Tests for Anions (Carbonates, Halides, Sulfates)
Get to grips with Tests for Anions (Carbonates, Halides, Sulfates) for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. A separate (triple) required practical: test for carbonates (fizzing), halides (silver nitrate) and sulfates (barium chloride).
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Topic overview
Three types of negative ion, or anion, can be identified by specific tests. This is a Triple-only required practical.
Carbonates: add dilute acid. The mixture fizzes and the gas produced turns limewater milky, showing carbon dioxide. Halides: add dilute nitric acid then silver nitrate solution. Chloride gives a white precipitate, bromide a cream one, and iodide a yellow one.
Sulfates: add dilute hydrochloric acid then barium chloride solution. A white precipitate forms. The acid is added first in both the halide and sulfate tests to remove any carbonate ions, which would otherwise form a precipitate and give a false positive.
Revision notes
Testing for carbonates
Add dilute acid to the sample.
If carbonate ions are present, the mixture fizzes and carbon dioxide is produced. Bubbling this gas through limewater turns it milky, confirming the result.
Testing for halides
Add dilute nitric acid, then silver nitrate solution.
Chloride gives a white precipitate, bromide gives a cream precipitate, and iodide gives a yellow precipitate. The three colours must be learned in that order.
Testing for sulfates and why acid is added first
Add dilute hydrochloric acid, then barium chloride solution. A white precipitate confirms sulfate ions.
The acid is added first in both tests to remove any carbonate ions present, which would otherwise form a precipitate and give a false positive result.
Key points
- Carbonates fizz with dilute acid producing carbon dioxide.
- Halides are tested with nitric acid then silver nitrate.
- Chloride gives white, bromide cream, iodide yellow.
- Sulfates are tested with hydrochloric acid then barium chloride.
- Sulfates give a white precipitate.
- Acid is added first to remove carbonate ions.
Worked examples
Example 1
Describe the test for sulfate ions and the positive result. [2 marks]
Model answer
Add dilute hydrochloric acid, then barium chloride solution (1 mark). A white precipitate forms if sulfate ions are present (1 mark).
Example 2
State the precipitate colours for chloride, bromide and iodide ions with silver nitrate. [3 marks]
Model answer
Chloride gives a white precipitate (1 mark). Bromide gives a cream precipitate (1 mark). Iodide gives a yellow precipitate (1 mark).
Example 3
Explain why dilute acid is added before the silver nitrate or barium chloride. [2 marks]
Model answer
The acid removes any carbonate ions present in the sample (1 mark), which would otherwise form a precipitate and give a false positive result (1 mark).
Common mistakes
Forgetting to add acid first.
Carbonate ions would give a false positive precipitate.
Using the wrong acid.
Nitric acid for halides, hydrochloric acid for sulfates.
Confusing the halide precipitate colours.
White, cream, yellow — in the order chloride, bromide, iodide.
Saying carbonates give a precipitate.
Carbonates fizz and produce carbon dioxide gas.
Exam tips
- Learn which acid goes with which test.
- Memorise the three halide colours in order.
- Always explain why the acid is added first.
- Remember this is a Triple-only required practical.
Key terms
- Anion
- A negatively charged ion.
- Halide
- A chloride, bromide or iodide ion.
- Barium chloride
- The reagent used to test for sulfate ions.
- False positive
- A result suggesting an ion is present when it is not.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.