AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2021: Question 10

8 marks · Standard Demand difficulty · Extended Answer

Plan an investigation to determine the Rf value of a dye in food colouring using paper chromatography, and answer questions about chromatography phases and variables.

Practise this question

Question

Question 10 consists of three parts. Part 10.1 shows the formula Rf = distance moved by substance / distance moved by solvent and asks students to plan an investigation to determine the Rf value of a dye in a food colouring using a beaker, solvent, and chromatography paper for 6 marks. Part 10.2 asks why two students found different Rf values for the same dye, with four multiple-choice options: different length of paper used, different period of time used, different size of beaker used, or different solvent used. Part 10.3 asks to identify the stationary phase in paper chromatography from four options: Beaker, Dye, Paper, or Solvent.
Question text

10 This question is about paper chromatography.

A food colouring contains a dye.

10.1 Plan an investigation to determine the Rf value for the dye in this food colouring.

distance moved by substance

Rf =

distance moved by solvent

Your plan should include the use of:

• a beaker

• a solvent

• chromatography paper.

[6 marks]

10.2 Two students investigated a dye in a food colouring using paper chromatography.

Each student did the investigation differently.

*34* The Rf values they determined for the same dye were different.

How did the students’ investigations differ?

[1 mark]

Tick ( ) one box.

Different length of paper used

Different period of time used

Different size of beaker used

Different solvent used

10.3 Paper chromatography involves a stationary phase.

What is the stationary phase in paper chromatography?

[1 mark]

Tick ( ) one box.

Beaker

Dye

Paper

Solvent

Mark scheme

Show the mark scheme Mark scheme for Question 10. Part 10.1 uses a 3-level mark scheme (5–6 marks for Level 3, 3–4 for Level 2, 1–2 for Level 1) with indicative content covering drawing a pencil start line, applying the dye spot, using a solvent below the pencil line in a covered beaker, allowing the solvent to run, marking the solvent front, measuring both distances, and calculating the Rf value. Part 10.2 gives the correct answer as 'different solvent used' (1 mark). Part 10.3 gives the correct answer as 'paper' (1 mark). Total is 8 marks.

Question 10

AO/

Question Answers Mark

Spec. Ref

10.1 Level 3: The design/plan would lead to the production of a valid 5‒6 AO1

outcome. All key steps are identified and logically sequenced. 4.8.1.3

RPA6

Level 2: The design/plan would not necessarily lead to a valid 3‒4

outcome. Most steps are identified, but the plan is not fully logically

sequenced.

Level 1: The design/plan would not lead to a valid outcome. Some 1‒2

relevant steps are identified, but links are not made clear.

No relevant content 0

Indicative content

Method

• draw (pencil) start line on (chromatography) paper

• place spot of food colouring on start line

• use of suitable solvent

• place solvent in beaker / container

• place (chromatography) paper in beaker / container

• so (chromatography) paper is in solvent

• but solvent is below start line

• use a lid

• wait for solvent to travel up the (chromatography) paper (until

near top)

• mark solvent front

• dry the (chromatography) paper

Measurements

• measure distance between start line and centre of spot

• measure distance between start line and solvent front

• use of measurements to determine Rf value

AO /

Question Answers Extra information Mark

Spec. Ref.

10.2 different solvent used 1 AO3

4.8.1.3

RPA6

Question 10 continued

AO /

Spec. Ref.

10.3 paper 1 AO1

4.8.1.3

RPA6

Total 8

How to answer it

Paper Chromatography & Rf Values

📌 What This Question Tests

This question assesses knowledge from AQA Specification 4.8.1.3 and Required Practical 6 (RPA 6):

  • Planning an experimental method to separate mixtures and calculate an Rf value.
  • Understanding factors affecting retention values (the effect of the stationary phase and mobile phase/solvent).
  • Identifying the stationary phase and mobile phase in paper chromatography.
Question 10.1

Planning an Investigation to Determine Rf Value

6 Marks • Extended Response (RPA 6)

✅ Model Method (Level 3: 5–6 Marks)

  1. Draw a baseline in pencil across chromatography paper (approx. 1–2 cm from the bottom).
  2. Spot the food colouring onto the pencil line using a capillary tube or pipette, and let it dry.
  3. Add a suitable solvent (e.g. water or ethanol) into a beaker to a depth below the pencil line.
  4. Place the paper in the beaker so the bottom is submerged, but the solvent remains below the baseline spot.
  5. Cover the beaker with a lid or watch glass to reduce evaporation of the solvent.
  6. Allow the solvent to rise up the paper until it is near the top.
  7. Remove the paper and mark the solvent front immediately in pencil, then leave to dry.
  8. Measure distances: measure the distance from the pencil baseline to the centre of the dye spot, and the distance from the baseline to the solvent front.
  9. Calculate Rf: use Rf = (distance moved by dye) ÷ (distance moved by solvent).

📐 How Rf Is Calculated

Rf formula:

Rf = distance moved by substance / distance moved by solvent

  • Step 1: Measure from the pencil baseline to the centre of the spot (in mm or cm).
  • Step 2: Measure from the pencil baseline to the solvent front (using the same unit).
  • Step 3: Divide spot distance by solvent front distance.
  • Check: An Rf value is a ratio; it has no units and must always be between 0 and 1.

🧠 Exam Technique & Examiner Insight

  • Level 3 requirement: The method must lead to a valid outcome. You must include both the experimental setup and the quantitative measurement steps.
  • The #1 missed mark: Many students write a great method for running the chromatogram but completely forget to explain how to calculate the Rf value at the end!
  • Always state pencil: Explain that the baseline and solvent front must be marked in pencil because pencil lead (graphite) is insoluble and will not dissolve in the solvent.

❌ Common Errors to Avoid

  • Solvent above baseline: If the solvent is above the spot, the dye washes off into the beaker instead of moving up the paper.
  • Drawing the line in pen: Ink from a pen contains dyes that separate and ruin the chromatogram.
  • Measuring from the paper edge: All measurements must be taken from the pencil baseline, never the bottom of the paper strip.
  • Measuring to the top edge of the spot: Measure to the centre of the spot for consistency.
Mark Scheme Breakdown:
Level 3 (5–6 marks): Clear, logically sequenced plan containing key practical setup steps AND explicit measurement details to calculate Rf.
Level 2 (3–4 marks): Most steps identified, but incomplete sequence (e.g. misses measurements or solvent level detail).
Level 1 (1–2 marks): Fragmented steps without a viable full method.
Question 10.2

Differences Causing Different Rf Values

1 Mark • Multiple Choice

✅ Correct Answer

☑ Different solvent used

The Rf value depends on the relative solubility of the substance in the mobile phase (solvent) compared to its attraction to the stationary phase (paper).

💡 Why the Other Options Are Incorrect

  • Different length of paper: Rf is a ratio. If the paper is longer and the solvent travels further, the dye also travels proportionally further; Rf stays the same.
  • Different period of time: Stopping earlier or later changes both distances proportionally, leaving the ratio Rf unchanged.
  • Different size of beaker: The container size has no chemical effect on dye solubility or partition.
Question 10.3

The Stationary Phase in Paper Chromatography

1 Mark • Recall

✅ Correct Answer

☑ Paper

In paper chromatography, the paper does not move, so it is the stationary phase.

💡 The Two Phases Defined

  • Stationary Phase: The material that remains fixed in place (the chromatography paper).
  • Mobile Phase: The liquid or gas that moves through the stationary phase (the solvent).
  • How separation works: Substances that are more soluble in the solvent spend more time in the mobile phase and travel further up the paper.

Topics

Chemistry · Required Practicals · C8: Chemical Analysis · Required Practicals

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Foundation), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.