Flame tests for metal ions, the use of sodium hydroxide to identify metal ions by precipitate colour, and the tests for hydrogen, oxygen, carbon dioxide, chlorine and ammonia.

A focused answer to OCR Gateway GCSE Chemistry A topic C4.2 on identifying substances, covering flame tests for metal ions, identifying metal ions using sodium hydroxide precipitates, and the tests for hydrogen, oxygen, carbon dioxide, chlorine and ammonia.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

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What this dot point is asking
Flame tests for metal ions
Identifying metal ions with sodium hydroxide
Tests for gases

What this dot point is asking

OCR wants you to identify metal ions by flame tests and by sodium hydroxide precipitates, and to carry out the chemical tests for gases: hydrogen, oxygen, carbon dioxide, chlorine and ammonia. These are core qualitative analysis skills, assessed in the C7 practicals and the written papers.

Flame tests for metal ions

The wire must be cleaned between tests so colours from a previous sample do not interfere, and a strong sodium (yellow) colour can mask others.

Identifying metal ions with sodium hydroxide

For example, copper(II) sulfate solution plus sodium hydroxide gives a blue precipitate of copper(II) hydroxide: $\text{Cu}^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq}) \rightarrow \text{Cu(OH)}_2(\text{s})$ . The precipitate forms because the metal hydroxide is insoluble in water: the metal ion and the hydroxide ion join to make a solid that drops out of solution. The number of hydroxide ions in the formula matches the charge on the metal ion, so a $2+$ ion combines with two hydroxide ions and a $3+$ ion (such as iron(III) or aluminium) combines with three.

The white precipitates of calcium, magnesium and aluminium look the same at first, so they are distinguished by adding excess sodium hydroxide: the aluminium hydroxide precipitate redissolves in excess to give a colourless solution, while the calcium and magnesium precipitates do not. A flame test can then separate calcium (orange-red flame) from magnesium (no colour).

Tests for gases

Take care to use the right splint: a lighted splint for hydrogen (it burns) and a glowing splint for oxygen (it relights).

The ammonia test links to a useful ion test: warming a solution that contains ammonium ions ( $\text{NH}_4^+$ ) with sodium hydroxide releases ammonia gas, which is identified because it turns damp red litmus paper blue. So the ammonia test doubles as the test for ammonium ions. Ammonia also has a sharp, choking smell, but smelling gases is not a reliable or safe identification method, so the damp red litmus test is used.

Identifying an unknown solution

An unknown solution gives a yellow flame test and forms a brown precipitate with sodium hydroxide solution. Identify the metal ions present, explaining each step.

step 1: Interpret the flame test

A yellow flame indicates sodium ions ( $\text{Na}^+$ ) are present. So the solution contains a sodium compound.

step 2: Interpret the sodium hydroxide test

A brown precipitate with sodium hydroxide indicates iron(III) ions ( $\text{Fe}^{3+}$ ) are present, because iron(III) hydroxide is brown.

step 3: Reconcile the two results

The flame test showing sodium and the precipitate showing iron(III) suggests the solution contains both ions, for example a mixture, or the dominant sodium flame is masking the iron. The brown precipitate is the decisive result for iron(III).

step 4: State the conclusion

The solution contains iron(III) ions (from the brown precipitate), and sodium ions are also present (from the yellow flame). To confirm iron(III) alone, the test could be repeated checking the precipitate colour carefully.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20184 marksA student carries out a flame test on four metal compounds. Describe how to carry out a flame test, and give the flame colour you would expect for lithium, sodium, potassium and copper ions.

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A C4.2 practical question. Reward the method: dip a clean (nichrome) wire loop in concentrated hydrochloric acid to clean it, dip it into the sample, then hold it in the edge of a blue (roaring) Bunsen flame and observe the colour. The expected colours: lithium gives a crimson (red) flame, sodium gives a yellow flame, potassium gives a lilac (purple) flame, and copper gives a green flame. Markers credit a clear method (clean wire, dip in sample, hold in flame) and the four correct colours: crimson lithium, yellow sodium, lilac potassium, green copper. A common error is to confuse the lithium and potassium colours.

OCR 20214 marksDescribe the chemical tests, including the result, for hydrogen gas, oxygen gas and carbon dioxide gas.

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A Higher tier recall question on gas tests. Reward: for hydrogen, hold a lighted (burning) splint at the mouth of the tube; hydrogen burns with a squeaky pop. For oxygen, insert a glowing splint into the gas; oxygen relights the glowing splint. For carbon dioxide, bubble the gas through limewater (calcium hydroxide solution); carbon dioxide turns the limewater cloudy (milky). Markers credit the correct test and result for each gas: squeaky pop with a lighted splint for hydrogen, relighting a glowing splint for oxygen, and cloudy limewater for carbon dioxide. A common slip is to mix up the lighted and glowing splint tests.

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Sources & how we know this

OCR GCSE (9-1) Gateway Science Suite Chemistry A (J248) specification — OCR (2016)