How do we test for common gases and ions?
Tests for hydrogen, oxygen, carbon dioxide and chlorine; flame tests for metal ions; tests for carbonates, halides and sulfates; and instrumental methods.
A focused answer to AQA GCSE Chemistry 4.8.1, covering the tests for hydrogen, oxygen, carbon dioxide and chlorine, flame tests for metal ions, tests for carbonates, halides and sulfates, and the advantages of instrumental methods.
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What this dot point is asking
AQA wants you to recall the tests for hydrogen, oxygen, carbon dioxide and chlorine, describe flame tests and tests for metal ions with sodium hydroxide, describe the tests for carbonate, halide and sulfate ions, and explain the advantages of instrumental methods. These tests are pure recall, so the exam reward comes from precise reagents and precise observations. The flame-emission and instrumental detail is partly Higher tier.
Tests for gases
Flame tests for metal ions
The sample is held on a clean wire loop in a blue Bunsen flame; the wire must be cleaned between tests so one colour does not mask another. Adding sodium hydroxide solution to a solution gives coloured precipitates for some metal ions: copper(II) blue, iron(II) green, and iron(III) brown. These precipitates are the metal hydroxides.
Tests for negative ions
- Carbonates: add dilute acid; they fizz, giving carbon dioxide that turns limewater milky.
- Halides (chloride, bromide, iodide): add dilute nitric acid then silver nitrate: chloride gives a white precipitate, bromide cream, iodide yellow.
- Sulfates: add dilute hydrochloric acid then barium chloride: a white precipitate (barium sulfate) forms.
In both the halide and sulfate tests, the dilute acid is added first to react away any carbonate, which would otherwise also form a precipitate and give a false result.
Instrumental methods
In flame emission spectroscopy, a sample is heated and the light it gives out is passed to a spectroscope; the pattern of lines (a line spectrum) identifies the metal ions present and their concentration, which a simple flame test cannot do.
Try this
Q1. Describe the test and result for carbon dioxide. [2 marks]
- Cue. Bubble through limewater; it turns milky (cloudy).
Q2. State the colour of the precipitate when silver nitrate is added to a chloride solution. [1 mark]
- Cue. White.
Q3. State the flame colour given by copper ions. [1 mark]
- Cue. Green.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksA white solid is thought to contain sodium ions and sulfate ions. Describe a test you could carry out to confirm the presence of each ion, including the reagents used and the positive result you would expect.Show worked answer →
A 4-mark question on confirming a metal ion and a negative ion.
Sodium ions (2 marks): carry out a flame test by dipping a clean wire (or nichrome loop) in the sample and holding it in a blue Bunsen flame; a yellow flame confirms sodium ions. Sulfate ions (2 marks): add a few drops of dilute hydrochloric acid, then add barium chloride solution; a white precipitate (of barium sulfate) confirms sulfate ions.
Markers reward naming the reagents and the correct positive result for each ion; the dilute acid before barium chloride is needed to remove carbonate interference.
AQA 20213 marksCompare chemical tests for ions with instrumental methods of analysis such as flame emission spectroscopy. Give two advantages of instrumental methods, and state one situation where they are especially useful.Show worked answer →
A 3-mark comparison question.
Advantages (2 marks, one each from): instrumental methods are faster; they are more sensitive (can detect very small amounts); they are more accurate; they can analyse mixtures of ions at once. Useful situation (1 mark): when the sample is very small, or when a mixture of several ions must be identified at once (for example forensic or environmental samples).
Markers reward two distinct advantages and a sensible situation, not just repeating "they are better".
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)