Flame tests, precipitate tests, gas tests and paper chromatography — every analytical technique for GCSE Chemistry with full results tables.
Chemical analysis is the area of GCSE Chemistry most suited to straightforward memorisation — the tests and their results follow fixed patterns, and once you know them, the marks are reliably available. But analysis questions also include interpretation tasks (what does this result tell us, is the sample pure, what is the Rf value) that require understanding as well as recall. This guide covers everything.
Flame tests identify the metal ion present in a compound. A clean nichrome wire loop is dipped into the sample and held in a Bunsen burner flame. The colour produced identifies the metal ion.
| Metal Ion | Flame Colour |
|---|---|
| Lithium (Li⁺) | Crimson red |
| Sodium (Na⁺) | Bright yellow/orange |
| Potassium (K⁺) | Lilac/violet |
| Calcium (Ca²⁺) | Brick red/orange-red |
| Copper (Cu²⁺) | Blue-green/green |
Sodium produces such an intense yellow flame that it can mask other colours. If sodium is present in a sample alongside another metal, the flame test may not reliably identify the other metal. This limitation is tested directly in exam questions about the reliability of flame tests.
Adding sodium hydroxide (NaOH) solution to a solution of a metal salt produces a precipitate of the metal hydroxide if the metal ion forms an insoluble hydroxide. The colour of the precipitate identifies the metal ion.
| Metal Ion | Precipitate Colour |
|---|---|
| Copper(II) (Cu²⁺) | Blue precipitate |
| Iron(II) (Fe²⁺) | Green precipitate |
| Iron(III) (Fe³⁺) | Orange/brown precipitate |
| Aluminium (Al³⁺) | White precipitate — dissolves in excess NaOH |
| Calcium (Ca²⁺) | White precipitate — does NOT dissolve in excess NaOH |
| Magnesium (Mg²⁺) | White precipitate — does NOT dissolve in excess NaOH |
The key distinction between aluminium and calcium/magnesium: aluminium hydroxide dissolves in excess NaOH (it is amphoteric — it reacts with both acids and alkalis). Calcium and magnesium hydroxides do not dissolve in excess NaOH. Adding excess NaOH is the test that distinguishes these white precipitates.
Add dilute nitric acid (to remove interfering ions), then add silver nitrate solution. The colour of the precipitate identifies the halide.
| Halide Ion | Precipitate with AgNO₃ | Solubility in ammonia |
|---|---|---|
| Chloride (Cl⁻) | White precipitate (AgCl) | Dissolves in dilute ammonia |
| Bromide (Br⁻) | Cream precipitate (AgBr) | Dissolves in concentrated ammonia |
| Iodide (I⁻) | Yellow precipitate (AgI) | Insoluble in ammonia |
Add dilute hydrochloric acid (to remove interfering carbonate ions), then add barium chloride solution. A white precipitate of barium sulfate (BaSO₄) confirms sulfate ions are present. Barium sulfate is insoluble — this makes the test very definitive.
Add dilute acid to the sample. If carbonate ions are present, carbon dioxide is produced (effervescence/fizzing). Pass the gas through limewater — if it turns milky/cloudy, CO₂ is confirmed and carbonate ions were present.
| Gas | Test | Positive Result |
|---|---|---|
| Hydrogen (H₂) | Burning splint held near the gas | Burns with a squeaky pop |
| Oxygen (O₂) | Glowing splint inserted into the gas | Splint relights |
| Carbon dioxide (CO₂) | Bubble through limewater | Limewater turns milky/cloudy |
| Chlorine (Cl₂) | Damp litmus paper held in the gas | Litmus is bleached white |
| Ammonia (NH₃) | Damp red litmus paper held near the gas | Litmus turns blue (ammonia is alkaline) |
The oxygen test uses a glowing splint — not a burning one. A burning splint tests for hydrogen. And chlorine bleaches litmus completely white — it doesn't just turn it blue. These distinctions are tested directly. Also note: limewater turning cloudy confirms CO₂ — but if you keep bubbling CO₂ through for longer, the limewater goes clear again (excess CO₂ reacts with calcium carbonate to form soluble calcium hydrogen carbonate). This is occasionally tested at Higher tier.
Chromatography separates mixtures based on how substances distribute themselves between a mobile phase (the solvent) and a stationary phase (the paper). Substances that are more soluble in the solvent travel further up the paper. Substances that adhere more strongly to the paper travel less far.
The Rf (retention factor) value identifies a substance. It is calculated as:
Rf = distance travelled by substance ÷ distance travelled by solvent front
Rf values are always between 0 and 1. A substance always has the same Rf value in the same solvent under the same conditions — this is what makes Rf useful for identification. By comparing the Rf of an unknown substance to those of known compounds run in the same solvent, you can identify it.
A single spot means the sample contains one substance (or one component that is soluble in the solvent). Multiple spots mean the sample is a mixture. If two spots (from different samples) are at the same height, they may be the same substance. To confirm identity, run a known standard alongside the unknown and compare Rf values.
A pure substance shows a single spot. This is one way chromatography is used to check purity — if you expect a pure substance but get multiple spots, impurities are present.
The AQA chemical analysis specification is at the AQA GCSE Chemistry specification page. Edexcel's is at the Edexcel GCSE Chemistry page.
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