AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2019: Question 3

✅ Correct answer

A mixture designed as a useful product.

💡 Key knowledge

• A formulation is not just any mixture.
• It is a mixture made for a specific purpose.
• Food colouring is a formulation because it is designed to colour food effectively.

🧠 Exam technique

• Use the phrase useful product or an equivalent idea.
• One clear definition is enough for the mark.

❌ Common errors

• Just saying “a mixture” is too vague.
• Writing about chromatography here does not answer the question.
• Don’t describe ingredients unless you link them to a designed product.

✅ Correct answer

Dyes are distributed differently between the stationary phase and the mobile phase. This means they move up the paper at different speeds or different distances.

💡 Key knowledge

• The paper is the stationary phase.
• The solvent is the mobile phase.
• Dyes have different solubilities and different attractions to the paper/solvent.
• Because of this, some dyes travel further than others.

🧠 Exam technique

• You needed two linked ideas for 2 marks:
• 1) dyes are distributed differently / have different solubilities or attractions,
• 2) so they move different distances or at different rates.

❌ Common errors

• “The dyes separate because they are different colours” is not enough.
• Do not talk about how to set up the experiment — the question says not to.
• “Different density” is wrong here.

✅ Correct answer

The chromatogram would show different dots/colours in a vertical column, above the original spot.

💡 Key knowledge

• If there is more than one dye, they separate into more than one spot.
• All spots come from the same starting point, so they appear in line vertically.
• More than one spot means more than one substance was present.

🧠 Exam technique

• For full marks, mention both:
• • more than one spot/colour, and
• • the spots are in a vertical line/column above the starting point.

❌ Common errors

• “The spot goes higher” is too vague.
• Talking about “more than one line” is incorrect.
• Not mentioning the vertical arrangement can lose a mark.

✅ Correct answer

Run known dyes and the food colouring as a chromatogram, then compare the distances moved or the R_f values. Any dye that moves the same distance or has the same R_f value as a known dye can be identified as that dye.

💡 Key knowledge

• Matching spots means the dyes are likely the same.
• R_f values are a reliable way to compare chromatograms.
• Use the same solvent and conditions when comparing.

🧠 Exam technique

• A full 3-mark answer usually needs:
• 1) compare with known dyes,
• 2) compare distance moved or R_f values,
• 3) identify dyes that match.

❌ Common errors

• Only saying “compare colours” is not enough.
• Not mentioning known dyes loses an important mark point.
• Confusing “same distance moved” with “same speed” is less precise.
• If you mention R_f, make sure it is written clearly.

💡 Key knowledge to remember

• Formulation = a mixture designed as a useful product.
• Stationary phase = paper.
• Mobile phase = solvent.
• Different dyes travel different distances because of different solubilities/attractions.
• More than one dye gives more than one spot.
• Identify unknowns by matching to known dyes using distance moved or R_f values.

🧠 How to score well

• Use chromatography vocabulary accurately.
• Be concise but complete.
• Link cause and effect: different attraction → different movement.
• For identification, always mention comparison with known samples.