How do we test for different ions and gases in the laboratory?
Flame tests and sodium hydroxide tests for metal ions, tests for halide, sulfate and carbonate ions, and the tests for hydrogen, oxygen, carbon dioxide and chlorine gases.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on qualitative analysis, covering flame tests and sodium hydroxide tests for metal ions, tests for halide, sulfate and carbonate ions, and the tests for hydrogen, oxygen, carbon dioxide and chlorine.
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What this dot point is asking
CCEA Double Award wants the standard laboratory tests: flame tests and sodium hydroxide tests for metal ions, tests for halide, sulfate and carbonate ions, and the four gas tests. These are pure recall marks if you learn the colours and results.
Flame tests for metal ions
The colour comes from the metal ion, so flame tests identify which metal is present.
Sodium hydroxide tests
Adding sodium hydroxide solution to a metal ion in solution can form a coloured precipitate (an insoluble solid). For example, copper(II) ions give a blue precipitate, iron(II) ions give a green precipitate, and iron(III) ions give an orange-brown precipitate. The colour identifies the metal ion.
This works because the metal ions react with the hydroxide ions to form an insoluble metal hydroxide. The reaction is useful when a flame test alone is not enough, and it gives a clear, named colour for each ion. Some metal hydroxides are white (such as calcium, magnesium and aluminium), so flame tests are used to tell those apart instead.
Tests for negative ions
Tests for gases
Examples in context
- Example 1. Fireworks colours
- The colours in fireworks come from metal salts - sodium for yellow, copper for green, lithium for red. This is the flame test put to spectacular use, and it shows the colour depends on the metal ion.
- Example 2. Checking for a sulfate in water
- Adding acid then barium chloride to a water sample and seeing a white precipitate confirms a sulfate is present. This is a routine analytical test, showing the negative-ion tests applied to real samples.
- Example 3. Identifying a salt from two tests
- Qualitative analysis usually combines a test for the metal ion (flame test or sodium hydroxide) with a test for the negative ion (silver nitrate, barium chloride or acid). Together the two results name the salt - for example a lilac flame plus a yellow silver nitrate precipitate identifies potassium iodide. Using two tests avoids the mistakes that one test alone can cause.
Try this
Q1. What is the flame test colour for potassium? [1 mark]
- Cue. Lilac.
Q2. How do you test for carbon dioxide? [1 mark]
- Cue. It turns limewater milky.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style4 marksDescribe the flame test colours for sodium, potassium, calcium and copper ions.Show worked answer →
Match each ion to its colour for four marks.
Sodium burns with a yellow/orange flame.
Potassium burns with a lilac flame.
Calcium burns with a brick-red (orange-red) flame.
Copper burns with a green/blue-green flame. Markers reward each ion paired with its correct colour.
CCEA-style3 marksDescribe the tests for hydrogen, oxygen and carbon dioxide gases.Show worked answer →
One test per gas for three marks.
Hydrogen: a lit splint gives a squeaky pop.
Oxygen: a glowing splint relights.
Carbon dioxide: it turns limewater milky (cloudy). Markers want the correct test and result for each gas.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)