Testing for Non-Metal Ions (Anions)
Part of Tests for Ions — GCSE Chemistry
This deep dive covers Testing for Non-Metal Ions (Anions) within Tests for Ions for GCSE Chemistry. Revise Tests for Ions in Chemical Analysis for GCSE Chemistry with 20 exam-style questions and 14 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 4 of 14 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 4 of 14
Practice
20 questions
Recall
14 flashcards
🔬 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.
Quick Check: An unknown solution gives a cream precipitate when silver nitrate and dilute nitric acid are added. Which halide ion is present?
Bromide (Br⁻). White = chloride, cream = bromide, yellow = iodide.