Identifying negative ions
Tests for negative ions
| Ion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | Add dilute acid; bubble the gas through limewater | Fizzing; limewater turns milky (CO₂) |
| Chloride, Cl⁻ | Add dilute , then solution | White precipitate |
| Bromide, Br⁻ | As for chloride | Cream precipitate |
| Iodide, I⁻ | As for chloride | Yellow precipitate |
| Sulfate, SO₄²⁻ | Add dilute hydrochloric acid, then barium chloride solution | White precipitate |
The carbonate test works because carbonates react with acids to make carbon dioxide: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
Key terms
Why acidify first?
In the halide and sulfate tests, the acid is added first to . Carbonate ions would react with silver nitrate or barium chloride to form a precipitate too, giving a result.
The choice of acid matters:
- For the test, use , not hydrochloric acid, which would add chloride ions.
- For the sulfate test, use hydrochloric acid, not sulfuric acid, which would add sulfate ions.
Key terms
Required practical: identifying ions
To identify the ions in an unknown ionic compound:
- Do a on the solid to look for metal ions.
- Dissolve some in water and add sodium hydroxide solution to look for coloured or white hydroxide precipitates.
- Test separate portions for carbonate (dilute acid and limewater), (nitric acid and silver nitrate) and sulfate (hydrochloric acid and barium chloride) ions.
- Combine the results to name the compound. A lilac flame and a cream precipitate with silver nitrate means potassium bromide.
Hazard: barium chloride is toxic and silver nitrate stains skin — wear gloves and eye protection.
Better results: use clean test tubes for each test so you don't carry ions over.