AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), June 2025: Question 6
11 marks · Standard Demand difficulty · Extended Answer
Explain mass changes when heating magnesium and magnesium carbonate, interpret predicted vs actual graphs, and determine the balanced equation for the reaction of iron with steam using mole ratios.
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Question text
06 Some reactions appear to involve a change in mass.
A student investigated the change in mass when solids were heated.
Figure 7 shows the apparatus.
Figure 7
The student determined the mass of the contents of the crucible at regular intervals.
06.1 The student used a Bunsen burner instead of a water bath to heat the crucible.
Suggest one reason why.
[1 mark]
The student planned to heat:
• 0.01 mol of magnesium
• 0.01 mol of magnesium carbonate.
The equations for the reactions are:
Mg + ½O2 → MgO
MgCO3 → MgO + CO2
Figure 8 shows the student’s predicted results.
Figure 8
06.2 Look at Figure 8.
Explain the two trends for the first 3 minutes.
[4 marks]
Magnesium
Magnesium carbonate
06.3 Figure 8 shows that both reactions are complete after 5 minutes.
Why is the final mass of the contents of the crucible the same for both reactions?
[1 mark]
06.4 The student heated magnesium carbonate.
Figure 9 shows the results.
Figure 9
The student concluded:
‘The difference between the predicted results and the actual results is caused
by solid escaping from the crucible.’
Give one reason why the results show the student is not correct.
[1 mark]
06.5 Iron reacts with steam (H2O) to produce an oxide of iron and hydrogen gas.
0.120 moles of iron reacts with 0.160 moles of steam.
Determine the balanced equation for the reaction.
You should determine:
• the simplest whole number mole ratio of Fe : H2O
• the formula of the iron oxide produced.
[4 marks]
Simplest mole ratio of Fe : H2O = :
Formula of the iron oxide =
Balanced equation for the reaction:
+ → +
Mark scheme
Show the mark scheme
Question 6
AO /
Question Answers Extra information Mark
Spec. Ref.
06.1 a Bunsen burner reaches a allow a water bath will not reach 1 AO3
higher temperature a high enough temperature 5.3.1.3
AO /
Spec. Ref.
06.2 (magnesium) 1 AO2
(contents of crucible) increase in allow a value in the range 0.38
mass (from 0.24 to 0.38 g) to 0.39 for 0.38
(because magnesium) gains 1 AO3
oxygen
(magnesium carbonate) 1 AO2
(contents of crucible) decrease allow a value in the range 0.44
in mass (from 0.84 to 0.44 g) to 0.45 for 0.44
(because) carbon dioxide is allow (because) carbon dioxide 1 AO3
released gas is produced
5.3.1.3
AO /
Spec. Ref.
06.3 both reactions produce 0.01 mol 1 AO3
of magnesium oxide 5.3.1.3
AO /
Spec. Ref.
06.4 (if solid escaped) the final mass allow (solid cannot have 1 AO3
would be lower for the actual escaped because) same mass 5.3.1.3
results than for the predicted of solid product formed
results
allow (solid cannot have
escaped because) both form
0.4 g of solid
allow (solid cannot have
escaped because) the final
mass of the contents (of the
– crucible) are the same – 8464/C/1H –
AO /
Spec. Ref.19
06.5 (ratio Fe : H2O) 3:4 1 AO2
5.1.1.1
5.3.1.1
(formula) Fe3O4 allow formula correctly 1 5.3.2.1
determined from incorrect ratio 5.3.2.2
5.3.2.3
3 Fe + 4 H2O → Fe3O4 + 4 H2 allow multiples 2 5.4.1.2
allow correct use of an
incorrectly determined formula
for iron oxide
allow 1 mark for Fe, H2O, Fe3O4
and H2
or
allow 1 mark for Fe, H2O, H2 and
an incorrectly determined
formula for iron oxide
Total Question 6 11
How to answer it
Mass Changes in Reactions & Mole Ratios
This question assesses your understanding of apparent mass changes during chemical reactions in open systems (oxidation vs thermal decomposition), explaining trends using graphical data, evaluating experimental claims, and using reacting mole ratios to deduce the empirical formula of an oxide and write a balanced equation.
Heating Apparatus Comparison
Explaining why a Bunsen burner was selected instead of a water bath
✅ Correct Answer [1 mark]
Any one of the following:
- A Bunsen burner reaches a higher temperature.
- A water bath will not reach a high enough temperature to decompose or react the solids.
💡 Key Knowledge
A standard water bath cannot exceed 100 °C (the boiling point of water). Thermal decomposition of metal carbonates (like MgCO₃) and burning magnesium in air require temperatures well above 500 °C.
Explaining Mass Trends from Graphs
Describing and explaining mass changes for Mg and MgCO₃ over the first 3 minutes
✅ Correct Answer [4 marks]
Magnesium [2 marks]:
- Mass increases from 0.24 g to 0.38 g (allow 0.38 to 0.39 g) [1 mark]
- Because magnesium reacts with / gains oxygen from the air [1 mark]
Magnesium carbonate [2 marks]:
- Mass decreases from 0.84 g to 0.44 g (allow 0.44 to 0.45 g) [1 mark]
- Because carbon dioxide gas is released / escapes into the air [1 mark]
🧠 Exam Technique: "Explain the trends"
When an exam question asks you to explain a trend from a graph:
- Describe the change: State clearly whether mass increases or decreases, and quote readings with units (e.g. 0.24 g to 0.38 g).
- Explain the chemical reason: State which gas was added from the air or which gas escaped into the surroundings.
❌ Common Errors
- Forgetting to quote specific data values at 0 and 3 minutes.
- Claiming that magnesium "creates mass" or violates conservation of mass (oxygen gas has mass!).
- Stating that magnesium carbonate "evaporates" rather than decomposing into carbon dioxide gas.
Final Mass of Solid Products
Why the final mass is identical (0.4 g) for both reactions
✅ Correct Answer [1 mark]
Both reactions produce 0.01 mol of magnesium oxide (MgO).
💡 Key Knowledge
Look at the equations given in the question prompt:
- 0.01 mol Mg + 0.005 mol O₂ → 0.01 mol MgO
- 0.01 mol MgCO₃ → 0.01 mol MgO + 0.01 mol CO₂
Since Mr of MgO = 24 + 16 = 40, mass = 0.01 mol × 40 g/mol = 0.40 g for both!
Evaluating Experimental Conclusions
Disproving the claim that solid escaped from the crucible
✅ Correct Answer [1 mark]
Any one of the following:
- If solid had escaped, the final mass would be lower for actual results than predicted.
- The final mass is the same for both curves (both finish at exactly 0.4 g).
🧠 Exam Technique: Look at the End Point
The actual curve drops faster because the reaction was simply faster than expected. However, both curves plateau at 0.4 g. If any solid had spat out or escaped, the final horizontal line would sit below 0.4 g.
Reacting Mole Ratios & Balancing Equations
Finding the ratio of Fe : H₂O, formula of iron oxide, and balanced equation
📐 Step-by-Step Calculation [4 marks]
- Find the simplest mole ratio:
Ratio Fe : H₂O = 0.120 : 0.160
Divide both by the smallest number (0.120) or divide by 0.040:
0.120 / 0.040 = 3
0.160 / 0.040 = 4
Simplest ratio = 3 : 4 [1 mark] - Deduce the formula of iron oxide:
3 Fe atoms react with 4 H₂O molecules. The 4 H₂O donate 4 oxygen atoms.
Therefore, the iron oxide contains 3 Fe and 4 O:
Formula = Fe₃O₄ [1 mark] - Write the balanced equation:
Reactants: 3 Fe + 4 H₂O
Products: Fe₃O₄ + H₂
Balance hydrogens: 4 H₂O has 8 H atoms → forms 4 H₂
3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂ [2 marks]
✅ Final Answers Summary
- Simplest mole ratio of Fe : H₂O = 3 : 4
- Formula of the iron oxide = Fe₃O₄
- Balanced equation:
3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂
• 1 mark for all correct formulas (Fe, H₂O, Fe₃O₄, H₂)
• 1 mark for correct balancing coefficients (3, 4, 1, 4)
❌ Common Errors
- Writing the ratio backwards (e.g. 4 : 3 instead of 3 : 4).
- Assuming the oxide is standard iron(III) oxide (Fe₂O₃) or iron(II) oxide (FeO) instead of using the calculated ratio.
- Forgetting that hydrogen gas is diatomic (writing H instead of H₂) or failing to balance hydrogen atoms.
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.