Tests for Carbonates, Halides and Sulfates | Chemistry

📖 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.

📐 Key Equations

Carbonate: acid + CO₃²⁻ → CO₂ (turns limewater milky)

Halides: Ag⁺ + X⁻ → AgX (Cl=white, Br=cream, I=yellow)

Sulfate: Ba²⁺ + SO₄²⁻ → BaSO₄ (white precipitate)

📌 Key Note

Carbonate: acid → CO₂ → limewater milky. Halides: acidify (HNO₃) + AgNO₃ → Cl=white, Br=cream, I=yellow precipitate. Sulfate: acidify (HCl) + BaCl₂ → white BaSO₄ precipitate. Always acidify first to remove interfering ions.

🎯 Matching Activity — Anion Test Results

Match each anion to the test reagent and result. — drag the symbols on the right to match the component names on the left.

Carbonate CO₃²⁻

Drop here

Chloride Cl⁻

Drop here

Bromide Br⁻

Drop here

Iodide I⁻

Drop here

Sulfate SO₄²⁻

Drop here

Add acid → CO₂ gas → limewater turns milky/cloudy

Acidify (HNO₃) + AgNO₃ → yellow AgI precipitate

Acidify (HNO₃) + AgNO₃ → white AgCl precipitate

Acidify (HCl) + BaCl₂ → white BaSO₄ precipitate

Acidify (HNO₃) + AgNO₃ → cream AgBr precipitate

⭐ Higher Tier Only

Write ionic equations for all anion identification tests: Ag⁺ + Cl⁻ → AgCl(s); Ba²⁺ + SO₄²⁻ → BaSO₄(s); CO₃²⁻ + 2H⁺ → H₂O + CO₂. Identify an unknown ionic compound from a combination of systematic cation and anion tests. Explain the need for acidification at each stage.

🔬 Triple Science Only

Tests for carbonates (4.8.3.3), halides (4.8.3.4) and sulfates (4.8.3.5) are chemistry-only — not in Combined Science.

🧪 Required Practical

🔬 RP Chemistry 4 (chemistry-only) — Test for anions using acid + limewater (carbonate), acidified AgNO₃ (halides), acidified BaCl₂ (sulfate).

Know the method, variables, equipment and how to analyse results.

🎯 Test Yourself

Question 1 of 2

1. Why must solutions be acidified with nitric acid before adding silver nitrate to test for halides?

2. A white precipitate forms when barium chloride is added to an acidified solution. What ion is present?

⭐ How Well Do You Understand This Topic?

Be honest with yourself — this helps you know what to revise!

Don't get it Getting there Nailed it!

🤖 Ask Mr Badmus AI

Stuck? Just ask! 💬

I'll use FIFA for calculations and flag Higher/Triple content clearly.

🧪 Tests for Carbonates, Halides and Sulfates

Test for Carbonate Ions (CO₃²⁻)

Test for Halide Ions (Cl⁻, Br⁻, I⁻)

Test for Sulfate Ions (SO₄²⁻)

🔬 RP Chemistry 4 (chemistry-only) — Test for anions using acid + limewater (carbonate), acidified AgNO₃ (halides), acidified BaCl₂ (sulfate).

Mr. Badmus AI