen ions gain electrons and form H₂ gas.
- Comparison: In copper chloride, copper was formed because copper is less reactive than hydrogen!
Topics
Chemistry · C3: Quantitative Chemistry · C4: Chemical Changes
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.