ARCH-C-13: Qualitative Ion Identification, Gravimetric Halide Precipitates & Strict Anion Test Sequence (CO3^2- → SO4^2- → X^-)

Full 15-Q Timed Drill

Step 1 of 6 • The Core Idea

ARCH-C-13: Why the 135s Classroom Method Fails Under the 90s Clock

Standard Classroom Method (Too Slow)~135s (Blows 90s Clock)

To avoid false-positive precipitates in sequential anion analysis, the strict order is **Carbonate

\rightarrow

Sulfate

\rightarrow

Halide using nitrates only**: (1) Add excess dilute

\text{HNO}_3\text{(aq)}

to test for and destroy

\text{CO}_3^{2-}

(preventing

\text{BaCO}_3

and

\text{Ag}_2\text{CO}_3

). (2) Add

\text{Ba(NO}_3)_2\text{(aq)}

to precipitate

\text{SO}_4^{2-}

\text{BaSO}_4\text{(s)}

(preventing

\text{Ag}_2\text{SO}_4

). (3) Add

\text{AgNO}_3\text{(aq)}

to test for halides (

\text{Cl}^-, \text{Br}^-, \text{I}^-

). Using

\text{HCl}

\text{H}_2\text{SO}_4

would contaminate the sample with

\text{Cl}^-

\text{SO}_4^{2-}

(~115s).

The Fast ShortcutTarget: 35s (Saves 100s)

**Line 1 (Strict Order

\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.
Line 2 (Nitrates Only): Use

\text{HNO}_3

and

\text{Ba(NO}_3)_2

so you never add

\text{Cl}^-

\text{SO}_4^{2-}

! (20s).

Where Students Lose Time: In 8 past UAT-UK questions (3.56% of Chemistry), candidates lose 100s by must acidify strictly with dilute hno3, and test in order: carbonate (hno3) -> sulfate (ba(no3)2) -> halide (agno3). Instead, apply C16 with Maths 1 skill M7.7.

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