AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2023: Question 6

9 marks · Standard Demand difficulty · Short Answer

Analyze a chromatography experiment setup, calculate a missing distance using Rf values, and explain how stationary and mobile phases affect Rf values.

Practise this question

Question

An exam page showing a chromatography question. Figure 3 shows a chromatography paper suspended in a beaker of water with a lid. The start line is labeled as 'Start line in ink' and is submerged below the water level. Table 3 lists chromatography results for red and yellow dyes on Type A and Type B papers, showing distance moved by dye, distance moved by water, and Rf values, with one distance value labeled as X. The questions ask to identify two mistakes in the setup, calculate X, explain why Rf values differ between the papers, and suggest another change that would alter the Rf value.
Question text

06 This question is about chromatography.

A student investigated an orange food colouring using two different types of

chromatography paper.

The food colouring:

• contained a mixture of red and yellow dyes

• was soluble in water.

This is the method used.

1. Draw a start line on a piece of type A chromatography paper.

2. Put a spot of orange food colouring on the line.

3. Put the paper into a beaker containing water as a solvent.

4. Wait for the water to travel up the paper.

5. Measure the distance above the start line moved by the red and yellow dyes and

the water.

6. Repeat steps 1 to 5 using type B chromatography paper.

Figure 3 shows how the student set up the apparatus.

Figure 3

06.1 The student made two mistakes when setting up the apparatus.

Give two mistakes the student made.

*18* [2 marks]

Another student set up the apparatus correctly.

Table 3 shows the results.

Table 3

Type A Type B

chromatography paper chromatography paper

Red dye Yellow dye Red dye Yellow dye

Distance moved by

4.8 6.6 5.4 X

dye in cm

Distance moved by

12.0 12.0 12.0 12.0

water in cm

*19*Rf value 0.40 0.55 0.45 0.60

06.2 Determine value X in Table 3.

[3 marks]

X = cm

Changing the type of chromatography paper resulted in different Rf values for the

red dye.

06.3 Explain why the Rf values for the red dye are different using the two types of

chromatography paper.

Use Table 3.

[3 marks]

06.4 What other change to the investigation could result in a different Rf value for the

red dye?

[1 mark]

Mark scheme

Show the mark scheme The mark scheme for Question 6. For 06.1, marks are given for identifying that the start line is drawn in ink and is below the water level. For 06.2, the calculation steps are shown: 0.60 = distance / 12.0, leading to 7.2 cm. For 06.3, marks are awarded for explaining that the Rf value is smaller for Paper A because the red dye is more attracted to Paper A and spends more time in it. For 06.4, the correct answer is to use a different solvent.

Question 6

AO /

Question Answers Extra information Mark

Spec. Ref.

06.1 the start line is drawn in ink allow the start line should be 1 AO3

drawn in pencil

4.8.1.3

RPA6

the start line is below the water allow the start line should be 1

level above the water level

AO /

Spec. Ref.

06.2 distance moved by dye 1 AO2

0.60 =

12.0 4.8.1.3

RPA6

(distance moved by dye =) 1

0.60 × 12.0

= 7.2 (cm) 1

AO /

Spec. Ref.

06.3 allow converse

the Rf value is smaller for Paper 1 AO2

A

(because the red dye) is more 1 AO3

attracted to Paper A (than to

Paper B)

(so the red dye) spends a 1 AO3

greater (proportion of the) time

distributed in Paper A (than in

Paper B) 4.8.1.3

RPA6

if no other mark awarded allow 1

mark for the dye has a different

attraction to each paper

AO /

Spec. Ref.

06.4 use a different solvent allow use ethanol (as the 1 AO1

solvent) 4.8.1.3 17

RPA6

Total Question 6 9

How to answer it

Mastering Paper Chromatography

AQA GCSE Chemistry Required Practical 6 (RPA6) Study Guide

What this question tests

This exam question assesses your practical knowledge of chromatography setups, your ability to perform and rearrange Rf value calculations, and your theoretical understanding of how substances partition between the stationary phase (paper) and the mobile phase (solvent).

Question Part 06.1

Identifying Experimental Setup Errors

Marks: 2

✅ Correct Answers

  • Mistake 1: The start line is drawn in ink.
  • Mistake 2: The start line is below the water level (solvent level).
Award 1 mark for each correctly identified mistake.

💡 Key Science Facts

  • Why pencil? Pencil lead is graphite, which is insoluble in water. Ink is soluble and will dissolve, run up the paper, and interfere with the chromatogram.
  • Why above water? If the start line is below the water level, the food colouring spots will dissolve directly into the water in the beaker instead of travelling up the paper.

🧠 Exam Technique

Always look closely at the diagram labels. In Figure 3, the label explicitly points to "Start line in ink". This is a massive clue! When analyzing experimental setups, compare what you see to the standard method you learned in class.

❌ Common Errors to Avoid

  • Do not write vague answers like "the line is in the wrong place" or "the paper is too low". You must be precise.
  • Do not say "the lid is on" as a mistake—having a lid is actually correct because it prevents solvent evaporation!

Question Part 06.2

Calculating the Missing Distance (Value X)

Marks: 3

📐 Step-by-Step Calculation

1
Recall the Rf formula:
Rf = distance moved by substance / distance moved by solvent
2
Substitute the known values from Table 3 for Yellow Dye on Type B paper:
Rf = 0.60 and Distance moved by solvent = 12.0 cm
0.60 = X / 12.0
1 Mark awarded for correct substitution
3
Rearrange the equation to solve for X:
X = 0.60 × 12.0
1 Mark awarded for correct rearrangement
4
Calculate the final value:
X = 7.2 cm
1 Mark awarded for correct final value

❌ Calculation Traps

The "Upside Down" Division: Students often divide the wrong way around when rearranging (e.g., doing 12.0 / 0.60 = 20 cm ). Always do a sanity check: the dye cannot travel further than the solvent front (12.0 cm), so any answer greater than 12.0 cm must be incorrect!

Question Part 06.3

Explaining Different Rf Values

Marks: 3

✅ Correct Explanation

  • The Rf value is smaller for Paper A than Paper B (or vice versa). [1 Mark]
  • This is because the red dye is more strongly attracted to Paper A than to Paper B. [1 Mark]
  • Therefore, the red dye spends a greater proportion of time in the stationary phase (Paper A) than in Paper B. [1 Mark]

💡 Stationary vs Mobile Phase

Chromatography separation depends on the distribution of substances between two phases:

  • Stationary Phase: The paper. If a substance has a high attraction to the paper, it moves slowly (lower Rf).
  • Mobile Phase: The solvent. If a substance is highly soluble in the solvent, it moves quickly (higher Rf).

🧠 How to get full marks here

Top-level answers explicitly link the data (the Rf values) to the physical interactions (attraction to the paper) and the time spent in that phase. If you only say "they have different attractions", you will only score 1 mark. You must compare Paper A and Paper B directly.

Question Part 06.4

Changing the Investigation

Marks: 1

✅ Correct Answer

Use a different solvent (for example, ethanol instead of water).

Award 1 mark for identifying a change in solvent.

💡 Why does this work?

Different solvents have different polarities. Changing the solvent changes how soluble the red dye is in the mobile phase, which alters how far it travels up the paper, resulting in a different Rf value.

Topics

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

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.