AQA A-Level Chemistry: Chromatography (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.3.16, Chromatography, of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams from June 2016 onwards. The section is marked “A-level only”, so none of it is AS content. It is organic chemistry content, listed under A-level Paper 2, and Paper 3 can assess any content and any practical skill, including Required practical 12.

Before you start, you may want the study guide or the revision notes. The AQA A-Level Chemistry hub and printable checklist cover the rest of the course, and the free diagnostics show where to focus.

Questions

1. For each of thin-layer chromatography, column chromatography and gas chromatography, describe the stationary phase and state the moving phase. [3]

2. On a TLC plate the solvent front is 7.80 cm above the baseline.

(a) Two spots have centres 2.34 cm and 5.46 cm above the baseline. Calculate the Rf value of each. [2] (b) A third substance has an Rf value of 0.45 in the same system. Calculate how far its spot moved from the baseline. [1]

3. Explain, in terms of the two phases, why chromatography can separate two different substances in a mixture. [2]

4. A mixture of compound X and compound Y is run on a TLC plate with a polar stationary phase and a solvent that is much less polar. X contains an –OH group. Y is a hydrocarbon.

(a) Deduce which compound has the larger Rf value. Explain your answer. [3] (b) Suggest how the Rf value of X would change if a much more polar solvent were used. [1]

5. A student runs a TLC plate of an unknown mixture alongside three pure standards, P, Q and R. The solvent front is 6.00 cm above the baseline.

Spot	Distance from baseline / cm
Standard P	1.50
Standard Q	4.20
Standard R	3.30
Mixture, spot 1	1.50
Mixture, spot 2	2.70
Mixture, spot 3	4.20

(a) Calculate the Rf value of each spot from the mixture. [2] (b) Deduce which standards are present in the mixture, and state what you can conclude about spot 2. [2] (c) Explain why a matching Rf value does not prove identity, and suggest how the student could strengthen the identification. [2]

6. Describe how column chromatography separates a mixture of two compounds and how each compound can be obtained separately. [4]

7. This question is about gas chromatography.

(a) Describe the stationary and moving phases, and the conditions in the column. [2] (b) Define the term retention time. [1] (c) The column temperature is raised. State and explain the effect on retention times. [2] (d) A peak from a sample has the same retention time as a standard run under identical conditions. State what this suggests and why it is not proof. [2]

8. A mixture is analysed by GC-MS. One component gives a molecular ion at m/z = 102.0678 in a high-resolution mass spectrometer.

Use H = 1.0078, C = 12.0000, N = 14.0031 and O = 15.9949.

(a) Explain why the mixture is passed through a GC column before it enters the mass spectrometer. [1] (b) Show which of C₅H₁₀O₂, C₆H₁₄O and C₄H₁₀N₂O is the molecular formula of this component. [3]

9. A student wrote this method for Required practical 12.

“Draw a line in ink 1 cm from the bottom of the plate. Spot the samples on the line. Pour solvent into a beaker to a depth of 2 cm and stand the plate in it. Leave the beaker open. When the solvent is near the top, remove the plate and leave it to dry overnight, then mark the solvent front.”

(a) Identify four errors in the method. For each, explain the problem it causes and give the correction. [8] (b) State one safety precaution needed because of the solvent. [1]

10. A chemist follows a reaction by TLC, spotting the starting material, pure product and the reaction mixture on one plate. The solvent front is 8.40 cm above the baseline.

Spot	Distance from baseline / cm
Starting material	5.88
Pure product	2.52
Reaction mixture after 1 hour	5.88 and 2.52
Reaction mixture after 3 hours	2.52 only

(a) Calculate the Rf values of the starting material and the product. [2] (b) State what the two reaction-mixture lanes show. [2] (c) The stationary phase is polar. Deduce whether the starting material or the product is more strongly retained, and explain what this suggests about their polarities. [2] (d) The chemist purifies the product by column chromatography with the same stationary phase and solvent. Predict which compound leaves the column first, and explain. [2]

11. A student uses TLC to identify the transition metal ions in a solution. Suggest why some spots can be seen without ultraviolet light or a developing agent, and state how the ions are identified. [2]

Answers

1. TLC: a plate coated with a solid; moving phase is a solvent (liquid) [1]. Column: a column packed with a solid; moving phase is a solvent [1]. GC: a column packed with a solid or a solid coated by a liquid; moving phase is a gas [1]. [3] Examiner insight: Each mark needs both phases for that technique; a liquid GC moving phase loses that mark.

2. (a) 2.34 / 7.80 = 0.30 [1]; 5.46 / 7.80 = 0.70 [1] (b) 0.45 × 7.80 = 3.51 cm [1] Examiner insight: An Rf with units, or a value above 1 from dividing the wrong way round, scores nothing; in (b) the unit cm is needed.

3. Each substance has a different balance between solubility in the moving phase and retention by the stationary phase [1]. So the substances move through or along the stationary phase at different rates and are separated [1]. [2] Examiner insight: An answer that mentions only one phase gets at most one mark.

4. (a) Y has the larger Rf value [1]. X can form hydrogen bonds with the polar stationary phase, so it is retained more strongly [1]. Y is held only weakly and is more soluble in the less polar solvent, so it moves further [1] (b) X’s Rf value increases, because X is more soluble in a polar moving phase [1] Examiner insight: The explanation marks in (a) need a comparison of X and Y, not a description of one compound alone.

5. (a) Spot 1: 1.50 / 6.00 = 0.25; spot 3: 4.20 / 6.00 = 0.70 [1]; spot 2: 2.70 / 6.00 = 0.45 [1] (b) Spots 1 and 3 match, so P and Q are present [1]. Spot 2 matches no standard, so the mixture also contains a substance that is not P, Q or R [1] (c) Two different substances can share an Rf value in one solvent [1]. Rerun mixture and standards together in a different solvent (or use GC-MS) [1] Examiner insight: Allow error carried forward in (b) from wrong Rf values in (a), as long as the matching logic is right.

6. The column is packed with a solid (stationary phase) and the mixture is added at the top [1]. Solvent is run down through the column [1]. The compound with less retention by the solid and greater solubility in the solvent moves down faster and leaves the bottom first [1]. Collect the liquid leaving the column in separate fractions, so each compound is in a different container [1]. [4] Examiner insight: “Solvent moves up the column” loses that mark.

7. (a) Stationary phase: a solid, or a solid coated by a liquid, packed in the column [1]. Moving phase: a gas, passed through under pressure at high temperature [1] (b) The time from injection of the sample until the component reaches the detector [1] (c) Retention times decrease [1]; the components spend more time in the gas phase, so they are carried through faster [1] (d) The sample probably contains that substance [1]; a different substance could have the same retention time on this column [1] Examiner insight: In (b), “the time it takes to come out” with no starting point is too vague.

8. (a) GC separates the components, so they enter the mass spectrometer one at a time and each gives its own spectrum [1] (b)

C5H10O2  : 5(12.0000) + 10(1.0078) + 2(15.9949)            = 102.0678
C6H14O   : 6(12.0000) + 14(1.0078) + 15.9949               = 102.1041
C4H10N2O : 4(12.0000) + 10(1.0078) + 2(14.0031) + 15.9949  = 102.0791

Correct method for precise masses [1]; all three values correct [1]; only C₅H₁₀O₂ matches 102.0678, so the formula is C₅H₁₀O₂ [1] Examiner insight: A correct formula chosen without any calculation scores only the final mark at best; the working is the evidence.

9. (a) Ink dissolves and separates, giving extra spots [1]; use pencil [1]. Solvent at 2 cm covers the 1 cm baseline, so samples dissolve into the solvent [1]; keep solvent below the baseline [1]. Open beaker: solvent evaporates from the plate, so the front rises unevenly [1]; use a lid to saturate the air with vapour [1]. After drying the front cannot be seen, so Rf cannot be calculated [1]; mark the front as soon as the plate is removed [1] (b) No naked flames (flammable solvent), or a fume cupboard if the vapour is harmful [1] Examiner insight: For each error, the problem and the correction are separate marks; a correction with no reason scores only one of the two.

10. (a) Starting material: 5.88 / 8.40 = 0.70 [1]; product: 2.52 / 8.40 = 0.30 [1] (b) After 1 hour both are present: the reaction is incomplete [1]. After 3 hours no starting material is detected: it has been used up [1] (c) The product has the smaller Rf, so it is more strongly retained by the polar stationary phase [1]; this suggests the product is more polar than the starting material [1] (d) The starting material leaves first [1]: it is less strongly retained and more soluble in the solvent, so it moves down faster [1] Examiner insight: In (b), “the reaction has finished” needs evidence: link it to the disappearance of the starting-material spot.

11. Many transition metal ions or complexes are coloured, so their spots are visible [1]. Identify each ion by comparing its Rf value with standards of known ions run on the same plate [1]. [2] Examiner insight: Rf values must be compared with standards run under the same conditions, not with values from elsewhere.

Where marks are usually lost

• Measuring from the plate edge instead of the baseline, or to the top of the spot instead of its centre.
• Explaining separation with only one phase instead of the balance between both.
• Saying the solvent moves up a column, or down a TLC plate.
• Defining retention time without saying it starts at injection.
• Claiming a matching Rf value or retention time proves identity.
• Picking a molecular formula from a high-resolution m/z value without showing the precise-mass sums.

Next steps

• Recap with the revision notes.
• Re-read any weak section in the study guide.
• More precise-mass practice: organic analysis questions.
• See the AQA A-Level Chemistry hub and the printable checklist.
• Try all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards (AQA). Section 3.3.16 Chromatography (A-level only), including Required practical 12.