Chemical analysis: GCSE Chemistry
Purity, formulations, chromatography, gas tests, flame tests, positive-ion tests and instruments
The short version
R_f = distance moved by spot ÷ distance moved by solvent. Gas tests: hydrogen squeaky pop, oxygen relights a glowing splint, carbon dioxide turns limewater milky.
Key idea
Everyday bottled water is potable / a mixture — not chemically pure H2O
Grade 2–3: Pure means one chemical. Paint and medicine are mixes made to a recipe.
Grade 5–6: I can name paints, alloys, medicines and fuels as formulations.
Grade 9: Food purity in everyday language is not the chemical definition. Chromatography (next) can show one spot vs many. Alloys are formulations again in Using resources.
Go a bit deeper
Both distances from the baseline. Measure to the centre of the spot. No units. Always ≤ 1
Grade 2–3: Draw a pencil line, dab the colour, stand the paper in a little water, watch the colours separate.
Grade 5–6: I can calculate R_f and I know one spot suggests a single substance in that solvent.
Grade 9: Mobile phase is the solvent; stationary phase is the paper. Different attractions to paper vs solvent. If two substances share an R_f, try another solvent. A 6-mark method is the sequence above.
Worked example
Worked example
Grade 2–3: Pure means one chemical. Paint and medicine are mixes made to a recipe. Grade 5–6: I can name paints, alloys, medicines and fuels as formulations. Grade 9: Food purity in everyday language is not the chemical definition. Chromatography (next) can show one spot vs many. Alloys are formulations again in Using resources.
Lessons in this topic
- Purity and formulations
- Chromatography (required practical)
- Gas tests
- Flame tests (Combined)
- Positive ion tests (Combined)
- Instrumental methods (mention)
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
Flame tests: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green. NaOH precipitates identify Cu²⁺, Fe²⁺, Fe³⁺, Ca²⁺, Mg²⁺ and Al³⁺.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
A chemically pure substance has
A formulation is
The baseline on chromatography paper should be drawn in
R_f =
Common mistakes
- Bottled water is not chemically pure H2O — it is a mixture that is safe to drink (potable). Do not write mix until it looks right for a formulation.
- Ink baseline dissolves and ruins the chromatogram. Always use pencil. Never use pen. Do not invert R_f (solvent ÷ spot).
- A flaming splint in oxygen just burns more brightly — the official test is a glowing splint. Damp litmus for chlorine.
- A dirty wire that last tested sodium will give a yellow flame for everything. Sodium colour also masks others.
Answers and working
Frequently asked questions
Is GCSE Chemical analysis on Combined Science and Triple?
Chemical analysis is on Combined Science and on Triple Chemistry. Progress is shared in the ReviseEasy app.
Which exam board is this Chemical analysis page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Chemical analysis Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
Learn Chemical analysis on ReviseEasy →Related revision
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