Acid–base titration defense
This is a live exam at the bench. The student fills the burette, conditions the glassware, and titrates a standardized base into an acid of unknown concentration — stopping at the first faint, lasting color change. Then the guide starts asking: why that endpoint and not one drop earlier, what the indicator is actually reporting, how the molarity falls out of the numbers, and how far the answer could be off. There is no worksheet to copy and no figure to look up: the student stands over the flask and defends the run out loud.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Technique & glassware handling | Misreads the burette, fails to rinse or condition glassware, leaves an air bubble in the tip. | Reads the meniscus and swirls, but conditioning or rinsing is inconsistent and delivery is jerky. | Conditions and rinses correctly, reads the meniscus at eye level, and delivers drop-by-drop with control near the endpoint. |
| Endpoint & indicator judgment | Overshoots well past the color change or stops far too early; cannot say what the indicator shows. | Lands near the endpoint but overshoots by several drops, or explains the indicator only in vague terms. | Stops at the first lasting color change, and explains that the endpoint approximates the equivalence point and why phenolphthalein turns where it does. |
| Molarity calculation & sig figs | Sets up the mole ratio wrong or reports digits the data cannot support. | Reaches a plausible molarity but mishandles the stoichiometric ratio or carries too many sig figs. | Derives the unknown molarity from C₁V₁ and the balanced ratio, and reports it to the correct number of significant figures. |
| Error & uncertainty awareness | Treats the result as exact; names no sources of error. | Lists a source or two but cannot say which way they push the answer. | Identifies real sources — burette reading, overshoot, indicator choice — and reasons about their direction and size on the final value. |
| Oral defense under questioning | Folds at the first follow-up or recites a memorized line that does not fit the run. | Answers some follow-ups, falters when asked to justify a choice or a number. | Handles unrehearsed follow-ups about this run with sound, on-the-spot reasoning. |
“I stopped at the first pink that lasted thirty seconds, because that’s the closest I can get to the equivalence point without overshooting. From C₁V₁ and the one-to-one ratio the unknown is 0.0982 molar — three sig figs, because my burette only reads to the hundredth of a milliliter.”
“I stopped when it went pink. The answer is somewhere around 0.1, I think. I’m not really sure how the numbers turned into the concentration.”
This assessment is AI-proof by design: it happens at the bench, with a real burette and a real flask, in real time. No chatbot can swirl to a clean endpoint, justify a number it did not measure, or hold up under a follow-up question about a color change it cannot see. Mastery is shown by doing and defending — not by submitting.