AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2021: Question 1

✅ Correct answers

Independent variable: concentration of hydrochloric acid

Control variable: one of these:

• temperature of hydrochloric acid
• volume of hydrochloric acid
• length of magnesium
• surface area of magnesium

You only needed one control variable for the mark.

💡 Key knowledge

• The independent variable is what the student changes.
• The control variable is what is kept the same to make it a fair test.
• Here, the concentration changes because the method says to repeat with higher concentrations.

🧠 Exam technique

• Write the variable exactly, not just “acid”.
• “Concentration” is better than “amount” here.
• For the control variable, be specific: temperature, not just “conditions”.

❌ Common errors

• Saying the independent variable is the volume of gas — that is the measured outcome.
• Writing “mass of magnesium” when the mark scheme expects length or surface area.
• Giving more than one control variable does not lose marks, but one clear correct one is enough.

✅ Correct answer

Plot all the points correctly:

0, 0

20, 48

40, 72

60, 90

80, 97

100, 98

120, 98

Then draw a smooth line of best fit through the points, and it must include 0,0.

📐 Calculations / plotting steps

1. Check the axes: time on the x-axis, volume of gas on the y-axis.
2. Use the correct scale: time in seconds up to 120, volume in cm³ up to 100.
3. Plot each point carefully.
4. Join with a curved line of best fit, not dot-to-dot straight lines.

The mark scheme allowed a tolerance of ± ½ a small square.

🧠 Exam technique

• Use a sharp pencil and small crosses for points.
• Make sure the line starts at the origin if the data supports it.
• The graph should show the reaction slowing down and then levelling off.

❌ Common errors

• Plotting the wrong pair of values, especially swapping time and volume.
• Forgetting the point 0,0.
• Drawing straight line segments instead of a smooth curve.
• Joining the points when a best-fit curve is needed.

✅ Correct answer

The rate decreases.

This is the only answer required for the mark.

💡 Key knowledge

From Table 1, the gas volume increases quickly at first, then more slowly, and finally stops increasing at 98 cm³. That shows the reaction is slowing down.

🧠 Exam technique

Look for the pattern in the numbers:

• 0 to 20 s: +48 cm³
• 20 to 40 s: +24 cm³
• 40 to 60 s: +18 cm³
• Later intervals: very small increases

Smaller increases per equal time = lower rate.

❌ Common errors

• Saying “the rate stays the same” because the graph has time intervals.
• Saying “the rate increases” because the gas volume keeps rising at first.

✅ Correct answer

The magnesium reacted more quickly.

This matches the mark scheme exactly.

💡 Key knowledge

• Higher concentration means more acid particles in the same volume.
• This leads to more frequent collisions with the magnesium.
• More collisions per second means a faster reaction.

🧠 Exam technique

If the question asks about rate, focus on speed of reaction, not energy changes.

This was a one-mark question: one clear correct statement was enough.

❌ Common errors

• Saying the activation energy is higher — concentration does not change activation energy.
• Saying the reaction finished at the same time.
• Saying the total gas volume was smaller — that is not the key effect of concentration here.

✅ Full-mark answer

The rate increases because particles have more energy, so they move faster. This means collisions happen more frequently, and more particles have enough energy to react.

The mark scheme accepts: “reaction happens faster”, “particles move faster”, and “more particles have energy greater than the activation energy”.

💡 Key knowledge

• Higher temperature gives particles more kinetic energy.
• They move faster.
• Collisions happen more often.
• More collisions are successful because more particles have energy at or above the activation energy.

🧠 Exam technique

• Use the chain: temperature up → particles have more energy → move faster → more frequent collisions → rate increases.
• For 3 marks, give at least 3 linked points.
• Use the word collisions clearly.

❌ Common errors

• Talking only about “more heat” with no particle explanation.
• Saying higher temperature changes the number of particles.
• Mixing up temperature with concentration.

💡 What top answers did well

• Named variables precisely.
• Plotted all points accurately and included the origin.
• Used the trend in the table to justify that the rate decreases.
• Explained rate changes using collision theory, not vague statements.

🧠 How marks were awarded

• 01.1: 1 mark for the independent variable, 1 mark for one correct control variable.
• 01.2: 2 marks for plotting the points, 1 mark for the line of best fit including 0,0.
• 01.3: 1 mark for “the rate decreases”.
• 01.4: 1 mark for “the magnesium reacted more quickly”.
• 01.5: 3 marks for linked particle/collision explanation.