Domain: Healthcare — Pharmaceutical Chemistry
Target concept: Determination of Para-Aminobenzoic Acid

Video generation prompt: A small amount of p-aminobenzoic acid powder was dissolved in 1 mL dilute hydrochloric acid and cooled to 0–5°C in an ice bath. Sodium nitrite solution was then added dropwise, the mixture was shaken evenly, and then alkaline β-naphthol reagent was added. Changes were observed.

Evaluation guide:
(i) Target concept: Diazonium coupling reaction producing a visible red azo dye precipitate.
Minimum mechanism: Cold acidic diazotization must precede alkaline coupling; precipitate formation requires both chemical steps and is inherently irreversible, flocculent, and red-hued under standard conditions.
(ii)Storyline: [Frame 0–2] White powder contacts colorless acid → dissolves fully → solution remains clear. [Frame 3–4] Tube placed into ice bath → visible condensation develops on outer wall. [Frame 5–6] Dropper tip positioned above tube → spherical droplet detaches → falls → impacts surface → merges. [Frame 7–8] Colorless-to-pale β-naphthol reagent dispensed → contacts solution → immediate localized red floc formation at entry point → expands downward → settles as dense red particles. All events occur from fixed frontal viewpoint; no object leaves frame; precipitate remains optically distinct from solution throughout.

Manual:
Experimental Procedure
1. Place white p-aminobenzoic acid powder into a clear test tube.
2. Add 1 mL dilute hydrochloric acid; swirl to dissolve completely.
3. Immerse tube in ice bath until external condensation forms.
4. Add sodium nitrite solution dropwise with visible spherical droplets entering solution.
5. Add alkaline β-naphthol reagent (colorless to pale yellow before contact with the solution) after nitrite addition completes.
Observation of Experimental Phenomena
Deep red or rust-red flocculent precipitate forms immediately after alkaline β-naphthol addition, settling as dense particles in colorless supernatant.
Summary of Experimental Principles
p-Aminobenzoic acid undergoes diazotization in cold acidic medium with NaNO₂, forming diazonium salt; subsequent coupling with alkaline β-naphthol yields a red azo dye precipitate — a confirmatory test for aromatic primary amines.

Prompt Grounding rubric:
level_1: None of p-aminobenzoic acid powder, dilute hydrochloric acid, the ice bath, sodium nitrite solution, or alkaline β-naphthol reagent appear on screen; no container is shown; no dissolving, cooling, dropwise addition, or final reagent-mixing step is depicted.
level_2: At least one of the required items appears (e.g., white powder, or a clear liquid in a tube, or an ice bath), but none of the required actions are shown: no dissolving of the powder, no sign of cooling, no dropwise addition of anything, and no later addition of a second reagent.
level_3: All five required items appear (white p-aminobenzoic acid powder, dilute hydrochloric acid in a clear tube, an ice bath around the tube, sodium nitrite solution being added from a dropper, and a separate alkaline β-naphthol reagent), and the four actions (dissolving, cooling, dropwise nitrite addition, final reagent addition) all happen, but something is out of order or misrepresented: e.g., the β-naphthol reagent is added before the sodium nitrite, the powder is shown as a color other than white, or the β-naphthol reagent itself is shown already deep red or orange before it ever touches the solution.
level_4: Beyond the level-3 requirements, all five items look right: white crystalline powder, colorless acid solution, a visibly frosted/condensation-covered tube, a clear sodium nitrite solution in the dropper, and a colorless-to-pale (not yet colored) alkaline β-naphthol reagent in its own labeled container; and the four steps occur in the correct order: powder dissolving in the acid, tube placed in the ice bath, sodium nitrite added dropwise with visible droplets entering the solution, then the β-naphthol reagent added afterward; at most one small flaw remains, such as the dropper held at an odd angle without affecting where the drops land, or a brief flicker in the shot during the addition.
level_5: Beyond the level-4 requirements, every item and action is unambiguous: the powder fully dissolves on contact with the acid; the tube sits in an ice bath that visibly develops frost or condensation; sodium nitrite arrives as distinct round droplets that fall one at a time into the solution; the β-naphthol reagent is visibly colorless or only faintly tinted right up until the moment it meets the diazonium solution, and it is added only after the sodium nitrite step is finished; nothing hides the dropper tip or the tube, no reagent container is mislabeled, and the whole sequence plays out in one continuous scene from a fixed viewpoint.

Scientific and Causal Correctness rubric:
level_1: The solution stays colorless for the entire clip, or any color/precipitate that does appear is unrelated to the reagent sequence: for example, a color change happens before the sodium nitrite or β-naphthol steps, or nothing changes even after both reagents have been added.
level_2: A color or precipitate does appear, but not the way the reaction demands: e.g., it shows up before the β-naphthol reagent is added, or before the sodium nitrite step, or a solid forms without any preceding color change at all.
level_3: A red, reddish-brown, or rust-toned change appears only after the β-naphthol reagent goes in, but it either spreads evenly through the whole tube at once rather than starting where the reagent enters, or no actual solid/floc is visible (just a color shift in the liquid), or a solid forms but is not red-toned (e.g., white or pale).
level_4: Beyond the level-3 outcome, a red or rust-toned solid forms only after the β-naphthol reagent is added, first appearing where the reagent enters the liquid and spreading outward from there rather than all at once; the liquid stays colorless until the solid appears; no solid or color shows up earlier in the sequence.
level_5: Beyond the level-4 outcome, the reaction reads as fully faithful to the diazonium coupling: as soon as the β-naphthol reagent touches the solution, a deep red or rust-red flocculent solid forms right at the contact point and then settles as dense particles; the solution shows no yellow/orange in-between color on the way to red; the settled solid keeps its red color against the colorless liquid above it rather than fading or browning by the end of the clip.

Spatiotemporal Consistency rubric:
level_1: A basic setup element breaks partway through the clip: the ice bath is missing right after a shot showed it fully surrounding the tube, with no reason given; the dropper appears already positioned above the tube with no hand or arm bringing it into place; the liquid level inside the tube jumps up or down between two shots with no addition step shown to explain it; the tube's size, shape, or tilt changes abruptly from one shot to the next; or the lighting or shadow direction suddenly flips between shots with no cause.
level_2: The overall scene holds together, but one clear break remains: the tube rotates or tilts through a large angle (roughly more than 45°) with no hand or holder visible to cause it; a droplet is shown already hanging fully formed at the dropper tip with no squeeze or release shown beforehand; frost on the tube's outer wall appears fully formed in one shot with no earlier shot showing it building up; or the red precipitate is shown already spread through the whole tube in one shot with no earlier shot showing it starting at a single point.
level_3: The tube, ice bath, and dropper keep the same relative arrangement from the opening shot to the final settling shot; frost/condensation on the tube's outer wall builds up gradually across shots rather than appearing all at once; each droplet of sodium nitrite is shown traveling from the dropper tip down to the liquid before merging in; the red precipitate is shown beginning at the spot where the β-naphthol reagent enters the liquid and spreading outward from there, with at most one small hiccup, such as a droplet's fall looking briefly uneven or the tube's outline losing sharpness for a moment during a camera adjustment.
level_4: Beyond the level-3 continuity, the motion reads as fully smooth: every droplet's fall from tip to liquid surface is shown as one unbroken motion; the red precipitate grows outward from its entry point as a single connected patch rather than in separate jumps; and the tube and ice bath never noticeably shift position once they are placed, with at most one brief, clearly noticeable hitch (such as the video visibly freezing for an instant) rather than any hidden or repeated defect.
level_5: Beyond the level-4 continuity, the clip holds together from start to finish: the tube keeps the same size, shape, and sharpness in every shot; frost keeps building steadily on the glass without ever resetting or vanishing; each droplet keeps a round shape as it falls and visibly merges into the solution on contact; and the red floc precipitate keeps expanding from its point of origin and settling downward without any part of it jumping location or blinking in and out of view.