AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2024: Question 4

13 marks · Standard Demand difficulty · Short Answer

Analyze paper chromatography results for orange dyes A and B, calculate Rf values, and compare with gas chromatography.

Practise this question

Question

A series of chromatography experiments are shown. Figure 4 displays two paper chromatography setups for orange dye A, one using water and one using ethanol, showing different separation patterns. Figure 5 shows a single chromatography setup for orange dye B using ethanol. The questions ask students to explain the separation of dyes, compare the purity of dyes A and B, calculate an Rf value, explain why Rf is constant, and list advantages of gas chromatography.
Question text

04 A student investigated an orange dye (A) using paper chromatography.

Figure 4 shows the results of Experiment 1 and Experiment 2 using orange dye A.

Figure 4

04.1 Explain why the yellow dye and red dye travel different distances in Experiment 1.

Refer to forces of attraction between the dyes and the chromatography paper in your

answer.

[2 marks]

04.2 The student used the same type of chromatography paper in Experiment 1 and in

Experiment 2.

Explain why the yellow dye is in different positions in Experiment 1 and in

Experiment 2.

Use Figure 4.

[3 marks]

Figure 4 is repeated below.

Figure 4 shows the results of Experiment 1 and Experiment 2 using orange dye A.

Figure 4

The student investigated a different orange dye (B).

Figure 5 shows the results of Experiment 3 using orange dye B.

Figure 5

04.3 Compare the purity of the orange dyes A and B.

Give reasons for your answer.

Use Figure 4 and Figure 5.

[2 marks]

04.4 The student calculated that the Rf value of the orange dye in the experiment shown in

Figure 5 was 0.48

Calculate the distance moved by the solvent front when the orange dye had moved

5.4 cm.

[3 marks]

Distance moved by solvent front = cm

04.5 Why is the Rf value of a dye not affected by how far the solvent front is allowed to

travel?

[1 mark]

04.6 Another type of chromatography is called gas chromatography.

Gas chromatography is an instrumental method of chemical analysis.

Scientists tested the orange dyes using gas chromatography.

Suggest two advantages of using the instrumental method of gas chromatography

rather than paper chromatography.

*15* [2 marks]

Mark scheme

Show the mark scheme The mark scheme provides the expected answers for each sub-question. It details the scientific reasoning required for explaining dye separation based on attraction to stationary and mobile phases, identifying purity based on the number of spots, the calculation steps for the Rf value, and the advantages of gas chromatography such as sensitivity, accuracy, and speed.

Question 4

AO /

Question Answers Extra information Mark

Spec. Ref.

04.1 allow converse argument AO1

4.8.1.3

allow stationary phase for paper

RPA6

the yellow dye travels further 1

(because the yellow) dye has a 1

weaker attraction to the

(chromatography) paper

if no other mark awarded

allow for 1 mark

the weaker the attraction to the

(chromatography) paper the

greater the distance travelled– – –

(by the dye)

AO /

Spec. Ref.

04.2 allow mobile phase for solvent AO3

4.8.1.3

RPA6

(in Experiment 2) the yellow dye 1

travels further

(because) the solvents are 1

different

(and) the yellow dye is more allow the yellow dye is less 1

soluble in ethanol (than water) soluble in water (than ethanol)

or

(and) the yellow dye is more allow the yellow dye is less

attracted to ethanol (than water) attracted to water (than ethanol)

OR

(in Experiment 1) the yellow dye

does not travel as far (1)

(because) the solvents are

different (1) 15

(and) the yellow dye is less allow (and) the yellow dye is

soluble in water (than ethanol) more soluble in ethanol (than

water)

or

(and) the yellow dye is less allow (and) the yellow dye is

attracted to water (than ethanol) more attracted to ethanol (than–– –

(1) water)

AO /

Spec. Ref.

04.3 A is an impure (substance) allow A is a mixture 1 AO3

4.8.1.1

and and

4.8.1.3

B is a pure (substance) B is a pure (substance) RPA6

16 (because) A contains two dyes allow (because) A produces two 1

and spots

B contains one dye and

B produces one spot

if no other mark awarded

allow 1 mark for

A contains two dyes (so) is

impure (substance)

or

A contains two dyes (so) is a

mixture

or

B contains one dye (so) is pure

(substance)

AO /

Spec. Ref.

04.4 5.4 1 AO2

0.48 = 4.8.1.3

distance moved by solvent

RPA6

(distance moved by solvent =)

5.4 1

0.48

=11.25 (cm) allow 11.25– correctly rounded to– 1 –

at least 2 significant figures

AO /

Spec. Ref.

04.5 the ratio / proportion of allow the distance travelled by 1 AO1

4.8.1.3

spot distance (moved) to solvent the spot relative to the distance

RPA6

distance (moved) is fixed / travelled by the solvent is

constant constant

allow the distance travelled by

the spot is (directly) proportional

to the distance travelled by the

solvent

AO /

Spec. Ref.

04.6 any two from: 2 AO1

• (more) sensitive allow small(er) sample 4.8.3.6

• (more) accurate allow greater resolution

• fast(er)

Total Question 4 13

How to answer it

Paper and Gas Chromatography Analysis

What this question tests

This question assesses your understanding of chromatography, specifically how substances separate based on their solubility in the mobile phase and attraction to the stationary phase. It also covers purity, Rf value calculations, and the advantages of instrumental analysis (gas chromatography).

Part (a) - Question 04.1

Explaining separation in a single solvent

💡 Key Knowledge

  • Stationary Phase: The paper.
  • Mobile Phase: The solvent (water).
  • Substances that travel further have a weaker attraction to the paper.

✅ Correct Answer

  • The yellow dye travels further. [1 mark]
  • Because the yellow dye has a weaker attraction to the paper. [1 mark]

🧠 Exam Technique

The question specifically asks you to refer to forces of attraction. If you only say "yellow is more soluble," you might miss the mark because the prompt directed you toward the paper's attraction. Always follow the specific instruction in the prompt!

Part (b) - Question 04.2

Comparing different solvents

✅ Correct Answer

  • In Experiment 2, the yellow dye travels further. [1 mark]
  • The solvents are different (water vs ethanol). [1 mark]
  • The yellow dye is more soluble in ethanol (or more attracted to ethanol). [1 mark]

❌ Common Errors

Students often forget to state the obvious: that the solvents are different. To get full marks, you must link the change in position to the change in the mobile phase (the solvent).

Part (c) - Question 04.3

Purity of Dyes A and B

💡 Key Knowledge

  • Pure substance: Produces exactly one spot.
  • Impure substance (mixture): Produces two or more spots.

✅ Correct Answer

A is impure and B is pure. [1 mark]

Reason: A produces two spots, whereas B produces only one spot. [1 mark]

Part (d) - Question 04.4

Calculating Solvent Front Distance

📐 Calculation Steps

  1. Recall the formula: Rf = distance moved by substance / distance moved by solvent
  2. Substitute the values: 0.48 = 5.4 / distance moved by solvent
  3. Rearrange the equation: distance = 5.4 / 0.48
  4. Final Answer: 11.25 cm

Award 3 marks for 11.25. Award 2 marks if the calculation is set up correctly but a math error occurs.

❌ Calculation Traps

A common mistake is multiplying the numbers (5.4 × 0.48). Remember: the Rf value is always a decimal less than 1 because the dye cannot travel further than the solvent. If your solvent distance is smaller than your dye distance, you've made a mistake!

Part (e) - Question 04.5

Why Rf is constant

🧠 Exam Technique

Think of Rf as a ratio or a percentage. If the solvent travels twice as far, the dye will also travel twice as far. The proportion stays the same.

✅ Correct Answer

The ratio of the distance moved by the spot to the distance moved by the solvent is constant. [1 mark]

Part (f) - Question 04.6

Advantages of Gas Chromatography

💡 Key Knowledge

Instrumental methods (like gas chromatography linked to mass spec) are almost always superior to manual paper chromatography for professional lab work.

✅ Correct Answer (Pick any two)

  • More accurate
  • More sensitive (can detect tiny amounts)
  • Faster
  • Requires a smaller sample

[2 marks]

Topics

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

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.