GCSE Chemistry Chemical Analysis Notes
Analysis lets chemists identify substances and check purity. Techniques range from chromatography and Rf values to simple gas and ion tests.
Key concepts
- Pure substance
- A single element or compound with a sharp, fixed melting/boiling point.
- Formulation
- A useful mixture with each component present in a measured quantity (e.g. paints, fuels, medicines).
- Chromatography
- Separates a mixture by how strongly components interact with the stationary vs mobile phases.
- Rf value
- Distance moved by spot ÷ distance moved by solvent (a value between 0 and 1).
- Hydrogen: burning splint pops. Oxygen: relights a glowing splint.
- Carbon dioxide: turns limewater milky. Chlorine: bleaches damp litmus paper.
- Halide tests: silver nitrate with dilute nitric acid — white (Cl⁻), cream (Br⁻), yellow (I⁻).
Key formulas
Rf value
Rf = distance moved by substance ÷ distance moved by solvent
Worked example
A spot moves 6.0 cm; the solvent front moves 8.0 cm. Calculate the Rf value.
- 1Rf = distance by spot ÷ distance by solvent.
- 2Rf = 6.0 ÷ 8.0.
- 3Rf = 0.75.
Answer: Rf = 0.75
Common mistakes
✗ Rf values greater than 1.
✓ Rf is always between 0 and 1 — check numerator and denominator.
✗ Drawing the start line in pen for chromatography.
✓ Use pencil — pen ink dissolves in the solvent and interferes.
✗ Missing that a pure substance melts at one temperature.
✓ Impure substances melt over a range, not sharply.
Quick check
Try these, then reveal the answers.
Test for hydrogen gas?Reveal answer
Lit splint gives a squeaky pop.
Colour of silver iodide precipitate?Reveal answer
Yellow.
What does a spot with Rf = 0 tell you?Reveal answer
It did not move — it is very strongly attracted to the paper (stationary phase).
Exam tips
- State the observation before the conclusion in test questions.
- Show the Rf calculation clearly and give the value with no units.
- For chromatography, mention 'pencil baseline' and 'solvent below the spots'.
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Frequently asked questions
What is the difference between a pure substance and a formulation?+
A pure substance has one component; a formulation is a mixture designed with precise quantities of each component for a specific use.
How can chromatography identify unknowns?+
By comparing Rf values (or spot heights) of unknowns with those of known reference samples run alongside.
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