How do we test for halide ions, sulfate ions and carbonate ions?
Describe tests for halide ions using silver nitrate, sulfate ions using barium chloride, and carbonate ions using dilute acid.
A focused answer to WJEC GCSE Chemistry topic 2.2, covering the tests for halide ions with acidified silver nitrate, sulfate ions with acidified barium chloride, and carbonate ions with dilute acid and limewater.
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What this topic is asking
WJEC topic 2.2 wants you to know the standard tests for three anions (negative ions): halide ions (chloride, bromide, iodide) using silver nitrate, sulfate ions using barium chloride, and carbonate ions using dilute acid. Each test gives a clear, identifying result.
Test for halide ions
Halide ions are the ions of the Group 7 elements: chloride (), bromide () and iodide ().
The nitric acid is added first to remove any carbonate ions, which would otherwise form a precipitate and give a false result.
Test for sulfate ions
Again the acid is added first, this time to remove carbonate ions that would also give a white precipitate and confuse the result.
Test for carbonate ions
This is the same gas behaviour you see when an acid reacts with a carbonate, since the carbonate releases carbon dioxide.
Why acidify first
In both the halide and sulfate tests you add an acid before the main reagent. This removes carbonate ions, which would otherwise react to form their own precipitate and make you think a halide or sulfate was present when it was not. Choosing nitric acid for the halide test and hydrochloric acid for the sulfate test avoids adding ions that would interfere.
Combining cation and anion tests
In a full analysis you often need to identify both the positive ion (cation) and the negative ion (anion) in an unknown salt, so these anion tests are used together with the flame tests and sodium hydroxide tests from the previous topic. For example, you might do a flame test to find the metal, then add silver nitrate to find a halide, building up the full name of the salt (such as sodium chloride or potassium bromide). It is important to do the tests in a sensible order and not to mix reagents that would interfere: for instance, you would not add silver nitrate and barium chloride to the same sample, because the chloride from the barium chloride would react with the silver nitrate and give a misleading result. Planning the order of tests carefully is a skill WJEC rewards in longer analysis questions.
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 test for chloride ions, including the reagents added and the result, and state how the result would differ for bromide ions.Show worked answer →
A Unit 2.2 structured question. Reward: add dilute nitric acid then silver nitrate solution to the sample. Chloride ions give a white precipitate (silver chloride). For bromide ions the precipitate is cream (silver bromide) instead. Markers credit nitric acid and silver nitrate, the white precipitate for chloride, and the cream precipitate for bromide. A common error is to forget to acidify with nitric acid first (which removes carbonate ions that would otherwise interfere).
WJEC sample3 marksDescribe how to test a solution for sulfate ions, including the reagents and the expected result.Show worked answer →
A Unit 2.2 method question. Reward: add dilute hydrochloric acid then barium chloride solution to the sample. A white precipitate (barium sulfate) shows that sulfate ions are present. Markers credit hydrochloric acid and barium chloride and the white precipitate. A common slip is to leave out the acid, which is added to remove carbonate ions that would also give a white precipitate.
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)