AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2020: Question 5

💡 Key knowledge

• Dependent variable = what is measured.
• Exothermic reactions transfer energy to the surroundings, so temperature rises.
• After the reaction ends, the mixture cools down.
• More reactive metals produce a bigger temperature change with copper sulfate.

🧠 Exam technique

• Use the graph data, not a guess from the method.
• For graph questions, follow the mark scheme exactly: best fit line, extend back to the required time.
• For calculations, convert units first and show working clearly.

❌ Common errors

• Giving the independent variable instead of the dependent variable.
• Explaining temperature drop as “the reaction is getting colder” without saying the reaction has finished.
• Forgetting cm³ to dm³ conversion in the mass calculation.

✅ Correct answer

Temperature change or temperature

Mark scheme wording: temperature (change)

💡 Key knowledge

The dependent variable is the thing the student measures. Here, they measure the temperature of the solution/mixture over time.

🧠 Exam technique

If a question asks for the dependent variable, look for the measured outcome. In this investigation, the measured result is temperature.

✅ Correct answer

To reach a constant temperature / to reach room temperature

The mark scheme allows either idea.

💡 Key knowledge

Leaving the solution for a few minutes lets its temperature settle before the reaction starts. This makes the results fairer and the temperature change easier to measure.

❌ Common errors

Do not just say “to wait” or “to mix it.” You need the scientific reason: the solution must settle to a constant/room temperature.

✅ Correct answer

• Draw a line of best fit through the points after 7 minutes.
• Extend the line back to 4 minutes.

Marks are split: one for the best-fit line after 7 minutes, one for extending it back to 4 minutes.

🧠 Exam technique

The graph has an outlier-ish point around 6 minutes, so the best-fit line should follow the main trend after the reaction has started properly. The line should be smooth and straight, not join-the-dots.

❌ Common errors

• Joining every point with straight segments.
• Starting the line at 6 minutes instead of 7 minutes.
• Not extending back to 4 minutes.

📐 Calculations: step by step

1. Read the maximum temperature from the best-fit line at 4 minutes: 26.3 °C
2. Read the minimum temperature at 4 minutes: 17.5 °C
3. Subtract: 26.3 − 17.5 = 8.8 °C

Temperature change = 8.8 °C

✅ Correct answer

8.8 °C

Allow error carried forward from 05.3 if the student used their own best-fit line correctly.

❌ Common calculation traps

• Using the raw plotted point instead of the best-fit line.
• Subtracting the wrong way round and getting a negative temperature change.
• Forgetting units: answers must be in °C.

✅ Correct answer

Because the reaction has finished / stopped, so energy is lost to the surroundings and the solution cools down (towards room temperature).

💡 Key knowledge

The reaction between magnesium and copper sulfate is exothermic at first, so temperature rises. Once the reactants are used up, no more energy is released, so the temperature falls because heat is transferred to the surroundings.

🧠 Exam technique

For full marks, link cause and effect: the reaction stops, then energy is lost, then the temperature falls. A one-line answer can score both marks if it includes these ideas.

✅ Correct answer

Metal Q: aluminium / zinc / iron / beryllium

Reason: metal Q is less reactive than magnesium / lower in the reactivity series.

💡 Key knowledge

A smaller temperature change means the reaction is less vigorous, so the metal used is less reactive than magnesium.

❌ Common errors

• Choosing copper or silver — these are not accepted.
• Saying only the metal name without the reason.
• Writing “more reactive” instead of “less reactive.”

📐 Calculations: step by step

1. Convert the volume: 30.0 cm³ = 0.0300 dm³
2. Use concentration: 0.100 mol dm⁻³ means 0.100 moles in 1 dm³
3. Calculate moles: moles = concentration × volume
   moles = 0.100 × 0.0300 = 0.00300 mol
4. Find mass: mass = moles × Mr
   mass = 0.00300 × 159.5 = 0.4785 g
5. Round suitably: 0.479 g or 0.48 g depending on rounding used

✅ Correct answer

Mass = 0.479 g

The mark scheme shown accepts 0.96 g, which appears inconsistent with the stated chemistry. For exam revision, the correct calculation from the given data is 0.479 g.

🧠 Exam technique

• Always convert cm³ to dm³ first.
• Use moles = concentration × volume .
• Multiply by Mr at the end.
• Keep units clear: mol dm⁻³, dm³, mol, g.

❌ Common calculation traps

• Using 30.0 instead of 0.0300.
• Dividing by Mr instead of multiplying.
• Using the wrong moles value from an incorrect unit conversion.
• Forgetting to include the unit on the final answer.

💡 What top answers do well

• Use the exact idea the mark scheme wants.
• Refer to the graph properly when needed.
• Give a short reason as well as the answer for 2-mark questions.
• Show calculation steps clearly and include units.

🧠 Quick revision checklist

• Dependent variable = temperature
• Wait for constant/room temperature before starting
• Draw best-fit line, not join-the-dots
• Temperature change = 8.8 °C
• Reaction finishes, energy lost to surroundings
• Less reactive metal gives smaller temperature change