Identification of Anions
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge IGCSE Chemistry (0620)
Tests for Anions
An anion is a negatively charged ion. Each anion below is identified by a specific chemical test.
| Anion | Test | Positive Result |
|---|---|---|
| Carbonate, CO₃²⁻ | React with dilute acid, then test the gas produced | Effervescence; gas turns limewater milky |
| Chloride, Cl⁻ Bromide, Br⁻ Iodide, I⁻ |
Acidify with dilute nitric acid, then add aqueous silver nitrate | Cl⁻: white precipitate Br⁻: cream precipitate I⁻: (pale) yellow precipitate |
| Nitrate, NO₃⁻ | Reduce with aluminium foil and aqueous sodium hydroxide, then test the gas produced | Ammonia gas given off - turns damp red litmus paper blue |
| Sulfate, SO₄²⁻ | Acidify with dilute nitric acid, then add aqueous barium nitrate | White precipitate |
| Sulfite, SO₃²⁻ | React with acidified aqueous potassium manganate(VII) | Purple potassium manganate(VII) is decolourised |
Carbonate Ions
Adding dilute acid to a carbonate produces carbon dioxide gas, seen as effervescence. Bubbling the gas through limewater (aqueous calcium hydroxide) confirms it: the limewater turns milky as insoluble calcium carbonate forms.
CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g)
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)
Halide Ions: Chloride, Bromide, Iodide
The solution is first acidified with dilute nitric acid (to remove any carbonate that might interfere), then aqueous silver nitrate is added.
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) (white)
Ag⁺(aq) + Br⁻(aq) → AgBr(s) (cream)
Ag⁺(aq) + I⁻(aq) → AgI(s) (pale yellow)
Dilute hydrochloric acid or sulfuric acid must never be used to acidify this test - they would introduce extra chloride or sulfate ions and give a false result. Dilute nitric acid is used because it introduces no ions that would interfere.
Worked Example
- Question (based on a real exam question): describe a test to identify the iodide ions in potassium iodide.
- Answer: acidify with dilute nitric acid, then add aqueous silver nitrate; a (pale) yellow precipitate of silver iodide confirms iodide ions.
Nitrate Ions
Add aluminium foil and aqueous sodium hydroxide, then warm gently. If nitrate ions are present, the aluminium reduces them to ammonia gas, which turns damp red litmus paper blue.
Sulfate and Sulfite Ions
Sulfate is tested by acidifying with dilute nitric acid and adding aqueous barium nitrate; a white precipitate of barium sulfate confirms sulfate ions.
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (white)
Sulfite is tested differently: reacting it with acidified aqueous potassium manganate(VII) decolourises the purple manganate(VII) solution, because the sulfite ion reduces it.
Worked Example
- Question (based on a real exam question): dilute nitric acid followed by aqueous barium nitrate is added to a solution, and no visible change is observed. What does this show about the solution?
- Answer: the solution does not contain sulfate ions - a white precipitate of barium sulfate would have formed if SO₄²⁻ were present.
Common Mistakes
- ❌ Using dilute hydrochloric acid to acidify before the silver nitrate (halide) test
- ✓ HCl itself contains chloride ions, which would react with the silver nitrate and give a false positive - always use dilute nitric acid instead
- ❌ Confusing sulfate and sulfite tests
- ✓ sulfate gives a white precipitate with acidified barium nitrate; sulfite decolourises acidified potassium manganate(VII) - they use completely different reagents
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