In Qualitative Analysis, we identify unknown anions in solution using specific chemical tests. This is an important topic in O-Level and IGCSE Chemistry.
Syllabus Objectives
(i) carbonate (by the addition of dilute acid and subsequent use of limewater)
(ii) chloride (by reaction of an aqueous solution with nitric acid and aqueous silver nitrate)
(iii) iodide (by reaction of an aqueous solution with nitric acid and aqueous silver nitrate)
(iv) nitrate (by reduction with aluminium in aqueous sodium hydroxide to ammonia and subsequent use of moist red litmus paper)
(v) sulfate (by reaction of an aqueous solution with nitric acid and aqueous barium nitrate)
Summary of Anion Tests and Observations
| Anion | Test | Observation & Inference |
|---|---|---|
| CO₃²⁻ | Add dilute hydrochloric acid. Pass the gas into limewater. | Effervescence is observed. Gas given off forms white precipitate with limewater. Carbon dioxide gas is given off. |
| NO₃⁻ | Add sodium hydroxide solution, then a piece of aluminium foil. Warm the mixture. Test gas given off with moist red litmus paper. | Effervescence is observed. Gas turns moist red litmus paper blue. Ammonia gas is given off. |
| SO₄²⁻ | Add dilute nitric acid, then barium nitrate solution. | White precipitate of barium sulfate is formed. |
| Cl⁻ | Add dilute nitric acid, then silver nitrate solution. | White precipitate of silver chloride is formed. |
| I⁻ | Add dilute nitric acid, then silver nitrate solution. | Yellow precipitate of silver iodide is formed. |
Alternative Tests Using Lead(II) Nitrate
Question:
Other than barium nitrate solution, what other solution could be added to test for sulfate ions?
Hint: Recall solubility table
Answer:
Lead(II) nitrate solution. White precipitate of lead(II) sulfate is formed.
Chemical Equation: Pb(NO3)2 (aq) + Na2SO4 (aq) → PbSO4 (s)+ 2 NaNO3 (aq)
Question:
Other than silver nitrate solution, what other solution could be added to test for chloride ions?
Hint: Recall solubility table
Answer:
Lead(II) nitrate solution. White precipitate of lead(II) chloride is formed.
Chemical Equation: Pb(NO3)2 (aq) + 2 NaCl (aq) → PbCl2 (s)+ 2 NaNO3 (aq)
Question:
Other than silver nitrate solution, what other solution could be added to test for iodide ions?
Answer:
Lead(II) iodide solution. Yellow precipitate of lead(II) iodide is formed.
Chemical Equation: Pb(NO3)2 (aq) + 2 NaI (aq) → PbI2 (s)+ 2 NaNO3 (aq)
Summary & Key Takeaways
- Always add dilute nitric acid first when testing with silver nitrate or barium nitrate to remove interfering ions.
- Carbonate and nitrate tests involve gas production.
- Chloride, iodide, and sulfate tests involve precipitate formation.
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One response to “Qualitative Analysis: Identifying Anions (Carbonate, Chloride, Iodide, Nitrate, Sulfate)”
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