AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2022: Question 1

✅ Correct answers

Any two pairs of error + problem from the list below score full marks.

• Error: start line drawn in ink
  Problem: the ink would mix with the solvent / move up the paper.
• Error: solvent is above the start line
  Problem: the colours / ink would dissolve into the solvent immediately.
• Error: no lid on the tank
  Problem: the solvent would evaporate.

💡 Key knowledge

• The start line should be drawn in pencil, not ink.
• The solvent level must be below the start line.
• The tank should have a lid to stop the solvent evaporating.
• In chromatography, substances separate because they have different solubilities.

🧠 Exam technique

• Always use the phrase "because" or "so" to link the error to the problem.
• Write your answer in clear pairs:
  Error 1 → Problem 1
  Error 2 → Problem 2
• The examiner awards marks for a correct scientific consequence, not just for spotting the picture detail.

❌ Common errors

• Saying only “the setup is wrong” without explaining what happens.
• Writing “use pencil” as the answer, when the student actually used ink.
• Forgetting the consequence, e.g. “solvent above the line” without saying the ink dissolves.
• Choosing “the lid is missing” but not stating evaporation.

✅ Correct answers

You can choose any two of these:

• The brown ink contains blue, yellow and red.
• The brown ink contains an unknown colour.
• The brown ink does not contain green.
• Blue is the most soluble / has the highest Rf.
• Red is the least soluble / has the lowest Rf.

💡 Key knowledge

• If a spot in the brown ink ends at the same height as a known colour, they are the same substance.
• Different heights mean the substances are different.
• Higher spot = usually more soluble in the solvent.
• Lower spot = usually less soluble.

🧠 Exam technique

• Use evidence from the spots shown in Figure 2.
• Good answers compare the brown ink spots with the single-colour spots.
• For top marks, use scientific wording like same height, same substance, soluble, and Rf.

❌ Common errors

• Saying the brown ink contains green, because a green spot is shown on the reference line — it does not.
• Calling the unknown spot “another colour” without linking it to evidence.
• Mixing up the meaning of high and low spots.

✅ Correct answer

The green colour was insoluble in the solvent.

💡 Key knowledge

• If a substance is insoluble, it does not dissolve in the solvent.
• So it cannot move up the paper.

🧠 Exam technique

• For multiple-choice science questions, eliminate the options that do not explain the result.
• The key idea here is dissolving: if it does not dissolve, it stays at the start line.

❌ Common errors

• “The experiment was left too long” — time does not explain a spot staying on the start line.
• “The green spot was too small” — size does not stop movement.
• “The green spot contained too many colours” — not the reason for staying still.

📐 Calculations — step by step

1. Write the equation:
   Rf = distance moved by colour ÷ distance moved by solvent
2. Substitute the numbers:
   0.24 = 1.8 ÷ distance moved by solvent
3. Rearrange to find the solvent distance:
   distance moved by solvent = 1.8 ÷ 0.24
4. Calculate:
   distance moved by solvent = 7.5 cm

✅ Correct answer

7.5 cm

💡 Key knowledge

• Rf values are always a ratio.
• Use the same unit for both distances — here, cm.
• Final answers should be given with the correct unit.

🧠 Exam technique

• If the question gives an equation, use it exactly.
• Show the rearrangement clearly to gain method marks.
• Check your answer: the solvent must travel further than the colour for Rf = 0.24, so 7.5 cm makes sense.

❌ Common errors

• Doing 1.8 × 0.24 instead of dividing.
• Using no units.
• Reversing the ratio.
• Rounding to an unsuitable value or forgetting that Rf has no unit, while distance does.

💡 Top-level response features

• Accurate chromatography vocabulary.
• Clear links between setup errors and their effects.
• Correct interpretation of spots as substances in the ink.
• Correct use of the Rf equation and rearrangement.

🧠 How marks were awarded

• 01.1: 2 marks available for each correct error-effect pair.
• 01.2: 1 mark for each valid conclusion from the chromatogram.
• 01.3: 1 mark for the correct explanation.
• 01.4: marks for correct method and final answer.

❌ Biggest trap

Students often describe what they can see in the diagram, but the examiner wants the scientific consequence of each mistake or result.