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A-level Chemistry required practicals

Separating species by thin-layer chromatography

AQA 3.3.16 · RP12

A-level Chemistry (7405) · Required practical 12 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.

Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works

Separate mixture components by thin-layer chromatography and use standards and Rf values as comparative evidence.

Apparatus

  • TLC plate with a suitable stationary phase
  • Developing solvent and covered chamber
  • Capillary spotting tubes
  • Pencil, ruler and forceps
  • Known standards and unknown or mixture samples
  • Suitable visualising method and eye protection

Apparatus & techniques (AT)

AT i · paper/TLC

Spots, develops and visualises a TLC plate, then calculates and compares Rf values.

AT k · safe handling

Handles volatile developing solvents, capillaries and visualising agents safely.

Safety

Hazard

Developing solvents may be volatile, harmful and flammable.

Control

Use small volumes in a covered chamber with good ventilation, keep away from ignition sources and replace the lid promptly.

Hazard

Visualising agents or ultraviolet light may be harmful.

Control

Use the specified enclosure and eye/skin protection; never look directly at an unshielded UV source.

Method

  1. 1Draw a light pencil baseline above the intended solvent level and label spotting positions without damaging the stationary phase.
  2. 2Use a capillary to apply small concentrated spots of the sample and standards; let each application dry before adding more at the same position.
  3. 3Stand the plate in a covered developing chamber with the solvent below the baseline and keep the plate upright without touching the sides.
  4. 4Remove the plate before the solvent front reaches the top, mark the front immediately in pencil and allow the plate to dry.
  5. 5Visualise the spots safely, mark their centres and compare the unknown with standards run on the same plate; use co-spotting if overlap needs testing.

CPAC focus (editorial)

This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.

  • CPAC 2: Editorial focus: select a solvent and spotting arrangement that can resolve the components.
  • CPAC 3: Editorial focus: control exposure to volatile solvent and any visualising reagent.
  • CPAC 4: Editorial focus: mark the front promptly and measure spots consistently from the baseline.
  • CPAC 5: Editorial focus: calculate Rf, compare only like conditions and avoid overclaiming identity.

Variables

Independent

Sample identity or mobile-phase composition when that factor is investigated

Dependent

Spot positions and calculated Rf values

Control

  • Stationary phase and plate batch
  • Baseline and solvent depth
  • Development distance, chamber conditions and spotting method

Results & processing

  • Measure from the baseline to each spot centre and from the baseline to the marked solvent front.
  • Calculate Rf = distance moved by spot / distance moved by solvent front; report it without units and between 0 and 1.
  • Use number of spots as evidence about components and compare Rf values only when stationary and mobile phases are the same.

Analysis skills

  • Calculate a unitless Rf from baseline-to-centre and baseline-to-front distances.
  • Interpret number of spots, standard matches and co-spots without treating one Rf as proof of identity.
  • Predict how changed mobile-phase affinity alters travel while recognising stationary-phase retention also matters.

Uncertainty

Sources

  • Finite spot width and choice of its centre
  • Delay or curvature when marking the solvent front
  • Ruler resolution and uneven plate development

Calculations

  • For Rf = spot distance/front distance, estimate percentage uncertainty by combining the percentage distance uncertainties.
  • Convert the percentage uncertainty to an absolute Rf uncertainty when reporting a result.

Interpretation

  • A wider spot makes its centre less certain even with a fine ruler.
  • Rf values from different solvents or stationary phases are not directly comparable.

Exam angles

  • Find and correct chamber, baseline, spotting or solvent-front faults.
  • Calculate Rf and identify the correct measurement points.
  • Explain co-spotting, component count and the effect of changing solvent using both mobile and stationary phases.

Where students lose marks

Explaining a spot position using polarity alone.

Fix: Discuss the balance between solubility in the mobile phase and retention by the stationary phase.

  • Examiner report: C2-17 · PDF p. 5

Placing the solvent above the baseline so samples dissolve directly into the reservoir.

Fix: Start with the solvent surface below every dry sample spot.

Claiming that matching Rf values prove two compounds are identical.

Fix: Treat a match under the same conditions as supporting evidence and combine it with a standard or co-spotting evidence.

Improve the method

  • Apply compact spots in repeated dry layers so bands do not overlap.
  • Keep the chamber covered and allow it to equilibrate so solvent composition and evaporation are more consistent.
  • Run standards and the unknown on the same plate and mark the solvent front immediately.

Source references

  • Specification: CSpec 3.3.16 · PDF p. 70

Try it — exam-style

Medium
ORIGINAL

A spot centre is 43.2 ± 0.5 mm from the baseline and the solvent front is 72.0 ± 0.5 mm from the baseline (each distance is a single ruler reading taken with the scale zeroed on the baseline). Calculate Rf and estimate its percentage and absolute uncertainty by adding percentage uncertainties.

[4 marks]

Total for this question: 4

Easy
ORIGINAL

An unknown and a standard each produce one spot with Rf = 0.42 on the same plate. Explain what can and cannot be concluded, and name one stronger comparison.

[3 marks]

Total for this question: 3

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

AQA 7405 specification coverage

Chromatography, organic analysis and practical interpretation.

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