Testing for Non-Metal Ions (Anions)

🔬 Testing for Non-Metal Ions (Anions)

1. Carbonate Ion (CO₃²⁻)

Test: Add dilute hydrochloric acid (HCl).

Positive result: Effervescence (fizzing). Collect the gas and bubble through limewater — limewater turns milky.

Equation: CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)

Confirming CO₂: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

2. Sulfate Ion (SO₄²⁻)

Test: Add barium chloride solution (BaCl₂) acidified with dilute hydrochloric acid (HCl).

Positive result: White precipitate of barium sulfate (BaSO₄) forms immediately.

Equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why add HCl? To acidify the solution and remove carbonate ions that would otherwise give a false positive white precipitate of barium carbonate (BaCO₃).

3. Halide Ions (Cl⁻, Br⁻, I⁻)

Test: Add silver nitrate solution (AgNO₃) acidified with dilute nitric acid (HNO₃).

Positive results:

• Chloride (Cl⁻): White precipitate of AgCl
• Bromide (Br⁻): Cream precipitate of AgBr
• Iodide (I⁻): Yellow precipitate of AgI

General equation: Ag⁺(aq) + X⁻(aq) → AgX(s)

Why add HNO₃? To acidify the solution and prevent other anions (especially carbonate) from precipitating with silver and giving false positives.