Chromatography & Purity

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CognitoChromatography and Rf values

Key facts & methods

How chromatography works

Separates mixtures based on different solubilities in the mobile and stationary phases.
Stationary phase: does not move — paper or silica (TLC plate).
Mobile phase: moves — solvent travels up the paper/plate by capillary action.
Substances with greater affinity for mobile phase travel further.
Pencil baseline (not ink — ink would move with solvent).
Solvent level BELOW the pencil line — or samples wash away.

Rf values

Rf = distance moved by substance / distance moved by solvent front.
Rf always between 0 and 1 (substance cannot travel further than solvent).
Each substance has a characteristic Rf value in a given solvent — used for identification.
Compare Rf values to known reference compounds run alongside.
A pure substance produces a single spot. A mixture produces multiple spots.

Purity in chemistry

A pure substance contains only one element or compound — not mixed with anything else.
Purity is tested by: melting point (pure substance has sharp, precise melting point; impurities lower and broaden the melting range).
Boiling point: impurities raise the boiling point and broaden the boiling range.
Chromatography: pure substance gives one spot; mixture gives multiple spots.

Formulations

A formulation is a mixture designed to have specific properties, with exact amounts of each component.
Examples: medicines (drug + excipient), paints (pigment + binder + solvent), fuels (additives for performance), alloys.
Each component has a specific purpose — the exact composition matters.
Different from ordinary mixtures where proportions are not critical.

Exam questions — 3 questions · 9 marks · AQA 8462 style

3 marksRf calculationAQA 8462 P1 style 🔢 Calculator

In a chromatography experiment, the solvent front travels 8.0 cm. A substance travels 6.0 cm. (a) Calculate the Rf value. (b) A reference sample of substance X has Rf = 0.90 in the same solvent. Are they the same substance? (3 marks)

Hint: Rf = substance distance / solvent front distance. Compare Rf values — identical Rf in the same solvent suggests same substance.

+30 XP

1 markPure substance testAQA 8462 style

How can chromatography show that a substance is impure?

A The substance travels less than 1 cm up the plate

B More than one spot appears on the chromatogram

C The Rf value is greater than 1

D The solvent front reaches the top of the paper

3 marksFormulation explanationAQA 8462 style

Explain what is meant by a formulation, and give one example. State why the exact proportions of each component matter. (3 marks)

+30 XP

1 markMelting point purityAQA 8462 style

A sample of aspirin has a melting range of 135-142°C. Pure aspirin melts at exactly 138°C. What does this suggest?

A The sample is pure and of high quality

B The sample is impure — impurities lower and broaden the melting range

C The sample is a different substance from aspirin

D The melting point depends on the mass of the sample used

Chromatography & Purity

How chromatography works

Rf values

Purity in chemistry

Formulations

Module complete! 🎉