C16: Qualitative Chemical Tests and Ion Identification
Official Cambridge & Imperial ESAT specification requirements for Qualitative Chemical Tests and Ion Identification on the Chemistry paper. Understand the syllabus boundaries, spot distractor traps engineered by examiners, and master the 2-step Sub-90 shortcuts that compress solve time well below the 88.8-second cutoff.
Official Syllabus Subtopics
Requirements specified in the UAT-UK framework
- 1Flame tests for metal cations: Li⁺ (crimson), Na⁺ (yellow), K⁺ (lilac), Ca²⁺ (orange-red), Cu²⁺ (green-blue)
- 2Aqueous sodium hydroxide tests: Cu²⁺ (blue ppt), Fe²⁺ (dirty green ppt), Fe³⁺ (rust brown ppt), Al³⁺ (white ppt dissolving in excess NaOH), Mg²⁺/Ca²⁺ (white ppt insoluble in excess)
- 3Strict anion testing sequence: 1. Carbonate (dilute acid ⟶ effervescence CO₂); 2. Sulfate (BaCl₂ in dilute HCl ⟶ white ppt); 3. Halides (AgNO₃ in dilute HNO₃: Cl⁻ white, Br⁻ cream, I⁻ yellow)
- 4Gas identification tests: H₂ (squeaky pop), O₂ (relights glowing splint), CO₂ (turns limewater cloudy), Cl₂ (bleaches damp litmus), NH₃ (turns damp red litmus blue)
Sub-90 Exam Speed Tips
How top candidates solve this spec under the 88.8s clock
Always follow the strict anion testing sequence: test for carbonate FIRST. Testing sulfate or halides before removing carbonate causes false white precipitates (BaCO₃ and Ag₂CO₃).
Aluminium hydroxide is amphoteric and dissolves in excess NaOH; magnesium hydroxide remains as an insoluble white precipitate.
Examiners engineer distractor options using common algebraic expansions and sign slips. Always test structural ratios or invariants before calculating arithmetic values.
Tested Topics in Specification C16
Master the individual question patterns and Sub-90 shortcuts for this specification.
Qualitative Ion Identification, Gravimetric Halide Precipitates & Strict Anion Test Sequence (CO₃²⁻ → SO₄²⁻ → X⁻)
Qualitative Anion Law: ALWAYS test Carbonate (HNO3 -> CO2 effervescence) -> Sulfate (Ba(NO3)2 -> white BaSO4) -> Halide (AgNO3 -> white AgCl / cream AgBr / yellow AgI) using NITRIC acid only.
Ready to Master Specification C16 Under Exam Conditions?
Practice timed questions under the authentic Pearson VUE interface with live split stopwatch pacing, dry-wipe scratchpad, and distractor forensics.