Tests for Anions — Chemical Tests | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable

Low

Stable8%

Ion tests, flame tests and gas tests all appear regularly; expect to recall observations.

Carbonate ion, CO₃²⁻

Add a few drops of dilute acid (HCl or HNO₃) to the solid or solution
(fizzing) is seen as carbon dioxide is released:

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

Confirm the gas is carbon dioxide by passing it through limewater — the turns milky as insoluble calcium carbonate forms:

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

The exam-mark wording is "bubble the gas through limewater; if carbonate is present the limewater turns milky" — saying only "limewater turns milky" is the result, not the test

Halide ions, Cl⁻ / Br⁻ / I⁻

Acidify the sample first with a few drops of dilute nitric acid — this destroys any carbonate impurity that would otherwise give a false-positive precipitate
Add a few drops of solution, AgNO₃(aq)
A silver halide precipitate forms whose colour identifies the halide:

Halide	Equation (with KX representing the metal halide)	Precipitate colour
Chloride, Cl⁻	KCl(aq) + AgNO₃(aq) → AgCl(s) + KNO₃(aq)	White
Bromide, Br⁻	KBr(aq) + AgNO₃(aq) → AgBr(s) + KNO₃(aq)	Cream
Iodide, I⁻	KI(aq) + AgNO₃(aq) → AgI(s) + KNO₃(aq)	Yellow

Use nitric acid (not hydrochloric) for the acidification step — HCl introduces chloride ions and would give a false positive
Memorise the colour order: white → cream → yellow as the halide moves down the group

Exam tip

Halide ion test — the acid choice matters

What comes up: Describe how to test for chloride, bromide or iodide ions, and state the result.

Write (two marks minimum): (1) Acidify the sample with dilute nitric acid, then add silver nitrate solution. (2) A white precipitate indicates chloride; cream indicates bromide; yellow indicates iodide.

Watch out: The mark scheme rejects hydrochloric acid and sulfuric acid as the acidifying agent — using HCl introduces chloride ions that would give a false positive, and H₂SO₄ is rejected outright. You must use nitric acid. Also: the cream (bromide) and white (chloride) results are only credited if silver nitrate has been added; stating just a "white precipitate" without naming silver nitrate scores nothing for the observation.

Sulfate ion, SO₄²⁻

Acidify the sample with dilute hydrochloric acid — this removes any carbonate impurity that would otherwise precipitate as BaCO₃ and confuse the result
Add a few drops of solution, BaCl₂(aq) (barium nitrate also works)
A white of barium sulfate forms if sulfate ions are present:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why HCl this time (not HNO₃): no chloride ion would interfere with the BaSO₄ result, but a carbonate would form BaCO₃ (also white) and give a false positive

Exam tip

Sulfate ion test — which acid to use

What comes up: Describe the test for sulfate ions.

Write (two marks): (1) Add acidified barium chloride solution (or barium nitrate solution) to the sample. (2) A white precipitate of barium sulfate forms if sulfate ions are present.

Watch out: The mark scheme rejects sulfuric acid as the acidifying agent — using H₂SO₄ introduces sulfate ions and would give a false positive. Any other named acid is accepted. The white precipitate mark depends on barium chloride or barium nitrate being named in the previous step; a bare statement of "white precipitate" without identifying the reagent scores nothing.

Chemical Tests

Exam Frequency Analysis

Carbonate ion, CO32−

Halide ions, Cl⁻ / Br⁻ / I⁻

Sulfate ion, SO42−

Carbonate ion, CO₃²⁻

Sulfate ion, SO₄²⁻