Qualitative Analysis: Anions and Gases
Learn and apply Qualitative Analysis: Anions and Gases in the published Chemistry course sequence.
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Qualitative Analysis: Anions and Gases: Orientation
The supplied notes include nitrite, sulfite and sulfur dioxide evidence, but 9476 does not require candidates to carry out tests involving hexane, sulfur dioxide gas, nitrite ions or sulfite ions. You may still need to interpret observations supplied in a question.
Anion and gas tests are predictable in Paper 4. The marks come from writing the steps and observations cleanly, not from memorising random “tricks”.
Use this with Paper 4 Skills: Planning, MMO, PDO, ACE and the Practical and QA (A Level) hub so method, data, and evaluation marks stay aligned.
Definitions (Must Know)
A. Acidify
To acidify means to add a dilute acid first (as instructed) to remove interfering ions before adding the test reagent.
B. Effervescence
Effervescence is fizzing/bubbling caused by a gas being produced.
C. Limewater
Limewater is aqueous Ca(OH)₂ used to test for CO₂(g) (turns milky due to CaCO₃(s)).
D. Confirmatory test
A confirmatory test is an additional test that uniquely supports an ion/gas identification (with a specific observation).
Detailed Explanations
A. Workflow: carbonate, CO₃²⁻
- Add dilute acid to the solid/solution.
- Observation: effervescence (gas produced).
- Test the gas with limewater.
Because CO₂(g) reacts with limewater Ca(OH)₂(aq) to form insoluble CaCO₃(s), therefore limewater turns milky.
B. Workflow: sulfate, SO₄²⁻
- Acidify (as instructed), then add a source of Ba²⁺ (e.g. Ba(NO₃)₂(aq)).
- Observation: white precipitate of BaSO₄(s).
C. Workflow: halides, Cl⁻, Br⁻, I⁻
- Acidify with dilute HNO₃(aq).
- Add AgNO₃(aq).
- Observe precipitate colour:
- AgCl(s): white
- AgBr(s): cream
- AgI(s): yellow
Because CO₃²⁻ (or OH⁻) would also form a precipitate with Ag⁺, therefore you acidify first to remove these interfering ions and avoid a false positive.
D. Nitrate, NO₃- (follow the question)
Exam questions may specify the test. A common idea is to reduce nitrate to NH₃(g) (e.g. using aluminium and NaOH(aq) on warming), then test NH₃(g) with damp red litmus (turns blue).
NH₃(g), Cl₂(g), and SO₂(g) are irritating/toxic. Avoid inhalation and use a fume cupboard if instructed. Many acids/oxidisers are corrosive/oxidising—wear goggles.
E. Gas tests (what to write)
| Gas | How it’s produced (common) | Test | Positive observation |
|---|---|---|---|
| NH₃(g) | warming ammonium salt with NaOH(aq) | damp red litmus | turns blue |
| CO₂(g) | carbonate + acid | limewater | turns milky |
| Cl₂(g) | bleach + acid / oxidising chloride | damp blue litmus | turns red then bleaches |
| SO₂(g) | sulfite + acid | acidified KMnO₄(aq) | purple decolourises |
F. Complete gas-test recall
| Gas | Positive test |
|---|---|
| NH₃ | turns damp red litmus paper blue |
| CO₂ | gives a white precipitate with limewater; it dissolves in excess CO₂ |
| Cl₂ | bleaches damp litmus paper |
| H₂ | gives a pop with a lighted splint |
| O₂ | relights a glowing splint |
| SO₂ | decolourises aqueous acidified potassium manganate(VII) |
For halides, the supplied notes add the ammonia-solubility evidence: AgCl is soluble, AgBr partially soluble and AgI insoluble in aqueous ammonia.
Worked Examples
Modelled example 1
Identify Chloride with Acidified Silver Nitrate
Problem
Study the worked solution
Record the test and observation
Method
State dilute nitric acid followed by aqueous silver nitrate, producing a white precipitate.Reason
Reagent order and precipitate colour are the observed evidence.Working
Observation: white precipitate.Infer the anion
Method
Identify chloride ions and silver chloride.Reason
The supplied halide pattern assigns white AgCl(s) to Cl-(aq).Working
Ag + (aq) + Cl-(aq) → AgCl(s).
Guided practice 2
Link Carbonate Effervescence to a Confirmatory Gas Test
Problem
Try this before viewing the solution
Hints
Hint 1: positive gas test
Hint 2: source anion
View solution step by step
Identify the gas
Method
Use milky limewater to identify carbon dioxide.Reason
Carbon dioxide reacts with limewater to form insoluble calcium carbonate.Working
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l).Infer the anion
Method
Identify carbonate ions in the original solid.Reason
Acid–carbonate reaction releases carbon dioxide, observed as effervescence.Working
CO₃²⁻(aq) + 2H + (aq) → CO₂(g) + H₂O(l).
Common misconception 3
Correct the Wrong Acid in a Halide Test
Learner method
Try this before viewing the solution
View solution step by step
Locate the contamination
Method
State that hydrochloric acid introduces chloride ions.Reason
Those ions react with added silver ions even if the original sample contained no chloride.Working
Ag + (aq) + Cl-(aq) → AgCl(s).Choose a non-interfering acid
Method
Acidify with dilute nitric acid before adding silver nitrate.Reason
Nitrate ions do not give the halide precipitate being tested.Working
Correct sequence: dilute HNO₃, then AgNO₃.
Examiner practice 4
Distinguish Hydrogen and Oxygen
Examination question
Try this before viewing the solution
View solution step by step
Test hydrogen
2 marksMethod
Apply a lighted splint to a small sample of gas.Reason
Hydrogen combusts rapidly in air.Working
Positive observation: a pop sound.Test oxygen
2 marksMethod
Insert a glowing splint into a sample of gas.Reason
Oxygen supports combustion.Working
Positive observation: the glowing splint relights.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark each test and its matching positive observation separately.
Challenge 5
Infer Nitrate from a Question-Specified Reduction Test
Anion transfer
Try this before viewing the solution
Hints
Hint 1: litmus evidence
Hint 2: follow the specified method
View solution step by step
Interpret the gas test
Method
Identify ammonia from the damp red litmus turning blue.Reason
Ammonia dissolves in the moisture and produces an alkaline response.Working
Observation → gas inference: NH₃(g).Infer the original anion
Method
State that the result supports nitrate ions.Reason
The supplied aluminium/alkali warming step reduces nitrate to ammonia.Working
Inference: NO₃-(aq) may be present.
Mind Stretchers
Mind stretcher 1Extension
A solution gives a white precipitate when Ba(NO₃)₂(aq) is added after acidifying. The precipitate is insoluble in dilute HNO₃(aq). Identify the anion and explain why the “acidify first” instruction matters.
Show Hint
The question asks what may be carried out, not everything printed in the supplied notes.
Show Answer
Mark scheme:
- Anion: SO₄²⁻ (white precipitate of BaSO₄(s), insoluble in dilute acid).
- Acidify first: removes interfering CO₃²⁻ (which would give BaCO₃(s)), preventing a false positive.
Mind stretcher 2: Using supplied notes without crossing the practical boundaryExtension
Question. The supplied Qualitative Analysis Notes list sulfite and a sulfur dioxide gas test. Explain how a 9476 candidate should respond if a question gives sulfur-dioxide observations, and whether the candidate is expected to carry out that gas test.
Show Hint
The question asks what may be carried out, not everything printed in the supplied notes.
Show Answer
The candidate may interpret supplied observations using the notes: sulfur dioxide decolourises acidified potassium manganate(VII), supporting sulfite or another stated source. However, 9476 explicitly says candidates are not required to carry out tests involving sulfur dioxide gas or sulfite ions. The distinction is between interpreting given evidence and performing the excluded test.