Qualitative Analysis: Tests for Anions
Tests for carbonate, chloride, iodide, nitrate and sulfate ions, including reagent order, observations, interference and ionic equations.
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The core idea
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Learning objectives
- describe tests to identify the following anions: carbonate (by the addition of dilute acid and subsequent use of limewater); chloride (by reaction of an aqueous solution with nitric acid and aqueous silver nitrate); iodide (by reaction of an aqueous solution with nitric acid and aqueous silver nitrate); nitrate (by reduction with aluminium in aqueous sodium hydroxide to ammonia and subsequent use of damp red litmus paper) and sulfate (by reaction of an aqueous solution with nitric acid and aqueous barium nitrate)
Anion tests depend on reagent order. Use a fresh portion for each test, name every reagent and condition, and record the observation before the inference.
1. Definition
An anion test uses a prescribed reagent sequence and positive observation to identify a negative ion. This course requires the tests for carbonate, chloride, iodide, nitrate and sulfate.
2. Key Ideas
| Anion | Test | Positive observation |
|---|---|---|
| carbonate, CO₃²⁻ | add dilute acid; test the gas with limewater | effervescence; limewater turns milky (white precipitate forms) |
| chloride, Cl⁻ | acidify with dilute nitric acid, then add aqueous silver nitrate | white precipitate |
| iodide, I⁻ | acidify with dilute nitric acid, then add aqueous silver nitrate | yellow precipitate |
| nitrate, NO₃⁻ | add aqueous sodium hydroxide, then aluminium foil; warm | ammonia gas is produced and turns damp red litmus paper blue |
| sulfate, SO₄²⁻ | acidify with dilute nitric acid, then add aqueous barium nitrate | white precipitate |
These five ions are the anions named in the O-Level qualitative-analysis syllabus. Bromide and silver-halide solubility in ammonia are not part of this core set.
3. Detailed Explanations
A. Why nitric acid is used first
Acid removes carbonate ions that could otherwise form misleading silver carbonate or barium carbonate precipitates. Nitric acid is chosen because nitrate ions do not give a precipitate with the later silver-nitrate or barium-nitrate reagent.
Do not acidify with hydrochloric acid before a chloride test because it adds Cl⁻. Do not acidify with sulfuric acid before a sulfate test because it adds SO₄²⁻. Use dilute nitric acid.
Always use a fresh portion for a new test. Reusing a portion after adding hydrochloric acid, sulfuric acid, silver nitrate or barium nitrate can contaminate later evidence.
B. Equations and representation levels
Carbonate
CO₃²⁻(aq) + 2H + (aq) → CO₂(g) + H₂O(l)
- Macroscopic: effervescence is seen; the evolved gas turns limewater milky.
- Particle level: carbonate ions react with hydrogen ions to form carbon dioxide molecules and water.
- Symbolic: atoms and total charge are balanced in the ionic equation.
Chloride and iodide
Ag + (aq) + Cl⁻(aq) → AgCl(s)
Ag + (aq) + I⁻(aq) → AgI(s)
The observation distinguishes them: silver chloride is white; silver iodide is yellow.
Sulfate
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
The 2 + and 2- charges cancel in a 1:1 ratio, producing a white solid.
C. Nitrate test sequence
- Add aqueous sodium hydroxide to a fresh portion of the sample.
- Add aluminium foil.
- Warm the mixture carefully.
- Test the evolved gas with damp red litmus paper.
- If the paper turns blue, ammonia has formed and nitrate ions are present.
Keep the apparatus away from the face. Use damp red litmus paper; do not inhale the gas directly.
4. Common Mistakes
- “A white precipitate forms” is not enough unless the reagent sequence is stated.
- Limewater turns milky; it does not simply “turn white”.
- The nitrate and ammonium tests both release ammonia with alkali, but the nitrate test also requires aluminium foil.
- A chloride test acidified with hydrochloric acid is invalid because the acid itself supplies chloride ions.
- The carbonate gas test identifies carbon dioxide; effervescence alone is not a complete confirmation.
5. Exam Tips
- Memorise each test as a sequence: prepare the sample, add the named reagent, state the positive observation.
- State “warm” for nitrate and “damp red litmus turns blue” for the evolved ammonia.
- Write an observation first and the ion second.
- Link precipitate formation to salt solubility rules and revise the six Gas Tests.
6. Worked Examples
Modelled example 1
chloride or iodide
Problem
Study the worked solution
Record the complete procedure
Method
State dilute nitric acid followed by aqueous silver nitrate.Reason
The acid removes interfering ions without adding a halide.Working
Acidified silver-nitrate test.Interpret precipitate colour
Method
Identify iodide ions.Reason
Silver iodide is the prescribed yellow precipitate.Working
I⁻ supported; AgI(s) is yellow.
Guided practice 2
nitrate or ammonium
Problem
Isolate the effect of aluminium
Hints
Hint 1: difference
Hint 2: control
View solution step by step
Run the control
Method
Warm a fresh portion with aqueous sodium hydroxide without aluminium foil.Reason
This checks whether ammonia forms from ammonium ions already in the sample.Working
Control: NaOH(aq) + warming only.Interpret comparatively
Method
Attribute extra ammonia with aluminium to the nitrate procedure only after the control.Reason
The comparison separates pre-existing ammonium evidence from aluminium-dependent nitrate reduction.Working
Ammonia without aluminium → ammonium may be present; aluminium-specific result supports nitrate.
Common misconception 3
invalid chloride evidence
Learner conclusion
Identify reagent contamination
View solution step by step
Identify the contamination
Method
State that hydrochloric acid added chloride ions.Reason
Those reagent-derived ions can form white AgCl even if the unknown originally contained none.Working
False-positive chloride source: HCl.Correct the procedure
Method
Repeat on a fresh portion using dilute nitric acid, then aqueous silver nitrate.Reason
Nitric acid acidifies without introducing a halide being tested.Working
Fresh portion: dilute HNO₃, then AgNO₃(aq).
Challenge 4
sulfate
Procedure transfer
Choose acid, reagent and observation
Hints
Hint 1: avoid adding sulfate
Hint 2: insoluble salt
View solution step by step
Prepare a fresh portion
Method
Acidify with dilute nitric acid.Reason
This removes interfering carbonate without introducing sulfate.Working
Fresh portion + dilute HNO₃.Add test reagent and observe
Method
Add aqueous barium nitrate and look for a white precipitate.Reason
Ba²⁺ and sulfate form insoluble white BaSO₄.Working
White precipitate → sulfate supported.
7. Mind Stretchers
Mind stretcher 1: Distinguish nitrate from ammoniumExtension
An unknown produces ammonia when warmed with aqueous sodium hydroxide and aluminium foil. Explain why this result alone may not prove that nitrate ions were originally present, and state a useful control.
Show Answer
Ammonium ions also release ammonia when warmed with aqueous sodium hydroxide. Test a fresh portion with aqueous sodium hydroxide and warming without aluminium foil. If ammonia is already released, ammonium ions interfere with the nitrate inference.
8. Quiz
Practise the five required anions, reagent order, observations, interference and equations.
Go to Anion Tests Quiz