Tests for Common Anions
IGCSE Chemistry tests for anions: carbonate, sulfate, chloride, bromide, iodide, and nitrate. Reagents, observations, and ionic equations for Cambridge 0620.
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Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Anion tests are a core part of the Cambridge 0620 practical syllabus and appear on Paper 6 (Alternative to Practical) as well as Paper 2 and Paper 4 theory questions. Each test follows a pattern: add a specific reagent and observe a characteristic result.
Carbonate ion (CO₃²⁻)
Test
Add dilute hydrochloric acid to the solid or solution.
Positive result
Effervescence (bubbling) occurs. The gas produced turns limewater milky.
Explanation
The acid reacts with the carbonate to produce carbon dioxide:
CO₃²⁻(s/aq) + 2H⁺(aq) → H₂O(l) + CO₂(g)
The CO₂ reacts with limewater (calcium hydroxide solution) to form insoluble calcium carbonate:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
Example
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Observation: effervescence; gas turns limewater milky/cloudy.
Sulfate ion (SO₄²⁻)
Test
Add dilute nitric acid (to remove any carbonate ions that could give a false positive), then add barium nitrate solution (Ba(NO₃)₂).
Positive result
A white precipitate forms that is insoluble in excess acid.
Explanation
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Barium sulfate is insoluble in water and in acid — the white precipitate does not dissolve.
Why add acid first?
Barium carbonate (BaCO₃) is also a white precipitate. Adding dilute nitric acid first reacts with any carbonate present, removing it so that only the sulfate gives a precipitate with barium nitrate.
Halide ions (Cl⁻, Br⁻, I⁻)
Test
Add dilute nitric acid (to remove interfering ions such as carbonate and sulfite), then add silver nitrate solution (AgNO₃).
Results
| Halide ion | Precipitate colour | Formula |
|---|---|---|
| Chloride, Cl⁻ | White | AgCl |
| Bromide, Br⁻ | Cream | AgBr |
| Iodide, I⁻ | Yellow | AgI |
Ionic equations
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Ag⁺(aq) + Br⁻(aq) → AgBr(s)
Ag⁺(aq) + I⁻(aq) → AgI(s)
Why add nitric acid first?
Other anions (e.g. carbonate, sulfite) also form insoluble silver salts. Adding dilute nitric acid first decomposes these, so that only the halide produces a precipitate with silver nitrate.
Distinguishing white from cream
In practice, the white (AgCl) and cream (AgBr) precipitates can look similar. An additional test — adding dilute ammonia solution — can help: AgCl dissolves in dilute ammonia, while AgBr requires concentrated ammonia to dissolve.
Nitrate ion (NO₃⁻) — Supplement
Test
There is no single simple test for nitrate at IGCSE level. One method involves adding sodium hydroxide solution and aluminium foil (or Devarda’s alloy), then warming. The nitrate is reduced to ammonia.
Positive result
Ammonia gas is produced, which turns damp red litmus paper blue.
This test is rarely asked at Core level but may appear in Supplement questions.
Summary table
| Anion | Reagent(s) | Observation |
|---|---|---|
| CO₃²⁻ | Dilute HCl | Effervescence; gas turns limewater milky |
| SO₄²⁻ | Dilute nitric acid then Ba(NO₃)₂(aq) | White precipitate (BaSO₄) |
| Cl⁻ | Dilute HNO₃ then AgNO₃(aq) | White precipitate (AgCl) |
| Br⁻ | Dilute HNO₃ then AgNO₃(aq) | Cream precipitate (AgBr) |
| I⁻ | Dilute HNO₃ then AgNO₃(aq) | Yellow precipitate (AgI) |
Order of testing in qualitative analysis
When given an unknown salt to identify:
- Test a portion with dilute HCl — if effervescence occurs and limewater turns milky, carbonate is present
- Dissolve a portion in water, acidify with dilute nitric acid, and add Ba(NO₃)₂ — test for sulfate
- Dissolve a portion in water, acidify with dilute HNO₃, and add AgNO₃ — test for halides
Worked exam question
A white solid dissolves in water. Adding dilute nitric acid followed by silver nitrate solution produces a cream precipitate. (a) Identify the anion. [1] (b) Write the ionic equation for the formation of the precipitate. [1] (c) Explain why dilute nitric acid is added before the silver nitrate solution. [1]
(a) Bromide ion / Br⁻ [1]
(b) Ag⁺(aq) + Br⁻(aq) → AgBr(s) [1]
(c) To react with and remove any carbonate/sulfite ions that could form precipitates with silver nitrate and give a false positive result [1]
Common exam mistakes
- Using the wrong acid — sulfate test uses HCl (not HNO₃); halide test uses HNO₃ (not HCl, which would introduce Cl⁻ and give a false positive).
- Confusing the precipitate colours — white = chloride, cream = bromide, yellow = iodide. Many candidates mix up cream and yellow.
- Forgetting to state that the gas must be tested with limewater for the carbonate test — just saying “bubbles form” is not enough to confirm CO₃²⁻.
- Writing the precipitate formula incorrectly — silver chloride is AgCl, not Ag₂Cl or AgCl₂.
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