AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2023: Question 6

11 marks · Standard Demand difficulty · Extended Answer

Identify phases in chromatography, explain separation mechanisms, calculate Rf values, and plan a paper chromatography experiment to identify components of black ink.

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Question

The image shows five parts of a chemistry exam question about chromatography. Part 06.1 is a multiple-choice question asking for the 'other phase' in chromatography. Part 06.2 asks for an explanation of why substances separate. Part 06.3 asks how many spots a pure compound produces. Part 06.4 provides data (blue colour moved 4.77 cm, solvent moved 5.30 cm) to calculate an Rf value. Part 06.5 is a 6-mark extended response question asking for an experimental plan to separate and identify colours in black ink.
Question text

06 Chromatography is used to separate mixtures.

Chromatography involves a mobile phase and one other phase.

06.1 What is the other phase in chromatography?

[1 mark]

Tick ( ) one box.

Moving phase

Recycled phase

Stationary phase

Viscous phase

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

[1 mark]

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

[1 mark]

29 Number of spots =

06.4 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]

30 R value =

f

06.5 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 provides answers for question 6. 06.1 is 'stationary phase'. 06.2 accepts reasons like different speeds or solubilities. 06.3 is '1'. 06.4 shows the calculation 4.77 divided by 5.30 equals 0.9. 06.5 uses a three-level marking grid (1-2, 3-4, 5-6 marks) with indicative content covering the experimental setup (pencil line, solvent level, lid) and analysis (measuring distances, calculating Rf, comparing to known values).

Question 6

AO /

Question Answers Extra information Mark

Spec. Ref.

06.1 stationary phase 1 AO1

5.8.1.3

RPA12

AO /

Spec. Ref.

06.2 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.

06.3 1 1 AO1

5.8.1.3

RPA12

AO /

Spec. Ref.

06.4 (Rf = ) AO2

4.77 5.8.1.3

1 RPA12

5.30

= 0.9 – OMBINED SCIENCE: TRILOGY – 1 2F –

AO /

Question Answers Mark

Spec. Ref.

06.5 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 6 11

How to answer it

Mastering Paper Chromatography

What this question tests

This question assesses your understanding of Required Practical 12. You need to know the terminology of phases, why substances separate, how to calculate Rf values, and how to plan a valid investigation to identify unknown substances in a mixture.

Part 06.1

Chromatography Phases

Correct Answer

Stationary phase

Key Knowledge

  • Mobile Phase: The solvent that moves (e.g., water or ethanol).
  • Stationary Phase: The material that does not move (e.g., the chromatography paper).
Parts 06.2 & 06.3

Separation and Purity

Why do they separate?

Substances separate because they have different solubilities in the mobile phase or different attractions to the stationary phase (the paper). This causes them to move at different speeds.

1 Mark: Mention speed, solubility, or attraction.

Pure Compounds

A pure compound will produce exactly 1 spot on a chromatogram.

1 Mark: Number of spots = 1

Part 06.4

Calculating Rf Values

Step-by-Step Calculation

  1. Identify the formula: Rf = distance moved by substance ÷ distance moved by solvent
  2. Plug in the numbers: Rf = 4.77 cm ÷ 5.30 cm
  3. Calculate: 0.9

Final Answer: 0.9

Common Calculation Traps

  • Upside down: Never divide the big number by the small number. An Rf value is always less than 1.
  • Units: Ensure both measurements are in the same units (cm) before dividing.

Examiner Tip

Even if you get the final answer wrong, show your working! You get 1 mark for the correct substitution (4.77 / 5.30) and 1 mark for the final answer.

Part 06.5

6-Mark Practical Plan: Black Ink

How to structure a Level 3 response (5-6 marks)

The Method

  1. Setup: Draw a pencil line near the bottom of the chromatography paper. (Pencil is used because it is insoluble and won't run).
  2. Spotting: Place a small dot of the black ink on the pencil line.
  3. Solvent: Place the paper into a beaker containing a small volume of solvent (water). Crucial: The solvent level must be below the pencil line.
  4. Development: Put a lid on the beaker to prevent evaporation. Allow the solvent to move up the paper.
  5. Finishing: Remove the paper just before the solvent reaches the top. Mark the solvent front with a pencil.
  6. Analysis: Measure the distance from the start line to the middle of each spot and the distance to the solvent front. Calculate Rf values and compare them to known data.

What distinguishes top answers?

Top-level students don't just say "do chromatography." They use logical sequencing. They mention specific details like using a lid, using pencil for the line, and the specific measurements needed for Rf values.

Common Errors

  • Drawing the start line in ink (it will smudge and ruin the results).
  • Letting the solvent level touch the ink spot (the ink will just dissolve into the beaker).
  • Forgetting to mention comparing the Rf values to identify the colours.

Diagram Description

If you were to draw this, you should show a beaker with a horizontal line representing the water. The paper should be dipping into the water, with a pencil line drawn above the water level. A single dot of ink should be on that pencil line.

Topics

Chemistry · C8: Chemical Analysis

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