4.2.2Physical Chemistry
Identifying Anions and Cations — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 100 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Tests for Anions
Carbonate
- Carbonate Add dilute hydrochloric acid;
- fizzing occurs and the gas turns limewater milky, CO₃²⁻ + 2H⁺ → CO₂ + H₂O.
Sulfate
- Sulfate Acidify with dilute HCl, then add barium chloride;
- a white precipitate of BaSO₄ forms, Ba²⁺ + SO₄²⁻ → BaSO₄.
Halides
- Halides Acidify with dilute nitric acid, then add silver nitrate; a precipitate of AgCl, AgBr or AgI forms.
Hydroxide
- Hydroxide Test with red litmus paper or universal indicator;
- red litmus turns blue and the pH is strongly alkaline.
Why acidify
- Why acidify Acid removes carbonate and hydroxide, which would also form precipitates with barium or silver ions.
Section 2 of 5Halide Ion Identification
Halide Ion Identification
Chloride
- Chloride Gives a white precipitate of silver chloride that dissolves in dilute ammonia solution.
Bromide
- Bromide Gives a cream precipitate of silver bromide that dissolves only in concentrated ammonia solution.
Iodide
- Iodide Gives a yellow precipitate of silver iodide that does not dissolve in dilute or concentrated ammonia.
Ionic equation
- Ionic equation Ag⁺(aq) + X⁻(aq) → AgX(s), where X is Cl, Br or I.
| Halide | Colour of AgX | Dissolves in ammonia? |
|---|---|---|
| Cl⁻ | White | Dilute NH₃(aq) |
| Br⁻ | Cream | Concentrated NH₃(aq) only |
| I⁻ | Yellow | Neither |
Section 3 of 5Tests for Cations
Tests for Cations
Ammonium
- Ammonium Warm with sodium hydroxide; ammonia gas turns damp red litmus paper blue, NH₄⁺ + OH⁻ → NH₃ + H₂O.
Magnesium
- Magnesium Forms a white precipitate with sodium hydroxide, which remains in excess, and with ammonia solution.
Calcium to barium
- Calcium to barium Sulfuric acid gives a white precipitate for Ca²⁺, Sr²⁺ and Ba²⁺, but not Mg²⁺, which stays as a colourless solution.
Barium
- Barium No precipitate forms with sodium hydroxide, but a dense white precipitate forms with sulfate ions.
Section 4 of 5Order of Testing an Unknown Salt
1
Dissolve any solid in deionised water to make an aqueous solution before testing.
2
Test for carbonate first with dilute acid, because carbonate forms precipitates with both barium and silver ions.
3
Test for sulfate next, adding acid first and then barium chloride solution.
4
Test for halides last with nitric acid and silver nitrate, since silver sulfate is sparingly soluble.
5
Test for ammonium and hydroxide ions separately with sodium hydroxide and litmus paper.
Section 5 of 5Exam Checklist & Tips
4.2.2 – Key Points for the Exam
- Anion A negatively charged ion.
- Cation A positively charged ion.
- Precipitate An insoluble solid formed when two solutions react.
- Limewater A solution of calcium hydroxide that turns milky in the presence of carbon dioxide.
Common Mistakes
- Adding silver nitrate to a halide test without first acidifying with nitric acid.
- Testing for halides before carbonate or sulfate, giving false positive precipitates.
- Using hydrochloric acid to acidify the halide test, which adds chloride ions.
- Stating that the gas turns red litmus blue as carbon dioxide; it is ammonia that does this.
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