Timed qualitative analysis
The student is handed two or three labeled unknowns and a clock. Working against time, they run a flame test, add reagents to force precipitates, and apply solubility rules to narrow the field — deciding which test to run next based on what the last one showed. At the end they name each ion or salt and justify it from the evidence on the bench. There is nothing to copy and no key to consult: the unknowns are real, the time is real, and the identification has to hold up.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Systematic procedure | Runs tests at random and loses track of which unknown is which. | Has a rough plan but skips a confirmatory step or repeats tests inefficiently. | Follows a logical sequence, letting each result decide the next test, and tracks every unknown without confusion. |
| Flame-test interpretation | Cannot connect a flame color to an ion, or contaminates the wire loop. | Reads obvious colors (yellow, lilac) but hesitates on close calls like strontium vs. lithium. | Cleans the loop between samples and reads flame colors confidently — sodium yellow, potassium lilac, copper green, strontium crimson — allowing for masking. |
| Precipitation & solubility reasoning | Adds reagents with no prediction; cannot say why a precipitate formed. | Predicts some precipitates but misapplies a solubility rule or misreads a color. | Uses solubility rules to predict and confirm precipitates — e.g. white AgCl with chloride, yellow PbI₂ with iodide — and explains each one. |
| Safety & technique under time pressure | Rushes into spills, mislabels tubes, or ignores goggles and reagent hazards. | Generally safe but gets sloppy as the clock runs — overfills tubes or crowds the workspace. | Stays deliberate under pressure: goggles on, tubes labeled, small volumes, clean workspace, reagents handled correctly throughout. |
| Identification & justification | Guesses an identity unsupported by the tests run. | Names the unknown but cites only one piece of evidence, or hedges between two ions. | Names each ion or salt and justifies it by chaining the flame, precipitation, and solubility evidence into a single conclusion. |
“The flame burned lilac and the solution gave no precipitate with chloride, so it’s a potassium salt, not sodium — sodium would have flooded the flame yellow. I ran the confirming test before I committed to the ID.”
“It turned a color in the flame, so it’s some kind of metal. I’d have to guess which one without looking it up.”
This assessment is AI-proof by design: it happens at the bench, with real unknowns, against a real clock. No chatbot can clean a wire loop, watch a precipitate drop, or decide the next test while the timer runs. The unknowns differ from student to student, so there is no answer to look up — mastery is shown by identifying and justifying in person, not by submitting.