Topic 2 of 4
Identify anions with the correct reagents
Acidification matters because contaminants can imitate the result.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Identify anions with the correct reagents
Acidification matters because contaminants can imitate the result.
| Anion | Procedure | Positive observation |
|---|---|---|
| CO32- | Add dilute acid; pass evolved gas into limewater | Effervescence; gas forms a white precipitate in limewater |
| Cl- | Acidify solution with dilute nitric acid, then add aqueous silver nitrate | White precipitate of AgCl |
| SO42- | Acidify solution with dilute nitric acid, then add aqueous barium nitrate | White precipitate of BaSO4 |
| NO3- | Add aqueous NaOH and aluminium foil, then warm carefully; test the gas | Ammonia turns damp red litmus blue |
Nitric acid removes interfering carbonate before the silver or barium test. Do not use hydrochloric acid to acidify for chloride: it introduces the ion being tested. Likewise, sulfuric acid would introduce sulfate. Record the original sample result using clean apparatus and separate portions; do not carry reagents from one test into another.
For nitrate, aluminium in alkaline solution reduces nitrate to ammonia. Ammonium ions also release ammonia with warm NaOH, even without aluminium. If ammonium could be present, check that separately and account for it before attributing ammonia to nitrate. Formulae of complex products are not needed for this identification.