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UAT-UK SPEC: C16MODULE: CHEMISTRYTOPIC ID: ARCH-C-13

Qualitative Ion Identification, Gravimetric Halide Precipitates & Strict Anion Test Sequence (CO₃²⁻ → SO₄²⁻ → X⁻)

Master Qualitative Ion Identification, Gravimetric Halide Precipitates & Strict Anion Test Sequence (CO₃²⁻ → SO₄²⁻ → X⁻) on the ESAT Chemistry paper. Traditional classroom derivations waste valuable seconds. Using the 2-step Sub-90 invariant elimination method compresses solve time to 35s, saving +100s per question.

Standard Classroom Time
135s
Textbook multi-line derivations bleed exam clock
Sub-90 Target Pace
35s
Comfortably below the 88.8s per-question limit
Time Saved for Review
+100s
Banks clock buffer across 27 questions

2-Step Sub-90 Shortcut Formula

How to eliminate intermediate algebra and solve under 45 seconds.

Launch Interactive Lesson
STEP 1 • SETUP INVARIANT (LINE 1)10–15s
Line 1 (Strict Order CO32−→SO42−→X−\text{CO}_3^{2-} \rightarrow \text{SO}_4^{2-} \rightarrow \text{X}^-): Each step removes the ion that would give a false precipitate in the next step.

Establish the structural invariant and eliminate impossible options immediately.

STEP 2 • FAST SOLVE (LINE 2)15–25s
Line 2 (Nitrates Only): Use HNO3\text{HNO}_3 and Ba(NO3)2\text{Ba(NO}_3)_2 so you never add Cl−\text{Cl}^- or SO42−\text{SO}_4^{2-}! (20s).

Evaluate the simplified expression using integer factoring or unit elimination.

Distractor Trap Forensics

The predictable cognitive traps engineered by examiners into the wrong options.

TRAP #1multi_statement_linchpin_foil

Adding HCl or H2SO4 before Ba²⁺ or Ag⁺ tests (introducing foreign Cl⁻ or SO₄²⁻ ions that cause false-positive precipitates)

Must acidify strictly with dilute HNO3, and test in order: Carbonate (HNO3) -> Sulfate (Ba(NO3)2) -> Halide (AgNO3).

Sub-90 Fix: Test invariants before expanding equations.
TRAP #2table_column_pruning_trap

Reading all multi-step precipitates before pruning table rows via Flame Test colour (Li+=Crimson, Na+=Yellow, K+=Lilac, Ca2+=Orange-Red, Cu2+=Blue-Green) or AgX colour (White=Cl, Cream=Br, Yellow=I)

Flame test or AgX colour column eliminates 75% of table rows in 5 seconds.

Sub-90 Fix: Test invariants before expanding equations.
TRAP #3multi_statement_linchpin_foil

Failing to distinguish Al³⁺, Ca²⁺, and Mg²⁺ with NaOH(aq) (Al(OH)3 white precipitate dissolves in excess NaOH to form a colourless solution; Ca(OH)2 and Mg(OH)2 do not)

Amphoteric Al(OH)3 redissolves in excess NaOH, whereas Mg(OH)2 and Ca(OH)2 remain insoluble.

Sub-90 Fix: Test invariants before expanding equations.

Try 1 Timed Question: Qualitative Ion Identification, Gravimetric Halide Precipitates & Strict Anion Test Sequence (CO₃²⁻ → SO₄²⁻ → X⁻)

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Full Lesson All 15 Questions (ARCH-C-13)
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ARCH-C-13: Qualitative Ion Identification, Gravimetric Halide Precipitates & Strict Anion Test Sequence (CO₃²⁻ → SO₄²⁻ → X⁻) (Sample)Chemistry
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SUB-90 ZONE (<50S)
0.0s50s Sub-9090s ESAT Budget150s Classroom
Question 1 of 1ARCH-C-13 (6-Card Lesson)Format: Table-Row Matching
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Question 1 of 1 • 90s Target

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Chemistry•ARCH-C-13•Table-Row Matching
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