AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), November 2020: Question 5
9 marks · Low Demand difficulty · Short Answer
Identify mixtures and formulations, match phases in paper chromatography, interpret a chromatogram, and calculate an Rf value.
Practise this questionQuestion
Question text
05 This question is about mixtures.
05.1 Which substance is a mixture?
[1 mark]
Tick ( ) one box.
Air Gold Methane Nitrogen
05.2 Food colourings are often mixtures of dyes.
What name is given to mixtures that are designed as useful products?
[1 mark]
A student investigated a purple food colouring, Y, using chromatography.
The student compares Y with dyes A, B and C.
05.3 Figure 8 shows the apparatus used.
Figure 8
Chromatography involves a stationary phase and a mobile phase.
Draw one line from each phase to what is used for that phase.
*18* Use Figure 8.
[2 marks]
Phase What is used
Beaker
Chromatography paper
Mobile phase
Food colouring
Stationary phase
Pencil line
20 Solvent
Figure 9 shows the student’s results.
Figure 9
05.4 What three conclusions can you make about the dyes in food colouring Y?
[3 marks]
05.5 In a different experiment a student recorded these results:
Distance moved by dye G = 60 mm
Distance moved by solvent = 80 mm
Calculate the Rf value of dye G.
distance moved by dye G
Rf =
distance moved by solvent
[2 marks]
Rf =
Mark scheme
Show the mark scheme
AO /
Question Answers Extra information Mark
Spec. Ref.
05.1 air 1 AO1
5.8.1.1
05.2 formulation(s) 1 AO1
5.8.1.2
05.3 AO2
5.8.1.3
additional line from a box on the left negates the mark for that box
allow colour for dyes
05.4 AO3
Y contains 3 dyes 1 5.8.1.3
Y contains 2 known dyes allow Y contains A and C 1
Y contains an unknown dye allow Y does not contain dye B 1
alternative approach:
Y contains 3 dyes (1)
Y contains 1 known dye (1) allow Y contains dye C
Y contains 2 unknown dyes (1) allow Y does not contain dyes
A and B
05.5 60 AO2
(Rf =) 1
80 5.8.1.3
= 0.75 ignore units 1
Total 9
How to answer it
Mixtures, Formulations and Paper Chromatography
- Substance classification: Distinguishing pure elements and compounds from mixtures.
- Formulations: Recalling the key definition of a formulation.
- Chromatography basics: Identifying the stationary phase and mobile phase in paper chromatography.
- Interpreting chromatograms: Matching dye spots by height and determining pure vs mixture samples.
- Rf calculations: Using the given formula to calculate retention factors accurately.
Identifying a Mixture
Question: Which substance is a mixture? (Air / Gold / Methane / Nitrogen)
✅ Correct Answer
Air
💡 Key Knowledge
- Air is a mixture of unbonded gases (nitrogen, oxygen, argon, carbon dioxide, water vapour).
- Gold (Au) and Nitrogen (N₂) are elements.
- Methane (CH₄) is a chemical compound.
❌ Common Errors
Students sometimes confuse nitrogen (an element making up ~78% of air) with air itself, or mistake methane for a mixture because it has two elements bonded together.
Useful Mixtures
Question: What name is given to mixtures that are designed as useful products?
✅ Correct Answer
Formulation (or formulations)
💡 Key Knowledge
A formulation is a complex mixture designed with exact quantities of specific chemicals to create a product with desired properties (e.g. paints, medicines, fuels, fertilisers, foods).
🧠 Exam Technique
This is direct AQA specification recall. Memorise the exact phrase: "A formulation is a mixture that has been designed as a useful product."
Phases in Chromatography
Question: Draw one line from each phase to what is used for that phase.
✅ Correct Matching
| Phase | What is used |
|---|---|
| Mobile phase | ➔ Solvent |
| Stationary phase | ➔ Chromatography paper |
💡 Key Knowledge
- Mobile phase: Moves through the setup carrying the dissolved substances (the liquid solvent, e.g. water or ethanol).
- Stationary phase: Does not move (the paper).
🧠 Exam Technique & Traps
Read instructions carefully: "Draw one line from each phase". If you draw two lines from a single left-hand box, you negate (lose) the mark for that box even if one is correct!
Interpreting the Chromatogram
Question: What three conclusions can you make about the dyes in food colouring Y?
✅ Correct Answers (Any three)
- Y contains 3 dyes (or 3 colours).
- Y contains dye C (the lowest spot aligns exactly with Dye C).
- Y contains dye A (the middle spot aligns with Dye A) OR state that Y contains 2 known dyes.
- Y contains an unknown dye (the top spot does not match A, B, or C).
- Y does not contain dye B (no spot at the height of B).
🧠 How to Read a Chromatogram
- Count the spots: Each spot directly above a sample represents a different dye. Column Y has 3 vertical spots = 3 dyes.
- Check horizontal alignment: If a spot in Y lines up horizontally with a spot from a known dye, Y contains that dye.
- Look for missing spots: No spot lines up with dye B, so dye B is absent.
❌ Common Errors
- Saying "Y is a pure dye" (it clearly separates into three distinct spots, so it is a mixture).
- Forgetting to state that one of the dyes is unknown or not from A, B, or C.
Calculating Retention Factor (Rf)
Question: Calculate the Rf value of dye G.
Given data: Distance moved by dye G = 60 mm | Distance moved by solvent = 80 mm
🔧 Step-by-Step Calculation
- Write down the formula:
Rf = distance moved by substance ÷ distance moved by solvent - Substitute the numbers:
Rf = 60 ÷ 80 - Calculate and simplify:
Rf = 0.75 (or 3/4 )
💡 Key Knowledge: Rf Rules
- No units: Rf is a ratio of two distances, so it has no units.
- Range check: An Rf value must always be between 0 and 1. If you get a value greater than 1, you divided upside down!
❌ Common Calculation Traps
- Dividing solvent by dye ( 80 ÷ 60 = 1.33 ). Always double-check that your Rf ≤ 1.
- Trying to convert units when both are already in the same unit ( mm ).
Topics
Chemistry · C8: Chemical Analysis
Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Foundation), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.