AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2024: Question 4

💡 Key knowledge

• A mixture can contain more than one dye or pigment.
• In chromatography, a substance moves only if it is soluble in the solvent.
• If a dye is insoluble, it stays at the start line.
• R_f = distance moved by substance ÷ distance moved by solvent.

🧠 Exam technique

• Use the exact science words: insoluble , solvent , start line .
• For calculations, rearrange the formula clearly and include units.
• When asked to suggest a reason, one valid point is enough.

❌ Common errors

• Saying the red colour “evaporated” or “disappeared”.
• Mixing up the substance distance with the solvent distance.
• Forgetting to round to 2 significant figures.
• Thinking every ink colour must always appear as a separate spot.

✅ Correct answer

The red colour was insoluble in the solvent, so it did not move with the solvent and stayed on the start line.

💡 Key knowledge

• Only substances that dissolve in the solvent are carried up the paper.
• If a dye is insoluble, it stays where it was placed.

🧠 Exam technique

• This is a 2-mark explanation, so give both linked ideas:
  1) insoluble
  2) does not move with the solvent
• Marks are for explanation, not just naming the colour.

❌ Common errors

• “It was too heavy” — not the chromatography explanation.
• “It got stuck” — too vague.
• Only saying “it was insoluble” may not gain full marks unless the movement idea is also made clear.

📐 Calculations: step-by-step

1. Use the formula: R_f = distance moved by substance ÷ distance moved by solvent
2. Substitute the values:
   0.87 = 6.4 ÷ distance moved by solvent
3. Rearrange:
   distance moved by solvent = 6.4 ÷ 0.87
4. Calculate:
   6.4 ÷ 0.87 = 7.356...
5. Round to 2 significant figures:
   7.4 cm

✅ Correct answer

7.4 cm

Final answer should include the unit cm and be rounded to 2 significant figures.

💡 Key knowledge

• R_f values are ratios, so they have no unit.
• Distances in chromatography are usually measured in cm.
• Always use the values from the question.

❌ Common calculation traps

• Using 6.4 ÷ 0.87 the wrong way round.
• Writing 7.356 cm without rounding.
• Forgetting that the solvent travelled further than the dye.
• Giving the answer as 7 instead of 7.4 cm .

✅ Correct answer

One reason is that two of the dyes had the same R_f value, so they moved to the same position and appeared as one spot.

💡 Key knowledge

• Different dyes do not always separate into different spots.
• If two dyes travel the same distance, they overlap.
• Other acceptable reasons: one dye was insoluble, or one colour was white/colourless and so not seen.

🧠 Exam technique

• This is a 1-mark suggest question, so give one clear reason.
• Any one of the following is acceptable:
  • two dyes had the same R_f
  • two spots were in the same place
  • two colours were insoluble
  • one colour was white

❌ Common errors

• “There were only three colours in the ink” — this ignores the question’s information.
• “The student made a mistake” — too vague.
• Talking about colourlessness without linking to visibility.

✅ Correct answer

Use a different solvent.

💡 Key knowledge

• Different solvents dissolve different dyes better.
• A new solvent can separate spots that previously overlapped.

🧠 Exam technique

• Keep it short and direct for a 1-mark suggestion question.
• Useful phrasing: “use a different solvent so the dyes separate differently”.

❌ Common errors

• “Use more ink” — this does not solve the separation problem.
• “Make the paper bigger” — irrelevant.
• “Heat it more” — not the required chromatography method.

💡 Remember

• Insoluble means it does not dissolve in the solvent.
• R_f uses the formula: substance distance ÷ solvent distance.
• Same spot can mean different dyes have the same R_f.
• Different solvent can reveal more colours.

🧠 Full-mark habits

• Use scientific terms exactly.
• Show working for calculations.
• Give answers in the form the mark scheme expects.