AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2021: Question 3

8 marks · Standard Demand difficulty · Extended Answer

Plan an investigation to determine the Rf value of a food colouring dye using paper chromatography, and identify how varying conditions and phases affect chromatography.

Practise this question

Question

Question 3 contains three parts about paper chromatography. Part 03.1 asks students to plan an investigation to determine the Rf value for a dye in food colouring using a beaker, solvent, and chromatography paper, showing the Rf equation: Rf = distance moved by substance divided by distance moved by solvent (6 marks). Part 03.2 presents a multiple-choice question asking why two students obtained different Rf values for the same dye, with options: Different length of paper used, Different period of time used, Different size of beaker used, or Different solvent used (1 mark). Part 03.3 asks to identify the stationary phase from four checkboxes: Beaker, Dye, Paper, and Solvent (1 mark).
Question text

03 This question is about paper chromatography.

A food colouring contains a dye.

03.1 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]

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

Each student did the investigation differently.

*10* 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

03.3 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 3. Question 03.1 gives a 3-level response rubric for 6 marks. Level 3 (5–6 marks) requires a logically sequenced plan leading to a valid outcome, including method steps (drawing pencil start line, spotting dye, using suitable solvent below the line, covering with lid, marking solvent front) and measurement steps (measuring distance moved by spot and solvent front to calculate Rf). Question 03.2 awards 1 mark for 'different solvent used'. Question 03.3 awards 1 mark for 'paper'. Total marks: 8.

Question 3

AO/

Question Answers Mark

Spec. Ref

03.1 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.

03.2 different solvent used 1 AO3

4.8.1.3

RPA6

Question 3 continued

AO /

Spec. Ref.

03.3 paper 1 AO1

12 4.8.1.3

RPA6

Total 8

How to answer it

Paper Chromatography & Rf Values

What this question tests

AQA Required Practical 6 (Chromatography) & Separation Techniques (Spec 4.8.1.3)

  • Method planning (AO1): Designing a complete, logically sequenced practical to separate a mixture and calculate an Rf value.
  • Variables and experimental errors (AO3): Understanding why Rf values vary and identifying the role of different solvents.
  • Chromatography theory (AO1): Identifying the stationary and mobile phases and knowing why specific apparatus steps matter (e.g. pencil lines, solvent level).
Question 03.1 • 6 Marks

Planning an Investigation to Determine an Rf Value

Extended response: Writing a logically ordered experimental method

Examiner Diagram Mental Model: Picture a 250 cm³ beaker containing a shallow layer of solvent. Inside rests a vertical strip of chromatography paper suspended from a glass rod or resting against the side, with a watch glass/lid on top. The pencil start line is clearly above the solvent level, with a single spot of food colouring on it. The solvent front is marked near the top.

✅ Model Method (Full 6/6 Marks)

  1. Preparation: Draw a horizontal baseline on the chromatography paper in pencil (approx. 1–2 cm from the bottom).
  2. Spotting: Use a capillary tube or pipette to place a small spot of the food colouring onto the pencil line and let it dry.
  3. Solvent setup: Pour a suitable solvent (such as water or ethanol) into a beaker to a depth of around 1 cm (ensuring it will sit below the pencil baseline).
  4. Assembly: Place the paper into the beaker so it dips into the solvent, ensuring the solvent does not submerge the dye spot. Cover the beaker with a lid or watch glass.
  5. Running: Allow the solvent to travel up the paper until it is near the top (e.g. 1–2 cm from edge).
  6. Measurements: Remove the paper and immediately draw a pencil line across the solvent front. Let the paper dry.
  7. Calculation: Measure the distance from the baseline to the centre of the dye spot, and from the baseline to the solvent front. Calculate:
    Rf = distance moved by substance ÷ distance moved by solvent

💡 Key Practical Essentials

  • Why use a pencil line? Pencil lead (graphite) is insoluble in the solvent and will not dissolve or run up the paper to produce false spots. Ink from pens contains soluble dyes that ruin the chromatogram.
  • Why keep the solvent level below the spot? If the spot is submerged, the dye dissolves straight into the bulk solvent in the beaker rather than traveling up the paper.
  • Why cover with a lid? Prevents solvent from evaporating before it ascends the strip, which keeps the atmosphere saturated.
  • Measurement datum: All distances must be measured from the pencil start line, not from the bottom edge of the paper!

🧠 Exam Technique: Securing Level 3 (5–6 Marks)

This is a banded "Level of Response" question. The mark scheme is split into two halves:

  • Part 1: The Practical Method: Setting up the paper, solvent, lid, and waiting for separation.
  • Part 2: The Measurements & Data: Stating what to measure with a ruler (spot distance & solvent front) and linking back to the Rf formula.
Top Tip: Students who only described running the chromatography paper scored a maximum of 3–4 marks (Level 2). To reach Level 3, you must state how to measure both distances and plug them into the equation to calculate Rf!

❌ Common Errors to Avoid

  • Missing the pencil rule: Forgetting to specify that the start line must be drawn in pencil.
  • Submerging the dye: Saying "fill the beaker with water and submerge the paper" without stating the baseline must sit above the solvent level.
  • Waiting until the solvent runs off: If the solvent reaches the very top edge, you cannot determine the solvent front accurately.
  • Measuring from the wrong place: Measuring distances from the bottom of the paper instead of from the baseline, or measuring to the edge of the spot rather than its centre.
Question 03.2 • 1 Mark

Understanding Factors Affecting Rf Values

Why would two students get different Rf values for the same dye?

✅ Correct Answer

[ ✓ ] Different solvent used

The Rf value is characteristic of a substance only for a specific solvent and stationary phase system.

💡 Why the other options are incorrect

  • Different length of paper / time: Rf is a ratio ( spot ÷ solvent ). If the solvent travels further over a longer time or paper, the spot travels proportionally further. The ratio remains unchanged.
  • Different size of beaker: The physical size of the container does not affect the affinity between the chemicals and the phases.
  • Different solvent: If one student uses water and another uses propanone or ethanol, the dye's solubility and partitioning differ completely, producing a different Rf.
Question 03.3 • 1 Mark

Chromatography Phases

Identifying the Stationary Phase

✅ Correct Answer

[ ✓ ] Paper

1 Mark: Exactly matching the mark scheme key term.

💡 Core Phase Recall

  • Stationary phase: Does not move. In paper chromatography, this is the chromatography paper (or cellulose fibres).
  • Mobile phase: Moves through the stationary phase. In this experiment, it is the liquid solvent.
  • Separation mechanism: Substances that have a higher solubility in the mobile phase and lower attraction to the stationary phase 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 (Higher), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.