AQA A Level Chemistry
AQA A Level Chemistry 3.3.16 Chromatography Exam Questions
Free AQA A Level Chemistry exam questions for 3.3.16 Chromatography, with a mark scheme for every pack. Covers stationary and mobile phases, thin-layer chromatography and Rf values, column chromatography and gas chromatography with retention times.
3.3.16 Chromatography exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.3.16 Chromatography
Chromatography is a set of separation techniques used to identify and purify substances. On the AQA A Level course you need to explain separation in terms of a stationary phase and a mobile phase, carry out thin-layer chromatography and calculate Rf values, and describe column chromatography and gas chromatography, including the use of retention times and combination with mass spectrometry. Required practical 12 is the separation of species by thin-layer chromatography. The topic links to 3.3.13 for identifying amino acids and to 3.3.6 and 3.3.15 for identifying unknown compounds. Examiners test experimental details, Rf calculations and explanations of why components separate.
What you need to know
- Principle All chromatography uses a stationary phase and a mobile phase. Components separate because they have different relative affinities for the two phases, such as different adsorption to the stationary phase or solubility in the mobile phase.
- Thin-layer chromatography The stationary phase is a thin layer of silica or alumina on a plate; the mobile phase is a solvent. Spots are made colourless compounds visible with UV light or a locating agent such as iodine or ninhydrin.
- Rf values Rf = distance moved by the spot / distance moved by the solvent front. Rf values depend on the solvent and stationary phase, so compare with a known sample run on the same plate.
- Column chromatography A column is packed with a solid stationary phase, such as silica or alumina, and the solvent carries the mixture down. Components leave at different times and can be collected separately, so this method purifies substances.
- Gas chromatography A gas mobile phase carries a vaporised sample through a column containing a liquid or solid stationary phase. Each component has a characteristic retention time, and the peak area shows the relative amount. GC can be combined with mass spectrometry to identify each component.
- Required practical 12 Draw the start line in pencil, keep it above the solvent level, cover the tank to saturate the atmosphere with solvent vapour, mark the solvent front immediately, and handle the plate by the edges.
Common mistakes in 3.3.16 Chromatography
- Drawing the start line in ink. Ink would dissolve and run; use pencil.
- Placing the start line below the solvent level, so the samples dissolve into the solvent instead of moving up the plate.
- Measuring to the top or bottom of a spot. Measure from the start line to the centre of the spot.
- Assuming the same Rf in different solvents. Rf depends on the conditions, so run standards alongside the unknown.
- Saying components separate because they are 'heavier'. Separation depends on relative affinity for the stationary and mobile phases.
Try three 3.3.16 Chromatography exam-style questions
1. A spot moved 2.4 cm from the start line and the solvent front moved 6.0 cm. Calculate the Rf value. (1 mark)
Show the mark scheme answer
Rf = 2.4 / 6.0 = 0.40.
2. Explain why the start line in TLC is drawn in pencil and above the level of the solvent. (2 marks)
Show the mark scheme answer
Pencil (graphite) does not dissolve in the solvent, whereas ink would separate and give extra spots; if the line is below the solvent, the samples would dissolve into the solvent in the tank instead of moving up the plate.
3. Explain how gas chromatography separates the components of a mixture and how the components are identified. (3 marks)
Show the mark scheme answer
The components have different affinities for (solubility in) the stationary phase compared with the gas mobile phase; so they travel through the column at different rates and have different retention times; components are identified by comparing retention times with known samples under the same conditions, or by passing them into a mass spectrometer.
3.3.16 Chromatography FAQs
What does AQA A Level Chemistry 3.3.16 Chromatography cover?
It covers the principles of separation using stationary and mobile phases, thin-layer chromatography and Rf values, column chromatography and gas chromatography. It includes Required practical 12.
Which paper is 3.3.16 Chromatography examined in?
It is examined in Paper 2, and Paper 3 can also include it through practical questions.
What practical details do I need to remember for TLC?
Pencil start line above the solvent, a lid on the tank, mark the solvent front straight away, avoid touching the plate surface, and use UV or a locating agent to see colourless spots. Each of these can be asked as a separate mark.
