Domain: Healthcare — Nutrition and Food Hygiene
Target concept: Benzoic Acid Determination

Video generation prompt: A 10 mL beverage sample containing excessive benzoic acid was placed in a test tube. Bromocresol green reagent was added, and the pH was adjusted to 4 with acetic acid. The mixture was then shaken and allowed to stand.

Evaluation guide:
(i) Target concept: Visual confirmation of pH 4 acidification via bromocresol green colorimetric response in a beverage matrix.
Minimum mechanism: Bromocresol green must exist in two spectrally distinct, stable protonation states (blue-green deprotonated, yellow protonated), with equilibrium shift driven solely by [H+] concentration, requiring both reagent presence and acid addition to trigger irreversible color transition under observation time.
(ii)Storyline: Frame 1: Static shot of empty test tube → Frame 2: 10 mL beverage poured in (meniscus visible) → Frame 3: Dropper dispenses bromocresol green (blue-green liquid entering tube) → Frame 4: Acetic acid added (clear liquid, no color change yet) → Frame 5: Hand shakes test tube (liquid sloshes, no splashing) → Frame 6: Test tube placed upright on bench, fully still → Frame 7: Yellow color uniformly fills tube, stable for ≥1 s. All frames share fixed frontal bench-level viewpoint; no occlusion of tube body; lighting constant; no external interference (e.g., air currents, vibration).

Manual:
Experimental Procedure
1. Place 10 mL beverage sample into a clean test tube.
2. Add bromocresol green reagent using a dropper.
3. Add acetic acid dropwise until pH ≈ 4 (visualized by color shift).
4. Cap and shake vigorously for uniform mixing.
5. Stand test tube upright and observe color stabilization.
Observation of Experimental Phenomena
Bromocresol green changes from blue-green (basic/neutral) to yellow (acidic, pH ≤4), confirming acidic condition suitable for benzoic acid extraction.
Summary of Experimental Principles
Bromocresol green is a pH indicator (pKa ≈ 4.7); its yellow form dominates below pH 4.0, providing visual confirmation that the sample environment is properly acidified for subsequent benzoic acid isolation via solvent extraction.

Prompt Grounding rubric:
level_1: No test tube is shown; no beverage sample is visible inside it; no bromocresol green reagent or dropper appears; no acetic acid container is shown; no pH-adjustment step, shaking, or standing phase is depicted.
level_2: A vessel appears but is the wrong type (e.g., a beaker or flask rather than a test tube); the beverage sample is shown but not clearly held as a small (about 10 mL) sample; bromocresol green is shown but never actually added to the tube; acetic acid is shown but no addition step follows; the shaking or standing phase is missing or replaced by an unrelated action such as pouring or stirring with a rod.
level_3: A test tube holding roughly 10 mL of beverage sample is shown; a dropper visibly adds bromocresol green reagent into the tube; a container of acetic acid is visibly used to add acid to the tube; the sequence clearly separates an active phase (adding reagents and shaking) from a passive phase (the tube standing still); no vessel is substituted for the test tube at any point.
level_4: Everything required at the 3-point level is present, AND the physical layout is plausible: the reagent containers sit on the same bench surface next to the test tube, the dropper tip is held directly over the tube's mouth while dispensing, the test tube stays upright and fully visible while being shaken, and during the standing phase no hand touches the tube and no motion blur appears.
level_5: Everything required at the 4-point level is present, AND the steps unfold in the correct order with no repeats or skips (beverage sample placed in the tube, then bromocresol green added, then acetic acid added, then shaking, then the tube set down and left undisturbed), all captured from one unbroken camera angle with no cut, zoom, or repositioning, and with no object entering or leaving the frame partway through.

Scientific and Causal Correctness rubric:
level_1: The liquid in the tube shows no color at all, or stays the same color throughout with no shift toward yellow; bromocresol green itself never shows its characteristic blue-green color before being added; the mixture looks the same before and after the acetic acid is added.
level_2: Bromocresol green is added but the tube's liquid looks colorless or an implausible hue (not blue-green); after acetic acid is added the liquid still shows no color shift; shaking produces no visible blending of color; the standing phase shows no settled, uniform appearance (e.g., visible streaks or an uneven color persist).
level_3: Before addition, the bromocresol green reagent itself is visibly blue-green; once it and the acetic acid have both been added to the beverage sample and the tube is shaken, the liquid turns a visibly yellow color, consistent with bromocresol green in its acidic (roughly pH 4) form; shaking blends the color evenly with no streaks; during standing, the liquid stays a uniform yellow with no layering, cloudiness, or precipitate.
level_4: Everything required at the 3-point level is present, AND the color change is tied to the actual chemistry rather than to the order the reagents were added: the liquid is still blue-green right after bromocresol green is added (before any acid goes in), and only turns yellow once acetic acid has actually brought the mixture to about pH 4; this holds regardless of whether the bromocresol green or the acetic acid was added first, since the final color reflects the resulting pH, not the addition order; the standing phase shows continuous stillness with no bubbling, fizzing, or sediment forming.
level_5: Everything required at the 4-point level is present, AND the indicator's two colors never blur together at the wrong time: it stays only blue-green while sitting in its own dropper or bottle, and only turns yellow once mixed into the acidified beverage sample; no in-between shade (e.g., teal, olive, or orange) appears at any point, and the yellow color looks uniform across the whole liquid volume during standing.

Spatiotemporal Consistency rubric:
level_1: The test tube or dropper vanishes from view and then reappears; the tube jumps from one spot on the bench to another between frames with no hand carrying it there; the liquid's level or color changes abruptly with no visible cause; the tube appears to rotate through a large angle with no visible hand motion causing it; the lighting in the shot suddenly brightens or dims between frames with no reagent addition, shaking, or other visible cause.
level_2: The test tube's position on the bench shifts noticeably between the shaking and standing phases with no visible hand movement to explain it; a reagent bottle leaves the frame and comes back; the liquid's level changes without a visible pour or shake causing it; the shadows on the bench swing to a new direction with no change in the light itself; background objects flicker or distort.
level_3: The test tube stays visible and in the same spot on the bench relative to its surroundings throughout; the tube, dropper, and bottles keep the same size and orientation across the whole sequence; shaking shows a smooth hand motion with the liquid visibly sloshing in response; once the tube is set down, the liquid surface stays flat and still with no ripples or drift.
level_4: Everything required at the 3-point level is present, AND the shaking motion looks physically continuous (starting from rest, building up, and settling back to stillness), with no motion blur so heavy it hides the liquid's color; no frames repeat or stutter; the light on the tube and its glass surface looks the same in direction and strength from the first frame to the last, without sudden glare or flat-looking patches appearing.
level_5: Everything required at the 4-point level is present, AND the glass of the test tube does not visibly warp or bend anything seen behind it; the liquid's meniscus keeps a natural curve throughout; the dropper tip never appears to merge into or pass through the tube's rim; no part of the tube, hand, or bottles is awkwardly cut off at the frame's edge; the hand's shaking motion stays within a normal, comfortable range for holding a small test tube.