How do we test for common gases and identify metal ions by flame tests and with sodium hydroxide?
Describe tests for hydrogen, oxygen, carbon dioxide and chlorine, flame tests for metal ions, and tests for metal cations using sodium hydroxide solution.
A focused answer to WJEC GCSE Chemistry topic 2.2, covering the standard tests for hydrogen, oxygen, carbon dioxide and chlorine, flame tests for metal ions, and identifying metal cations by the colour of their hydroxide precipitates with sodium hydroxide.
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What this topic is asking
WJEC topic 2.2 wants you to know the standard tests for gases (hydrogen, oxygen, carbon dioxide, chlorine), the flame test colours for some metal ions, and how to identify metal cations from the colour of the hydroxide precipitate they form with sodium hydroxide solution.
Tests for gases
These four gas tests come up constantly, so learn the test (what you do) and the result (what you see) for each.
Flame tests for metal ions
Some metal ions give a characteristic colour when held in a flame. Clean a wire (often by dipping in acid), dip it in the sample, and hold it in a non-luminous Bunsen flame:
A flame test only works well for one metal ion at a time, because a strong colour (such as sodium's yellow) can mask others.
Tests for cations with sodium hydroxide
Adding sodium hydroxide solution to a solution of a metal salt forms an insoluble metal hydroxide precipitate, and the colour identifies the metal ion:
For example, copper(II) sulfate solution plus sodium hydroxide gives a blue precipitate of copper(II) hydroxide: .
Why these tests work
The cation tests work because the metal hydroxides are insoluble and each has its own colour. When you add sodium hydroxide, the hydroxide ions react with the metal ions to make an insoluble coloured solid that drops out of solution, so the colour tells you which metal ion was there. The flame tests work for a different reason: when a metal ion is heated, its electrons absorb energy and jump to higher shells, then release that energy as light of a particular colour as they fall back. Because each metal has a different electron arrangement, each gives a different flame colour. Understanding the reason behind a test, not just the result, helps you answer the "explain" versions of these questions and avoid mixing the colours up under exam pressure.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample4 marksDescribe the chemical tests for hydrogen gas and for carbon dioxide gas, including the result of each test.Show worked answer →
A Unit 2.2 structured question. Reward: for hydrogen, hold a lighted splint at the mouth of the test tube; the gas burns with a squeaky pop. For carbon dioxide, bubble the gas through limewater; the limewater turns cloudy (milky). Markers credit the lighted splint and squeaky pop for hydrogen, and bubbling through limewater turning cloudy for carbon dioxide. A common error is to swap the tests or to give the wrong observation.
WJEC sample3 marksA solution forms a blue precipitate when sodium hydroxide solution is added. Identify the metal ion present and write the colour you would expect for an iron(III) solution instead.Show worked answer →
A Unit 2.2 identification question. Reward: a blue precipitate with sodium hydroxide indicates copper(II) ions, . An iron(III) solution would instead give a brown (orange-brown) precipitate. Markers credit copper(II) for the blue precipitate and brown for iron(III). A common slip is to confuse the iron(II) green precipitate with the iron(III) brown one.
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)