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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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 are the five anions required in this course. 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.
Test unknown anions with fresh portions. Try barium nitrate on a carbonate before and after acidifying to see why nitric acid comes first.
Unknown 2: a solution of the sodium salt of one anion. Nothing tested yet.
- Reagent added to this portion
- 0.00 cm³
- Portions tested
- 0
- Tests done
- 0
- Still possible
- 5
Try this
0 of 4 doneTest an unknown cation until only one ion in the Notes fits. (not done yet)
Some cations need both reagents: Al³⁺ and Zn²⁺ both give a white ppt. that dissolves in excess NaOH(aq), but only Zn(OH)₂ dissolves in excess NH₃(aq).
Test an unknown anion until only one ion in the Notes fits. (not done yet)
Each anion test is specific: a halide precipitates with acidified Ag⁺, sulfate with acidified Ba²⁺, a carbonate fizzes with acid and a nitrate gives ammonia with Al and NaOH(aq).
Identify an unknown gas with the tests for gases. (not done yet)
One test can mislead: SO₂ also turns limewater milky, so CO₂ needs limewater and SO₂ needs acidified potassium manganate(VII).
Add Ba(NO₃)₂(aq) to the carbonate without acidifying it first. (not done yet)
Barium carbonate is also a white ppt., so a carbonate could pass for a sulfate. Acidifying with nitric acid first removes carbonate as CO₂.
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.
- Damp red litmus turning blue confirms ammonia. To attribute it to nitrate, first check that a fresh portion warmed with sodium hydroxide without aluminium does not also produce ammonia.
Aluminium enables nitrate to be reduced to ammonia in alkaline conditions. Ammonium ions can produce ammonia with sodium hydroxide alone. If the no-aluminium control also gives ammonia, this positive gas test cannot on its own establish nitrate. Both ions could be 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
A sample releases ammonia when warmed with aqueous sodium hydroxide and aluminium foil. What control is needed before attributing this ammonia to nitrate, and what would ammonia in that control suggest?
Isolate the effect of aluminium
Hints
Hint 1: difference
The nitrate procedure differs from the ammonium test by the added aluminium.
Hint 2: control
Repeat the shared alkaline-warming conditions while omitting aluminium.
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
Use the presence or absence of ammonia in the control before making the nitrate inference.
Reason
A negative control followed by ammonia in the aluminium test supports nitrate. A positive control shows ammonium interference; the presence of ammonia in both tests does not establish whether nitrate is also present.
Working
Ammonia without aluminium → ammonium may be present; ammonia only with aluminium supports nitrate.
Common misconception 3
invalid chloride evidence
Learner conclusion
A learner acidifies an unknown with hydrochloric acid, adds silver nitrate and concludes that a white precipitate proves chloride in the original sample. Explain why the evidence is invalid and correct the method.
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).
Guided practice 4
sulfate
Complete the reagent sequence
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
Two unknowns are tested on fresh portions under the correct alkaline-warming conditions. P produces no ammonia without aluminium, but produces ammonia with aluminium. Q produces ammonia both with and without aluminium.
Which ions are supported in each unknown? Can you conclude that Q contains no nitrate?
Show Answer
P supports nitrate: ammonia appears only in the aluminium-dependent test. Q supports ammonium because ammonia appears with sodium hydroxide and warming alone. Q’s nitrate status remains unresolved by these presence/absence results: it could contain ammonium alone or both ammonium and nitrate. A positive ammonium control is not proof that nitrate is absent.
8. Practise and check
Practise the five required anions, reagent order, observations, interference and equations.
Practise and check qualitative analysisSyllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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