AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2022: Question 6

10 marks · Standard Demand difficulty · Short Answer

Analyze a chromatography experiment to identify setup errors, interpret results for ink composition, and calculate solvent distance using the Rf value formula.

Practise this question

Question

The image presents a chromatography investigation. Figure 10 illustrates a flawed experimental setup where the start line is drawn in ink and the solvent level is above this line. Figure 11 displays the results of a correct experiment, showing the separation of red, blue, green, yellow, and brown inks. The brown ink separates into four distinct spots, while the green ink remains on the start line. The final part asks for a calculation of solvent distance using the provided Rf formula and values.
Question text

06 A student investigated the colours in a brown ink using chromatography.

06.1 Figure 10 shows the apparatus used.

Figure 10

Give two errors made by the student.

Describe the problem each error would cause.

[4 marks]

Error 1

Problem 1

Error 2

Problem 2

A different student set up the apparatus correctly.

Figure 11 shows the results.

Figure 11

06.2 Give two conclusions the student can make from Figure 11 about the four colours

in the brown ink.

[2 marks]

06.3 Why was the green colour still on the start line at the end of the experiment?

[1 mark]

Tick ( ) one box.

The experiment was left for too long.

The green colour was insoluble in the solvent.

The green spot contained too many colours.

The green spot was too small.

06.4 A student calculated the Rf value of a colour to be 0.24

The colour moved 1.8 cm from the start line.

Calculate the distance the solvent moved.

Use the equation:

distance moved by colour

Rf =

distance moved by solvent

[3 marks]

Distance moved by solvent = cm

Mark scheme

Show the mark scheme The mark scheme details the 10 marks available. For 06.1, it awards marks for identifying errors like the ink start line and high solvent level, along with their effects. For 06.2, it lists valid conclusions about the brown ink's components. 06.3 identifies the green ink as insoluble. 06.4 shows the calculation steps: 0.24 = 1.8 / distance, leading to distance = 1.8 / 0.24, resulting in 7.5 cm.

Question 6

AO /

Question Answers Extra information Mark

Spec. Ref.

06.1 any two pairs from: 4 AO3

5.8.1.3

RPA12

(start) line drawn in ink (1) allow (start) line should be

drawn in pencil

(so ink) will mix with solvent (1) allow the ink will move up the

paper

the solvent is above the (start) allow the solvent should be

line (1) below the (start) line

(so) colours / ink will dissolve (1)

no lid on tank (1)

(so) solvent will evaporate (1)

AO /

Spec. Ref.

06.2 any two from: 2 AO3

5.8.1.3

• (the brown ink) contains the RPA12

blue, yellow and red (colours)

• (the brown ink) contains an

unknown colour

• (the brown ink) does not

contain green ink

• blue (colour) is the most allow blue (colour) has the

soluble highest Rf value

or

red (colour) is the least allow red (colour) has the lowest

soluble Rf value

ignore green colour is insoluble

AO /

Spec. Ref.

06.3 the green colour was insoluble 1 AO2

in the solvent 5.8.1.3

RPA12

AO /

Spec. Ref.

06.4 1.8 AO2

0.24 = 1

distance moved by solvent 5.8.1.3

RPA12

(distance moved by solvent =)

1.8 1

0.24

= 7.5 (cm) 1

Total Question 6 10

How to answer it

Chromatography: Setup and Analysis

What this question tests

This question assesses your knowledge of Required Practical 12 (Paper Chromatography). You need to identify experimental errors, interpret a chromatogram to identify components of a mixture, and use the Rf equation to calculate distances.

Part (a) - Question 06.1

Identifying Experimental Errors

Spotting mistakes in the setup

✅ Correct Answers (Any 2 pairs)

  • Error: Start line drawn in ink.
    Problem: The ink will run/mix with the solvent.
  • Error: Solvent level is above the start line.
    Problem: The ink spots will wash off/dissolve into the solvent.
  • Error: No lid on the tank.
    Problem: The solvent will evaporate.

🧠 Exam Technique

When asked for an "error" and a "problem," make sure they match! If you say the line is in ink, the problem must explain what happens to that ink. Don't just list facts; link the cause to the effect.

❌ Common Errors

Students often forget that the start line must be in pencil because pencil (graphite) is insoluble and won't move. Another common mistake is saying "the experiment won't work" — you must be specific about why (e.g., "the spots will dissolve into the solvent").

Part (b) - Question 06.2

Interpreting Results

Comparing spots to identify mixtures

💡 Key Knowledge

  • If two spots travel the same distance, they are likely the same substance.
  • A pure substance produces one spot.
  • A mixture (like the brown ink) produces multiple spots.

✅ Correct Conclusions (Any 2)

  • Brown ink contains blue, yellow, and red.
  • Brown ink contains an unknown colour (the top spot).
  • Brown ink does not contain green.
  • Blue is the most soluble (it travelled furthest).
Part (c) - Question 06.3

Solubility and Movement

✅ Correct Answer

The green colour was insoluble in the solvent.

🧠 Examiner Insight

If a spot stays on the start line, it means it did not dissolve in the mobile phase (the solvent). Top-tier students use the word insoluble rather than just saying "it didn't stick to the water."

Part (d) - Question 06.4

Calculating Distance

Using the Rf Equation

📐 Step-by-Step Calculation

1. Identify the values:
Rf = 0.24
Distance moved by colour = 1.8 cm

2. Rearrange the formula:
Distance moved by solvent = Distance moved by colour / Rf

3. Substitute and solve:
1.8 / 0.24 = 7.5

Final Answer: 7.5 cm

❌ Calculation Traps

The most common mistake is multiplying the numbers (0.24 × 1.8) instead of dividing. Always check: the solvent distance must be larger than the colour distance, because Rf is always less than 1.

Mark Breakdown:
1 Mark: Correct substitution into formula.
1 Mark: Correct rearrangement.
1 Mark: Correct final answer.

Topics

Chemistry · C8: Chemical Analysis

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.