UAT-UK SPEC CODE: C16 (Chemical tests)ASSUMED MATHS 1 PREREQUISITE: M7.7FREQUENCY: MEDIUM
Qualitative Ion Identification, Gravimetric Halide Precipitates & Strict Anion Test Sequence (CO3^2- → SO4^2- → X^-)
Deconstructed from 8 past UAT-UK questions (3.56% of all historical Chemistry papers). Students using standard A-Level classroom methods spend an average of 135s on this archetype—blowing past the 90s ESAT budget. Using the 2-line Sub-90 heuristic below compresses solve time to 35s (saving 100s per question).
Historical Frequency
8 Past Qs
3.56% of Chemistry
Classroom Trap Time
135s
+45s over 90s budget
Sub-90 Target Time
35s
Banks +100s on the clock
Dominant UI Format
Table Matching
Maths 1 Skill: M7.7
Card 1 & Card 2 Preview: The Classroom Time Trap vs. Sub-90 2-Line Rule
Excerpted from our interactive 6-Step Topic Lesson for ARCH-C-13.
CARD 1 OF 6 • THE CLASSROOM TIME TRAP (135s)
Why Students Bleed Time:
To avoid false-positive precipitates in sequential anion analysis, the strict order is **Carbonate Sulfate Halide using nitrates only**: (1) Add excess dilute to test for and destroy (preventing and ). (2) Add to precipitate as (preventing ). (3) Add to test for halides (). Using or would contaminate the sample with or (~115s).
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.
CARD 2 OF 6 • THE 2-LINE SUB-90 HEURISTIC (<35s)
**Line 1 (Strict Order ):** Each step removes the ion that would give a false precipitate in the next step.
Line 2 (Nitrates Only): Use and so you never add or ! (20s).
Sanity & Limit Check: Verify units/signs against
multi_statement_linchpin_foil (Adding HCl or H2SO4 before Ba²+ or Ag^+ tests (introducing foreign Cl^- or SO4²- ions that cause false-positive precipitates)) and table_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)).Examiner Distractor Blueprint for ARCH-C-13
FOIL #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).
FOIL #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.
FOIL #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.
FOIL #4factor_of_two_or_stoichiometry_trap
Using a 1:1 instead of 1:2 mole ratio when precipitating divalent metal halides MCl2 + 2AgNO3 -> 2AgCl(s) in gravimetric mass questions
Each mole of MCl2 or BaCl2 yields 2 moles of AgCl (M_r = 143.5).
Try 1 Live Sample Question Under the 90-Second Stopwatch
Press W to open the 2-line dry-wipe scratchpad, select A–E, and lock in to reveal the Dual-View solution.
sub90 ESATARCH-C-13: Qualitative Ion Identification, Gravimetric Halide Precipitates & Strict Anion Test Sequence (CO3^2- → SO4^2- → X^-) (Sample)Chemistry
00:00.0
SUB-90 ZONE (<35S)
0.0s35s Sub-9090s ESAT Budget150s Classroom
Right-click any option to cross out • Keys
A–E + EnterQuestion 1 of 1 • 90s Target
Ready to Start Question 1?
The question stem, diagram, and answer options are hidden behind this pre-start gate. When you click below, the question will reveal and your 90-second split stopwatch will immediately begin ticking.
Chemistry•ARCH-C-13•Standard 5-Option
or press Enter to start