AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2018: Question 5

✅ Correct answer

CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

💡 Key knowledge

• Marble chips are calcium carbonate, CaCO₃.
• Carbonates + acids produce salt + carbon dioxide + water.
• The salt here is calcium chloride, CaCl₂.

🧠 Exam technique

• Write the products first, then balance the equation.
• Make sure you use the correct formulae: CaCl₂ , CO₂ , H₂O .

❌ Common errors

• Writing H₂CO₃ instead of CO₂ + H₂O.
• Forgetting to balance the chlorine atoms, so leaving HCl unbalanced.
• Writing the wrong salt formula, e.g. CaCl or CaCl₂H₂.

✅ What a full-mark answer could say

The volume of gas increases quickly at first, for example from 0 cm³ at 0 s to about 40 cm³ at 10 s. After that it increases more slowly, reaching about 60 cm³ by 30 s, and then the graph levels off.

💡 Key knowledge

• A graph that becomes flatter shows the rate is decreasing.
• When the line levels off, no more gas is being produced.
• From the graph, the maximum volume is about 60 cm³.

🧠 Exam technique

• Use numbers from the graph, not just words like “it goes up”.
• Describe both the change in volume and the change in steepness.
• Good phrases: increases rapidly, less rapidly, levels off.

❌ Common errors

• Saying only “it increases” — this is not enough for 3 marks.
• Quoting values without a time, or times without a volume.
• Missing the final idea that the graph levels off.

✅ Correct method

Draw a tangent to the curve at 15 s, then find the gradient of that tangent.

Example gradient calculation: choose two points on the tangent and use gradient = change in volume ÷ change in time.

📐 Calculations / method

1. Draw a straight line that just touches the curve at 15 s.
2. Pick two clear points on the tangent line.
3. Work out change in volume in cm³.
4. Work out change in time in s.
5. Calculate gradient = Δvolume ÷ Δtime.

🧠 Exam technique

• For a rate from a curve, you need the tangent, not the gradient of the curve itself.
• Use a ruler and make the tangent touch the curve at one point.
• Marks are for the method, so show your working even if the final number is wrong.

❌ Common errors

• Calculating the gradient of the whole graph instead of the tangent.
• Drawing a line that cuts through the curve in lots of places.
• Forgetting that the tangent must be at 15 s.

✅ Correct answer

cm³ per second

You could also write cm³/s or cm³ s⁻¹.

💡 Key knowledge

• Rate = change in volume ÷ time.
• So the unit must be volume per time.

❌ Common errors

• Writing sec or s only — that is just time, not rate.
• Using non-SI shortcuts like cm³/sec if your teacher wants the standard form. The exam accepts cm³/s or cm³ s⁻¹.

✅ Correct answer

The rate increases as the marble chips get smaller.

💡 Key knowledge

• Smaller chips have a larger surface area for the same mass or volume.
• Larger surface area gives a faster reaction.

❌ Common errors

• Stating only “small chips react faster” without linking it to rate.
• Giving an explanation instead of a conclusion when the question only asks for the conclusion.

✅ Correct answer

Same amount of acid / same volume of acid / same number of moles of acid.

💡 Key knowledge

• The total amount of gas depends on the amount of the limiting reactant.
• Here, the acid amount is the same, so each experiment makes the same final amount of gas.

🧠 Exam technique

• If the final volume is the same, think: same number of moles of a reactant.
• You can also mention that the same mass of CaCO₃ was used, but the mark scheme mainly rewards the same acid amount.

❌ Common errors

• Writing “same rate” — that does not explain the same final volume.
• Saying “same concentration” without saying same volume or same moles, if the question is about the total gas.

✅ Correct answer

Surface area = 24 cm²

📐 Calculations: step by step

1. Find the area of one face: 2 × 2 = 4 cm²
2. A cube has 6 faces
3. So total surface area = 6 × 4 = 24 cm²

🧠 Exam technique

• Surface area means the area of all outer faces.
• For a cube, use 6 × area of one square face.

❌ Common errors

• Using volume instead of surface area.
• Writing 8 cm² because the volume is 8 cm³.
• Forgetting the units: surface area is in cm², not cm³.

✅ Correct answer

Smaller chips have a larger surface area for the same volume, so there are more frequent collisions between acid particles and the marble chips.

💡 Key knowledge

• Collision theory says reactions happen when particles collide.
• More surface area means more particles are exposed to the acid at once.
• That increases the rate of reaction.

🧠 Exam technique

• Always link surface area → collisions → faster rate.
• You do not need to mention energy of collisions for this question.

❌ Common errors

• Talking about particles moving faster, which is not the key idea here.
• Saying “more successful collisions” without mentioning why there are more collisions.

✅ Correct answer

The curve for the lower concentration acid is less steep, so the reaction is slower because there are fewer acid particles in the same volume. The graph also levels off at a lower volume, so less gas is produced.

💡 Key knowledge

• Lower concentration = fewer acid particles in each cm³.
• Fewer particles means fewer collisions per second.
• That makes the initial slope smaller.

🧠 Exam technique

• Use the graph evidence: less steep and lower final volume.
• Make sure your explanation is about concentration, not “weak acid”.
• The mark scheme rewards a chain of reasoning: lower concentration → fewer particles → fewer collisions → slower rate.

❌ Common errors

• Saying the particles move slower — that is not the reason.
• Writing “weaker acid” instead of lower concentration.
• Forgetting to mention that less gas is produced by the end.

🧠 What top responses did well

• Used specific graph values instead of vague descriptions.
• Gave correct chemical formulae and balanced the equation.
• For explanation questions, linked ideas in a clear chain: surface area/concentration → collisions → rate.
• Used the right science vocabulary: tangent, gradient, concentration, surface area.

❌ Where students lost marks

• Not balancing the equation.
• Describing a graph without using numbers.
• Mixing up rate with amount of gas.
• Forgetting units, especially in the calculation question.