📖 In-Depth Theory
Test for Carbonate Ions (CO₃²⁻)
To test for CARBONATE IONS (CO₃²⁻):
1. Add dilute hydrochloric acid (or another dilute acid) to the sample.
2. If carbonate present: fizzing (effervescence) — CO₂ gas produced.
3. CONFIRM CO₂: bubble the gas through limewater (calcium hydroxide solution).
4. Limewater turns MILKY/CLOUDY if CO₂ is present.
EQUATION:
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O (limewater goes cloudy)
This test can also identify sulfites (SO₃²⁻) — they produce SO₂ with acid, which also turns limewater cloudy.
To distinguish: use acidified silver nitrate solution (sulfite gives precipitate in different conditions).
Test for Halide Ions (Cl⁻, Br⁻, I⁻)
To test for HALIDE IONS (chloride, bromide or iodide):
1. Add DILUTE NITRIC ACID (acidify to remove interfering ions like CO₃²⁻ or SO₄²⁻).
2. Add SILVER NITRATE SOLUTION (AgNO₃).
3. Observe the colour of any precipitate formed.
PRECIPITATE COLOURS:
Chloride Cl⁻: WHITE precipitate of silver chloride (AgCl).
Bromide Br⁻: CREAM precipitate of silver bromide (AgBr).
Iodide I⁻: YELLOW precipitate of silver iodide (AgI).
Memory: Cl=White, Br=Cream, I=Yellow
IONIC EQUATIONS:
Ag⁺(aq) + Cl⁻(aq) → AgCl(s) white
Ag⁺(aq) + Br⁻(aq) → AgBr(s) cream
Ag⁺(aq) + I⁻(aq) → AgI(s) yellow
Why nitric acid first? To remove CO₃²⁻ and SO₄²⁻ which could form their own precipitates with Ag⁺.
Test for Sulfate Ions (SO₄²⁻)
To test for SULFATE IONS (SO₄²⁻):
1. Add DILUTE HYDROCHLORIC ACID (acidify to remove interfering carbonate ions).
2. Add BARIUM CHLORIDE SOLUTION (BaCl₂).
3. A WHITE precipitate of BARIUM SULFATE (BaSO₄) confirms sulfate ions are present.
EQUATION:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (white precipitate)
Why HCl first? Carbonate ions also form a white precipitate with Ba²⁺ (BaCO₃). Adding HCl removes carbonate as CO₂ first — ensuring any white precipitate is specifically BaSO₄.
SUMMARY OF ANION TESTS:
CO₃²⁻: acid → CO₂ gas (turns limewater milky)
Cl⁻: HNO₃ + AgNO₃ → white precipitate (AgCl)
Br⁻: HNO₃ + AgNO₃ → cream precipitate (AgBr)
I⁻: HNO₃ + AgNO₃ → yellow precipitate (AgI)
SO₄²⁻: HCl + BaCl₂ → white precipitate (BaSO₄)
⚠️ Common Mistake
Always ACIDIFY before adding the precipitating reagent. For halides: use dilute nitric acid (NOT HCl — it would add Cl⁻ ions). For sulfates: use dilute HCl (to remove carbonates). Halide precipitate colours: white (Cl), cream (Br), yellow (I) — memorise in order.
🎯 Test Yourself
Question 1 of 2
1. Why must solutions be acidified with nitric acid before adding silver nitrate to test for halides?
Nitric acid provides extra silver ions to improve the sensitivity of the test
Silver nitrate is unstable in neutral conditions — acid preserves it
To remove carbonate and sulfate ions that would also form precipitates with silver nitrate — ensuring only halide precipitates are observed
Nitric acid makes the silver nitrate solution work faster as a catalyst
2. A white precipitate forms when barium chloride is added to an acidified solution. What ion is present?
Carbonate CO₃²⁻ — barium carbonate is also white
Sulfate SO₄²⁻ — Ba²⁺ + SO₄²⁻ → BaSO₄ (white precipitate)
Bromide Br⁻ — barium bromide is a white precipitate
Chloride Cl⁻ — barium chloride reacts with itself to form a precipitate