AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2024: Question 3
9 marks · Low Demand difficulty · Short Answer
Analyze a paper chromatography investigation of dyes and calculate an Rf value.
Practise this questionQuestion
Question text
03 This question is about paper chromatography.
A student investigated substance Y using paper chromatography.
This is the method used.
1. Draw a start line in ink on a piece of chromatography paper.
2. Put spots of four different dyes, A, B, C and D, and a spot of substance Y on the
start line.
3. Dip the paper into water so that the water level is below the start line.
4. Wait until the water has risen to near the top of the paper.
03.1 The student’s method contains a mistake in Step 1.
What is the mistake in Step 1?
Give one reason for your answer.
[2 marks]
Mistake
Reason
A different student used a method which gave valid results.
Figure 4 shows the results.
Figure 4
03.2 How many different dyes are in substance Y?
Use Figure 4.
[1 mark]
03.3 Which of the four dyes, A, B, C and D, could be in substance Y?
Give one reason for your answer.
Use Figure 4.
[2 marks]
Dye
Reason
03.4 Suggest why dye D remained on the start line at the end of the investigation.
Use Figure 4.
[1 mark]
03.5 The student determined that:
• the distance moved by the water was 6.0 cm
• the distance moved by dye A was 2.4 cm.
Calculate the Rf value of dye A.
Use the equation:
distance moved by dye A
Rf =
distance moved by water
[2 marks]
Rf =
03.6 Complete the sentence.
Choose the answer from the box.
[1 mark]
solute solution solvent
The water in step 3 is used as a .
Mark scheme
Show the mark scheme
Question 3
AO /
Question Answers Extra information Mark
Spec. Ref.
03.1 (drew start line in) ink 1 AO3
4.8.1.3
ink runs / smudges / dissolves 1 RPA6
OR
a pencil should be used (to draw
the start line) (1)
so that the ink does not run /
smudge / dissolve (1)
AO /
Spec. Ref.
03.2 2 1 AO3
4.8.1.3
RPA6
AO /
Spec. Ref.
03.3 (dye) AO2
A 1 4.8.1.3
RPA6
(reason)
(dye) A travelled the same allow (dyes) B, C, and D did not 1
distance as a spot from travel the same distance as
(substance) Y either spot from (substance) Y
or
(dye) A has the same Rf value allow (dyes) B, C, and D do not
as a spot from (substance) Y have the same Rf value as
either spot from (substance) Y
MP2 is dependent upon the– – –
award of MP1
AO /
Spec. Ref.
03.4 (dye) D is insoluble (in water) allow there is no attraction 1 AO3
between (dye) D and water 4.8.1.3
RPA6
AO /
Spec. Ref.
03.5 2.4 1 AO2
(Rf=)
6.0 4.8.1.3
RPA6
= 0.4 ignore units 1
AO /
Spec. Ref.
03.6 solvent 1 AO2
4.8.1.3
RPA6
Total Question 3 9
How to answer it
AQA GCSE Chemistry: Paper Chromatography Investigation
This question assesses fundamental practical and analytical skills from Required Practical 6 (RPA6) / Spec 4.8.1.3 (Chromatography):
- Evaluating the standard experimental setup (use of pencil vs. ink for the baseline).
- Interpreting a chromatogram to determine mixture composition.
- Comparing spots across lanes using relative positions and identical Rf values.
- Explaining differences in dye movement based on solubility in the mobile phase.
- Calculating an Rf value using the standard formula.
- Recalling basic chemical terminology (solute, solution, solvent).
Identifying Experimental Errors in Chromatography Setup
Method evaluation: Step 1 (drawing the start line in ink)
✅ Correct Answer
Mistake: Drew the start line in ink (or: a pencil should have been used).
Reason: The ink would dissolve in the water and run/smudge up the paper (interfering with results).
💡 Key Knowledge
- The start line (baseline) must always be drawn in pencil.
- Pencil lead is graphite, which is completely insoluble in water and other common chromatography solvents.
- Ink contains soluble dyes that move with the solvent front.
🧠 Exam Technique
This is a standard 2-mark recall question. Ensure both parts are linked:
- 1 mark for clearly naming what was wrong or what should be used instead.
- 1 mark for explaining the physical consequence (ink dissolves / runs).
❌ Common Errors
- Stating "pen" without stating why it is bad.
- Saying the pen makes the paper "dirty" or "messy" rather than explaining that the ink will dissolve or travel up the paper.
Counting Components in a Mixture
Interpreting spots in substance Y
✅ Correct Answer
2
💡 Key Knowledge
- Each separate, distinct spot in a vertical column represents at least one unique substance/dye.
- Column Y displays exactly two spots: one at mid-height and one near the solvent front.
🧠 Exam Technique
Carefully scan ONLY column Y from the start line to the top. Do not count spots in lanes A, B, C, or D.
❌ Common Errors
- Counting all spots across the whole chromatogram (giving an answer of 5 or 6).
- Confusing the start line mark with an actual dye spot.
Identifying Components by Comparison
Matching known single dyes (A–D) to mixture Y
✅ Correct Answer
Dye: A
Reason: Dye A has travelled the same distance (or has the same Rf value) as the lower spot in substance Y.
💡 Key Knowledge
- Substances in a mixture can be identified if they travel the same vertical distance up the paper as a known pure reference spot.
- Identical substances have the exact same Rf value when run in the same solvent on the same paper.
🧠 Exam Technique
Hold a ruler horizontally across the chromatogram. The lower spot of Y aligns horizontally with the spot for A.
Note: The reason mark (MP2) depends on correctly choosing A (MP1).
❌ Common Errors
- Selecting C because it is a single spot, without checking the height alignment with Y.
- Vague reasons such as "it looks similar" without referencing distance moved or Rf value.
Understanding Solubility and Stationary vs Mobile Phase
Why dye D did not move from the baseline
✅ Correct Answer
Dye D is insoluble in water (or has no attraction to the water / solvent).
💡 Key Knowledge
- Mobile phase: The liquid solvent (water) moving up the paper.
- Stationary phase: The chromatography paper.
- If a substance does not move at all, it cannot dissolve in the mobile phase (insoluble) or is too strongly attracted to the paper.
🧠 Exam Technique
Use clear scientific vocabulary. State directly: "Dye D is insoluble in the solvent". Keep it brief and unambiguous.
❌ Common Errors
- Saying "it is too heavy" to move (a very frequent misconception!).
- Saying "it ran out of time" or "it got stuck" without mentioning solubility.
Calculating Retention Factor (Rf)
Quantitative analysis using measured distances
📐 Step-by-Step Calculation
- State the formula:
Rf = distance moved by dye A / distance moved by water - Substitute the values:
Distance by dye A = 2.4 cm
Distance by water = 6.0 cm
Rf = 2.4 / 6.0 (1 mark) - Calculate final value:
Rf = 0.4 (1 mark)
🧠 Exam Technique & Units
- Rf has NO units! It is a ratio of two distances.
- An Rf value must always be between 0 and 1.
- If your answer is greater than 1 (e.g. 6.0 / 2.4 = 2.5), you have divided upside down!
❌ Common Errors
- Inverting the division: calculating 6.0 / 2.4 = 2.5.
- Adding units (e.g. "0.4 cm") – while the mark scheme ignores units here, it is bad practice.
- Rounding incorrectly if decimals arise.
Scientific Vocabulary: Solutions
Choose from: solute / solution / solvent
✅ Correct Answer
The water in step 3 is used as a solvent.
💡 Key Definitions to Remember
- Solvent: The liquid in which a substance dissolves (e.g. water, ethanol).
- Solute: The substance that dissolves in the liquid (e.g. dye, salt).
- Solution: The mixture formed when a solute completely dissolves in a solvent.
🧠 Exam Technique
When given a word bank, only choose words that appear in the box. Check the grammatical fit and chemical role of the water.
❌ Common Errors
- Confusing solute (the dye spot) with solvent (the liquid).
- Writing solution (water alone before mixing is a pure liquid/solvent, not a solution).
Topics
Chemistry · Required Practicals · C8: Chemical Analysis · Required Practicals
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Foundation), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.