AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2023: Question 2

11 marks · Standard Demand difficulty · Short Answer

Use chromatography knowledge to identify the stationary phase, explain why substances separate, calculate an Rf value, and describe a paper chromatography method for black ink.

Practise this question

Question

An exam question page about chromatography. Question 02.1 asks what the other phase in chromatography is, with four tick-box options: Moving phase, Recycled phase, Stationary phase, and Viscous phase. Question 02.2 asks why substances in a mixture separate in the mobile phase, with a blank line for a written answer. Question 02.3 asks how many spots are produced on the chromatogram of a pure compound, with a line to write the number. Question 02.4 states that a blue colour moved 4.77 cm and the solvent moved 5.30 cm, then asks the student to calculate the Rf value for the blue colour, with space for working and an answer line. Question 02.5 asks the student to plan a paper chromatography experiment to separate the colours in black ink and identify the colours from their Rf values, with several lines for a longer response. The total marks shown are 11 for the whole question.
Question text

02 Chromatography is used to separate mixtures.

Chromatography involves a mobile phase and one other phase.

02.1 What is the other phase in chromatography?

[1 mark]

Tick ( ) one box.

Moving phase

Recycled phase

Stationary phase

Viscous phase

02.2 Why do the substances in the mixture separate in the mobile phase?

[1 mark]

02.3 How many spots will be produced on the chromatogram of a pure compound?

[1 mark]

7 Number of spots =

02.4 In a chromatography experiment, a blue colour moved 4.77 cm.

The solvent moved 5.30 cm.

Calculate the Rf value for the blue colour.

[2 marks]

8 R value =

f

02.5 Black ink is a mixture of several colours.

Plan an experiment using paper chromatography to:

• separate the colours in black ink

• identify the colours from their Rf values.

[6 marks]

Mark scheme

Show the mark scheme The mark scheme shows that for 02.1 the correct answer is stationary phase for 1 mark. For 02.2 acceptable answers include that substances move at different speeds, have different solubilities, or have different attractions for the paper, worth 1 mark. For 02.3 the answer is 1 spot for a pure compound, worth 1 mark. For 02.4 the mark scheme shows the calculation Rf = 4.77 divided by 5.30, giving 0.9, with 1 mark for the method and 1 mark for the correct value. For 02.5 it gives level-based marking from 5 to 6 marks, with indicative points such as drawing a pencil line near the bottom of the paper, putting a small dot of ink on the line, placing the paper in solvent with the ink above the solvent level, leaving it to run, marking the solvent front, and calculating and comparing Rf values.

Question 2

AO /

Question Answers Extra information Mark

Spec. Ref.

02.1 stationary phase 1 AO1

5.8.1.3

RPA12

AO /

Spec. Ref.

02.2 any one from: 1 AO2

(the substances) 5.8.1.3

• move at different speeds RPA12

• have different solubilities

• have different attractions for

the (chromatography) paper

AO /

Spec. Ref.

02.3 1 1 AO1

5.8.1.3

RPA12

AO /

Spec. Ref.

02.4 (Rf = ) AO2

4.77 5.8.1.3

1 RPA12

5.30

= 0.9 – OMBINED SCIENCE: TRILOGY – 1 –

AO /

Question Answers Mark

Spec. Ref.

02.5 Level 3: The method would lead to the production of a valid

5–6 AO1

outcome. The key steps are identified and logically sequenced.

5.8.1.3

Level 2: The method would not necessarily lead to a valid 3–4

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

logically sequenced.

Level 1: The method 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

• draw a pencil line

• near the bottom edge of the (chromatography) paper

• put a small dot of black ink on the pencil line

• put a small volume of water / solvent in a beaker

• place the (chromatography) paper in the solvent

• the dots of ink should be above the level of the water / solvent

• support the paper in this position

• put a lid on the beaker

• leave to run

• mark position of solvent front

• remove from beaker and leave to dry

• determine Rf values

• measure distance of all dots from start line

• measure distance of solvent front from start line

• calculate Rf values for all dots

• compare with known Rf values

Total Question 2 11

How to answer it

Chromatography Basics and Paper Chromatography Method

What this question tests

This question tests your knowledge of chromatography vocabulary, why mixtures separate, how pure substances appear on a chromatogram, how to calculate an Rf value, and how to plan a clear paper chromatography method that would successfully separate and identify colours using Rf values.

Question overview • Total = 11 marks

💡 Key knowledge for the whole question

  • Chromatography has a mobile phase and a stationary phase .
  • Substances separate because they move at different speeds .
  • This is linked to their different solubilities and different attractions to the paper .
  • A pure substance gives one spot .
  • Rf = distance moved by substance ÷ distance moved by solvent.
  • For a method question, marks come from logical order , practical detail , and including how the colours are identified .

🧠 Big examiner message

Most of the short questions are simple recall, but the 6-mark method question needs a sequenced practical method. Top answers do not just list equipment — they explain what to do, in the right order, and include how to calculate and compare Rf values.

Part 02.1 — The other phase in chromatography

1 mark

✅ Correct answer

Stationary phase

1 mark for selecting stationary phase.

💡 Key knowledge

The solvent is the mobile phase because it moves. The paper is the stationary phase because it stays in place.

❌ Common errors

  • Choosing moving phase because it sounds sensible — but the question asks for the other phase, not the mobile phase.
  • Choosing a made-up sounding term like viscous phase or recycled phase.

Part 02.2 — Why substances separate

1 mark

✅ Correct answers

Any one of these gets the mark:

  • They move at different speeds.
  • They have different solubilities.
  • They have different attractions for the paper.
Only one clear valid idea is needed for 1 mark.

🧠 Exam technique

For a 1-mark explain question, keep it short and precise. A strong answer would be:

The substances have different solubilities, so they move at different speeds.

This combines two marking ideas and is very safe.

❌ Common errors

  • Saying only because they dissolve — too vague.
  • Saying because they are different colours — colour does not cause the separation.
  • Mentioning temperature or evaporation without linking to movement through the paper.

Part 02.3 — Pure compound spot count

1 mark

✅ Correct answer

1

A pure compound produces one spot on the chromatogram.

💡 Key knowledge

A pure substance contains only one compound, so there is only one spot. A mixture gives more than one spot.

❌ Common errors

Students sometimes think a pure substance makes no spot or many spots. Remember:

pure = one spot

Part 02.4 — Calculating the Rf value

2 marks

📐 Calculation

  1. Write the formula:
    Rf = distance moved by substance ÷ distance moved by solvent
  2. Substitute the values:
    Rf = 4.77 ÷ 5.30
  3. Calculate:
    Rf = 0.9
1 mark for setting up 4.77 / 5.30
1 mark for the answer 0.9

✅ Correct answer

Rf = 0.9

The mark scheme accepts the calculation and the final answer 0.9.

🧠 Exam technique

Always use:

  • distance moved by the spot
  • distance moved by the solvent front

Show the division clearly to secure the method mark even if the final answer is wrong.

❌ Common calculation traps

  • Doing the division the wrong way round: 5.30 ÷ 4.77 is wrong.
  • Using the total paper length instead of the solvent front distance.
  • Forgetting that Rf is usually written as a number with no unit.
  • Giving too many decimal places when the mark scheme simply wants 0.9 .

Part 02.5 — Plan a paper chromatography experiment

6 marks

💡 What the mark scheme is looking for

This is a levels question. To reach the top level, your method must be:

  • valid — it would actually work
  • detailed — key practical steps are included
  • logical — steps are in the correct order
  • complete — includes both separation and identification using Rf values
Level 3 = 5–6 marks: valid, clear, logically sequenced method
Level 2 = 3–4 marks: several steps included but sequence or detail weaker
Level 1 = 1–2 marks: some relevant ideas but unclear or incomplete

✅ Full-mark style answer

Draw a pencil line near the bottom of the chromatography paper. Put a small dot of black ink on the pencil line. Place a small volume of water or suitable solvent in a beaker. Put the paper in the beaker so that the solvent level is below the ink spot. Support the paper and put a lid on the beaker.

Leave the solvent to move up the paper. When it is close to the top, remove the paper, mark the solvent front in pencil and leave the paper to dry. Measure the distance moved by each coloured spot from the start line and the distance moved by the solvent front from the start line. Calculate the Rf value for each colour using:

Rf = distance moved by spot ÷ distance moved by solvent front

Compare the Rf values with known Rf values to identify the colours.

🧠 Examiner insight

  • Top answers are sequenced like a method, not scattered notes.
  • You must include the detail that the spot starts above the solvent. If the spot is in the solvent, the experiment may not work properly.
  • To answer the second bullet in the question, you must explain how to identify the colours from Rf values, not just how to separate them.
  • Better answers mention marking the solvent front before it evaporates away.

❌ Common errors

  • Using pen instead of pencil for the start line — pen may dissolve in the solvent.
  • Putting the ink spot below the solvent level.
  • Forgetting to mark the solvent front.
  • Describing separation only, but not explaining how to calculate and compare Rf values.
  • Giving a list of equipment with no method.

📐 How to include the Rf part clearly

  1. Measure the distance from the start line to each spot.
  2. Measure the distance from the start line to the solvent front.
  3. For each colour, calculate:
    Rf = distance moved by spot ÷ distance moved by solvent front
  4. Compare each Rf value with known Rf values to identify the substance or colour.

🧠 A strong 6-mark structure to memorise

  1. Start line: pencil line near the bottom.
  2. Sample: small dot of black ink on the line.
  3. Solvent: small amount in beaker.
  4. Set-up: paper in beaker, spot above solvent, cover with lid.
  5. Run: let solvent rise up the paper.
  6. Finish: remove paper, mark solvent front, dry.
  7. Identify: measure distances, calculate Rf values, compare with known values.

Quick full-answer summary

✅ Answers in one place

  • 02.1: stationary phase
  • 02.2: substances move at different speeds / have different solubilities / have different attractions for the paper
  • 02.3: 1
  • 02.4: Rf = 4.77 ÷ 5.30 = 0.9
  • 02.5: valid paper chromatography method including pencil baseline, ink spot above solvent, solvent front marked, distances measured, Rf values calculated, and comparison with known Rf values

💡 Final revision takeaway

If you remember just four things, remember these:

  • stationary phase
  • pure substance = 1 spot
  • Rf = spot distance ÷ solvent distance
  • pencil line, spot above solvent, compare Rf values

Topics

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

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