This topic summary covers Knowledge Organiser: Ion Tests 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 is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 15 of 15 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Ion Tests
NaOH Tests (Cations)
- Cu²⁺ = Blue precipitate
- Fe²⁺ = Green precipitate
- Fe³⁺ = Brown/orange precipitate
- Al³⁺ = White; redissolves in excess
- Ca²⁺ = White; stays cloudy
Anion Tests
- CO₃²⁻: add HCl → fizzing + milky limewater
- SO₄²⁻: BaCl₂ + HCl → white precipitate BaSO₄
- Cl⁻: AgNO₃ + HNO₃ → white AgCl
- Br⁻: AgNO₃ + HNO₃ → cream AgBr
- I⁻: AgNO₃ + HNO₃ → yellow AgI
Why Add Acid?
- BaCl₂ test: HCl removes CO₃²⁻ → prevents false white ppt
- AgNO₃ test: HNO₃ removes CO₃²⁻ → prevents false ppt
- Always acidify BEFORE adding the test reagent
Key Ionic Equations
- Ba²⁺ + SO₄²⁻ → BaSO₄(s)
- Ag⁺ + Cl⁻ → AgCl(s)
- Cu²⁺ + 2OH⁻ → Cu(OH)₂(s)
- CO₃²⁻ + 2H⁺ → CO₂ + H₂O
Common Mistakes
- Forgetting to acidify before anion tests: BaCl₂ test must be acidified with HCl first; AgNO₃ test must be acidified with HNO₃ — without acid, carbonate ions cause false positive white precipitates
- Confusing Fe²⁺ and Fe³⁺ NaOH colours: Fe²⁺ gives GREEN precipitate; Fe³⁺ gives BROWN/ORANGE precipitate — learn both; one mark lost for getting this wrong
- Saying Al³⁺ gives a white precipitate that stays: Aluminium hydroxide redissolves in EXCESS NaOH (forms aluminate) — Ca²⁺ also gives white but does NOT redissolve
- Getting halide precipitate colours wrong: Cl⁻ = white AgCl; Br⁻ = cream AgBr; I⁻ = yellow AgI — learn all three colours in order
Practice questions for Tests for Ions
Which reagents are used to test for carbonate ions in a solution?
Describe how sodium hydroxide solution can be used to distinguish between iron(II) ions and iron(III) ions in solution, including the expected observations.
Quick recall flashcards
What is a precipitation reaction?
A reaction where two soluble ionic compounds react to form an insoluble precipitate. General form: A⁺(aq) + B⁻(aq) → AB(s)
How do you test for sulfate ions (SO₄²⁻)?
Add barium chloride solution + dilute HCl. White precipitate of BaSO₄ forms. Equation: Ba²⁺ + SO₄²⁻ → BaSO₄(s)