How do we identify ions and gases in the laboratory?
Qualitative analysis: 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.
A CCEA GCSE Chemistry answer on qualitative analysis, covering flame tests and sodium hydroxide precipitate tests for metal ions, tests for halide, sulfate and carbonate ions, and the laboratory tests for hydrogen, oxygen, carbon dioxide and chlorine gases.
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What this dot point is asking
CCEA wants you to carry out and describe the standard qualitative tests: flame tests and sodium hydroxide tests for metal ions, tests for halide, sulfate and carbonate ions, and the gas tests for hydrogen, oxygen, carbon dioxide and chlorine, giving the method and the positive result each time.
Flame tests for metal ions
The wire is cleaned with acid first so a previous sample does not give a false colour. Flame tests work because heated metal ions emit light of a characteristic colour.
Sodium hydroxide tests for metal ions
Adding sodium hydroxide solution to a metal salt forms an insoluble metal hydroxide, and the colour identifies the metal ion:
- Copper(II): blue precipitate.
- Iron(II): green precipitate.
- Iron(III): orange-brown precipitate.
- Calcium, aluminium, magnesium: white precipitate (aluminium hydroxide redissolves in excess sodium hydroxide).
Tests for negative ions
The acid is added first to remove carbonate ions, which would otherwise react and give a misleading result.
Tests for gases
Worked example
Examples in context
- Example 1. Fireworks colours
- The colours in fireworks come from metal compounds emitting light when heated, exactly as in a flame test: lithium and strontium for red, copper for green, sodium for yellow. The flame-test chemistry studied here is what gives a fireworks display its palette.
- Example 2. Checking pool and tap water
- Water companies test for chloride and sulfate levels using silver nitrate and barium chloride to monitor quality. The same precipitate tests used in the lab underpin routine water analysis.
- Example 3. Confirming a gas in industry
- Limewater turning milky is used well beyond the classroom: it confirms carbon dioxide in fermenters, in checks on furnace gases, and in demonstrations of respiration. The simple, reliable colour change makes it the standard confirmatory test wherever carbon dioxide must be identified.
Building a systematic analysis
Real qualitative analysis combines several tests, because one result is rarely enough on its own. A flame test names the metal, a sodium hydroxide test can confirm it through the colour of the precipitate, and an anion test then identifies the negative ion. Working through the tests in a sensible order, and adding acid before the silver nitrate or barium chloride tests to remove interfering carbonate, lets you build up the full identity of an unknown salt step by step. CCEA questions often supply two or three observations and expect you to combine them, exactly as in the worked example, rather than relying on a single test.
Try this
Q1. State the test and positive result for oxygen gas. [2 marks]
- Cue. A glowing splint relights.
Q2. State the flame colour for potassium ions. [1 mark]
- Cue. Lilac.
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 20184 marksDescribe the test, and the positive result, for (a) carbon dioxide gas and (b) chloride ions in solution.Show worked answer →
Markers want both the method and the observation for each.
(a) Carbon dioxide: bubble the gas through limewater (calcium hydroxide solution). A positive result is that the limewater turns milky (cloudy white).
(b) Chloride ions: add dilute nitric acid then silver nitrate solution. A positive result is a white precipitate (silver chloride). The nitric acid removes carbonate ions that would otherwise give a false result.
Markers reward limewater turning milky for carbon dioxide, and silver nitrate (with nitric acid) giving a white precipitate for chloride.
CCEA 20214 marksA solution gives a lilac flame test and forms a white precipitate with sodium hydroxide that does not dissolve in excess. Identify the likely metal ion, and describe the test for hydrogen gas.Show worked answer →
The marks are for the flame colour link and the hydrogen test.
A lilac flame test colour indicates potassium ions (). (A flame test colour identifies the metal: lilac is potassium.)
For hydrogen gas: hold a lit splint at the mouth of the test tube. A positive result is a squeaky pop as the hydrogen burns.
Markers reward potassium from the lilac flame, and the lit splint giving a squeaky pop for hydrogen.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)