Chemistry · Lesson 23

Practical 5: anions, gas tests and mixtures

WAECNECO

What the syllabus asks

WAEC: tests for CO₃²⁻, HCO₃⁻, SO₄²⁻, SO₃²⁻, Cl⁻ (Br⁻, I⁻), NO₃⁻; gas tests CO₂, NH₃, O₂, H₂, SO₂, Cl₂, NO₂, HCl, H₂S, H₂O; mixtures; full salt-analysis strategy.

NECO: tests for CO₃²⁻, HCO₃⁻, SO₄²⁻, SO₃²⁻, Cl⁻ (Br⁻, I⁻), NO₃⁻; gas tests CO₂, NH₃, O₂, H₂, SO₂, Cl₂, NO₂, HCl, H₂S, H₂O; mixtures; full salt-analysis strategy.

What you will learn

Explain the order and conditions of anion tests.
Identify CO₃²⁻, HCO₃⁻, SO₄²⁻, SO₃²⁻, Cl⁻ and NO₃⁻.
Describe and interpret tests for common gases.
Test gases with correct technique.
Analyse two-component mixtures.
Plan a full salt analysis within the time.
Write ionic equations for anion and gas tests.
Check results for consistency.

Key points

Acid first: carbonate test.
HNO₃ before AgNO₃; HCl before BaCl₂.
Cl⁻ white ppt dissolves in NH₃.
BaSO₄ insoluble in acid.
SO₂: dichromate orange → green.
Brown ring for NO₃⁻.
Glowing splint O₂; burning splint H₂.

Start here: Anion tests: the order matters

After identifying the cation, Question 2 asks you to find the anion. Six anions cover most WAEC and NECO papers: CO₃²⁻ (trioxocarbonate(IV)), HCO₃⁻, SO₄²⁻ (tetraoxosulphate(VI)), SO₃²⁻ (trioxosulphate(IV)), Cl⁻ (chloride), and NO₃⁻ (trioxonitrate(V)). Some papers add I⁻, Br⁻ and S²⁻.

Carbonate is tested first, with dilute acid, because carbonate ions interfere with the precipitation tests: Ag₂CO₃ and BaCO₃ are both white precipitates. That is why the silver nitrate test is done in solution acidified with dilute HNO₃, and the barium chloride test in solution acidified with dilute HCl (or HNO₃). The acid destroys any carbonate before the reagent is added.

Why these particular acids?

Silver nitrate test: acidify with HNO₃, not HCl (which adds Cl⁻ and gives AgCl anyway) and not H₂SO₄ (Ag₂SO₄ is only sparingly soluble).
Barium chloride test: acidify with HCl or HNO₃, not H₂SO₄ (which adds SO₄²⁻).
Lead salts: use dilute HNO₃ to dissolve, since PbCl₂ and PbSO₄ are insoluble.

A question such as “why was dilute HNO₃ added before AgNO₃?” is worth two marks: to remove CO₃²⁻ (or SO₃²⁻) ions that would also precipitate with Ag⁺, and because HNO₃ adds no interfering anion.

A reliable routine for anions

Work through the anions as a decision tree, one fresh portion of solution per test:

Dilute HCl to the solid or solution. Effervescence? Test the gas: odourless and lime water milky means CO₃²⁻ (or HCO₃⁻); choking and dichromate green means SO₃²⁻. No gas: carbonate and sulphite are absent.
Dilute HNO₃ then AgNO₃. White precipitate dissolving in NH₃: Cl⁻. Cream: Br⁻. Yellow: I⁻. No precipitate: halides absent.
Dilute HCl then BaCl₂. White precipitate: SO₄²⁻.
If none of the above, suspect NO₃⁻ and do the brown-ring test or the copper/conc. H₂SO₄ test.

Remember that the paper usually tells you which tests to do. The routine helps when a question says “carry out a test to identify the anion”, or when you must suggest a confirmatory test, which is a common two-mark item.

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Practice questions

1. Effervescence with dilute acid giving a gas that turns lime water milky shows

CO₃²⁻ or HCO₃⁻
SO₄²⁻
Cl⁻
NO₃⁻

Show answer

A: CO₃²⁻ or HCO₃⁻
CO₂ is evolved.

2. With MgSO₄(aq) in the cold, a trioxocarbonate(IV) gives

no precipitate
a blue precipitate
a brown ring
a white precipitate

Show answer

D: a white precipitate
Hydrogentrioxocarbonate(IV) gives one only on boiling.

3. A white precipitate with BaCl₂(aq), insoluble in dilute HCl, shows

CO₃²⁻
Cl⁻
SO₄²⁻
SO₃²⁻

Show answer

C: SO₄²⁻
BaSO₄ is insoluble in acid.

4. SO₂ is identified by

forming dense white fumes with HCl
turning orange acidified K₂Cr₂O₇ green
relighting a glowing splint
popping with a lighted splint

Show answer

B: turning orange acidified K₂Cr₂O₇ green
Dichromate is reduced.

5. A white precipitate with acidified AgNO₃ that dissolves in dilute NH₃(aq) shows

Cl⁻
I⁻
Br⁻
SO₄²⁻

Show answer

A: Cl⁻
AgCl dissolves in ammonia.

Key words

Anion: Negatively charged ion, e.g. Cl⁻, SO₄²⁻.
Filtrate: Liquid that passes through filter paper.
Residue: Solid left on the filter paper.
Brown-ring test: Test for nitrate with FeSO₄ and conc. H₂SO₄.
Lime water: Aqueous calcium hydroxide.
Devarda’s alloy: Cu–Al–Zn alloy that reduces nitrate to ammonia in alkali.
Bleaching: Removal of colour, e.g. damp litmus by Cl₂.

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