AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2020: Question 4
8 marks · Low Demand difficulty · Short Answer
Calculate an Rf value, identify chromatography phases, determine a component ratio, and evaluate properties of ink chromatography.
Practise this questionQuestion
Question text
04 This question is about ink.
A student investigated green ink using paper chromatography in a beaker.
Figure 6 shows:
• the results the student obtained
• measurements A, B, C and D the student could make.
Figure 6
Diagram not to scale
04.1 The student calculated the Rf value of the blue dye.
The student measured:
• the distance moved by the blue dye = 2.7 cm
• the distance moved by the solvent = 9.0 cm
Calculate the Rf value of the blue dye.
Use the equation:
distance moved by dye
Rf =
distance moved by solvent
[2 marks]
Rf =
04.2 Which measurements on Figure 6 are needed to calculate the Rf value
of the yellow dye?
[1 mark]
*16* Tick ( ) one box.
A and B
A and C
B and D
C and D
04.3 Paper chromatography has a stationary phase and a mobile phase.
Draw one line from each phase to the identity of that phase in the
student’s investigation.
[2 marks]
Phase Identity
Beaker
Mobile phase Ink
Paper
Stationary phase Solvent
18 Start line
The green ink contains 85% yellow dye and 15% blue dye.
04.4 Determine the simplest whole number ratio of yellow dye : blue dye in the green ink.
[1 mark]
Yellow dye : Blue dye = :
04.5 Which word correctly describes the green ink?
[1 mark]
Tick ( ) one box.
Compound
Element
Formulation
Solvent
04.6 The student repeated the investigation using green ink containing 75% yellow dye and
25% blue dye.
What would happen to the Rf value of the yellow dye?
[1 mark]
Tick ( ) one box.
The Rf value would decrease.
The Rf value would increase.
The Rf value would stay the same.
Mark scheme
Show the mark scheme
Question 4
AO /
Question Answers Extra information Mark
Spec. Ref.
AO2
04.1 2.7 1 4.8.1.3
9.0 RPA6
= 0.3 ignore units 1
AO1
04.2 C and D 1 4.8.1.3
RPA6
04.3 AO1
4.8.1.3
RPA6
additional line from a box on the left negates the mark for that box
04.4 17 : 3 1 AO2
4.8.1.2
04.5 formulation 1 AO2
4.8.1.2
04.6 the Rf value would stay the 1 AO3
same 4.8.1.2
Total 8
How to answer it
Paper Chromatography & Formulations
This question assesses fundamental practical and theoretical skills from AQA Chemistry Topic 8 (Chemical Analysis - RPA 6):
- Calculating retention factors (Rf values) from experimental measurements.
- Correctly identifying measurement baselines (always from the pencil start line, not the paper edge).
- Distinguishing between the stationary phase and mobile phase.
- Simplifying percentage compositions into whole-number ratios.
- Recognising mixtures designed for a useful purpose as formulations.
- Understanding factors affecting Rf values (concentration does not alter Rf).
Calculating the Rf Value of the Blue Dye
2 Marks • AO2 Calculation
📐 Step-by-Step Calculation
- Identify formula & values:
Distance moved by dye = 2.7 cm
Distance moved by solvent = 9.0 cm - Substitute values into equation:
Rf = 2.7 / 9.0 [1 mark] - Evaluate:
Rf = 0.3 [1 mark]
❌ Common Errors
- Inverting the fraction: Doing 9.0 / 2.7 = 3.33. An Rf value can never be greater than 1.0!
- Adding units: Rf is a pure ratio (cm / cm), so it has no units. Writing "0.3 cm" is ignored by examiners, but always remember it has no units.
Identifying Correct Chromatogram Measurements
1 Mark • AO1 Practical Technique
✅ Correct Answer
C and D
- C: Distance from the pencil start line to the solvent front.
- D: Distance from the pencil start line to the centre of the yellow spot.
🧠 Exam Technique: Where to Measure
- Always measure from the pencil start line, NEVER from the bottom edge of the chromatography paper.
- Measurements A and B incorrectly start at the very bottom edge of the paper, making them invalid for Rf calculations.
Stationary and Mobile Phases
2 Marks • AO1 Recall
✅ Correct Matches
- Mobile phase → Solvent [1 mark]
- Stationary phase → Paper [1 mark]
💡 Key Knowledge
- Mobile phase: The phase that moves. In paper chromatography, this is the liquid solvent (e.g. water or ethanol) carrying the dyes up the strip.
- Stationary phase: The phase that does not move. In this setup, it is the absorbent chromatography paper.
❌ Examiner Warning
Drawing more than one line from a box on the left automatically negates the mark for that box. Only draw exactly one line from each phase.
Simplest Whole Number Ratio
1 Mark • AO2 Mathematical Skill
📐 Step-by-Step Calculation
- Write given ratio: Yellow dye : Blue dye = 85 : 15
- Find the highest common factor: Both numbers end in 5, so divide both by 5:
85 ÷ 5 = 17
15 ÷ 5 = 3 - Final whole-number ratio: 17 : 3
❌ Common Errors
- Reversing the order: Writing 3 : 17 instead of 17 : 3 (the question explicitly asks for yellow dye : blue dye).
- Failing to fully simplify: Leaving it as 85 : 15 scores 0 marks because the question requests the simplest whole number ratio.
Classifying the Ink: Formulations
1 Mark • AO2 Classification
✅ Correct Answer
Formulation
💡 What is a Formulation?
A formulation is a complex mixture designed as a useful product, prepared by mixing components in carefully measured, exact quantities to give the product specific desired properties.
Common examples include: paints, inks, medicines, cleaning agents, cosmetics, and fertilisers.
❌ Why Other Options Are Incorrect
- Compound: Chemical substances chemically bonded together in fixed ratios; ink components can be physically separated.
- Element: Pure substance containing only one type of atom.
- Solvent: The liquid used to dissolve substances (e.g. water), not the whole coloured ink mixture.
Effect of Concentration on Rf Value
1 Mark • AO3 Analysis & Understanding
✅ Correct Answer
The Rf value would stay the same.
💡 Key Knowledge: What Determines Rf?
An Rf value depends only on:
- The substance itself (its relative affinity/solubility).
- The mobile phase (solvent used).
- The stationary phase (paper used).
Changing the concentration or percentage of dye alters the spot's size or colour intensity, but does not change the Rf value.
🧠 Exam Strategy Tip
When asked what happens to an Rf value if you change sample size or concentration, remember that Rf is a characteristic physical constant for a specific substance in a specific solvent system. It remains constant!
Topics
Chemistry · Required Practicals · C8: Chemical Analysis · Required Practicals
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Foundation), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.