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
Memory Aid
- Cl⁻ = White
- Br⁻ = Cream
- I⁻ = Yellow
- Remember: "Why (Y) does Cream come between?" — White, Cream, Yellow