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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

7.2 Required Practicals

1 Make up a volumetric solution and carry out a simple acid–base titration 2 Measurement of an enthalpy change 3 Investigation of how the rate of a reaction changes with temperature 4 Carry out simple test-tube reactions to identify cations anions 5 Distillation of a product from a reaction 6 Tests for alcohol, aldehyde, alkene and carboxylic acid 7 Measuring the rate of reaction, by an initial rate method, by a continuous monitoring method 8 Measuring the EMF of an electrochemical cell 9 Investigate how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base 10 Preparation of, a pure organic solid and test of its purity, a pure organic liquid 11 Carry out simple test-tube reactions to identify transition metal ions in aqueous solution 12 Separation of species by thin-layer chromatography

Disclaimer:
The exact reagents, reactions, processes and equipment used in practical activities may vary between schools, colleges, and exam board exemplar methods. The essential techniques, skills, and learning objectives remain the same. Always follow the instructions, risk assessments, and safety guidance provided by your teacher or centre.


PRACTICAL 12 – Thin-Layer Chromatography (TLC)

Aim:
To separate and identify the components in various painkiller tablets (aspirin, caffeine, paracetamol, ibuprofen, and a combination tablet like Anadin Extra) using thin-layer chromatography (TLC).

Overview:
Thin-layer chromatography (TLC) separates compounds based on their differing affinities for the mobile phase (solvent) and stationary phase (silica or alumina). Compounds with stronger solvent affinity move further up the plate, while those strongly interacting with the stationary phase move less. The separation is quantified using the Rf value. AQA A-Level Chemistry expression showing Rf value = distance moved by substance divided by distance moved by solvent front.


Apparatus and Materials

Preparation of Samples

  1. Crush each tablet (e.g. aspirin) using pestle and mortar.
  2. Dissolve ~0.1 g of powder in 0.5 cm³ ethanol (7.0 cm³ for caffeine).
  3. Repeat for paracetamol, ibuprofen, and Anadin Extra (mixture).

Thin-Layer Chromatography Procedure

AQA A-Level Chemistry TLC plate with pencil baseline, sample spots marked evenly, applied using capillaries.
  1. Draw a baseline 1 cm from bottom of TLC plate in pencil.
  2. Mark 5 spots evenly along line, one for each sample.
  3. Spot each sample using capillary tube, drying between drops.
  4. Prepare developing chamber with ~10 cm³ ethyl acetate, cover with lid.
  5. Ensure solvent is below baseline, then place plate vertically in chamber.
AQA A-Level Chemistry TLC developing chamber with solvent rising up plate.
  1. Allow solvent to rise to ~1 cm from top, remove plate, mark solvent front.
  2. Dry plate in fume cupboard.
  3. Spray with locating agent (e.g. ninhydrin) or view under UV lamp.
  4. Draw circles around spots lightly in pencil.
AQA A-Level Chemistry TLC plate visualised under UV light or with a locating agent, with spots circled in pencil.
  1. Measure distances and calculate Rf values.
AQA A-Level Chemistry TLC showing measurement of Rf values based on distances travelled by spots relative to solvent front.

Safety Notes