📖 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?
To remove carbonate and sulfate ions that would also form precipitates with silver nitrate — ensuring only halide precipitates are observed
Silver nitrate is unstable in neutral conditions — acid preserves it
Nitric acid makes the silver nitrate solution work faster as a catalyst
Nitric acid provides extra silver ions to improve the sensitivity of the test
2. A white precipitate forms when barium chloride is added to an acidified solution. What ion is present?
Sulfate SO₄²⁻ — Ba²⁺ + SO₄²⁻ → BaSO₄ (white precipitate)
Chloride Cl⁻ — barium chloride reacts with itself to form a precipitate
Carbonate CO₃²⁻ — barium carbonate is also white
Bromide Br⁻ — barium bromide is a white precipitate