Domain: Engineering — Food and Bioprocess Engineering
Target concept: Determination of Yogurt Fermentation Effectiveness (pH Colorimetric Method)

Video generation prompt: Take three test tubes, and add 2 mL of unfermented milk, fermenting milk, and fermented milk to each, respectively. Add two drops of bromothymol blue indicator to each tube.

Evaluation guide:
(i) Target concept: Visual confirmation of yogurt fermentation stage via pH-dependent colorimetric response.
Minimum mechanism: Bromothymol blue must remain chemically intact and fully soluble in each tube; the three milk samples must represent successive points on one natural fermentation timeline (fresh milk → partially soured milk → finished yogurt), so pH falls steadily from roughly 6.6-6.8 (unfermented) through roughly 6.0-6.2 (fermenting, still inside bromothymol blue's ~6.0-7.6 color-change band) down to roughly 4.2-4.6 (fully fermented, well past the indicator's yellow threshold); indicator must fully dissolve and equilibrate to display a stable, non-diffusing color in each tube.
(ii)Storyline: [Frame 0–2] Static wide shot showing three empty test tubes aligned left-center-right. [Frame 3–4] Pipette/dropper enters frame from top, dispenses two drops into left tube; drops contact liquid surface and disperse. [Frame 5–6] Dropper moves smoothly rightward, dispenses two drops into center tube: same contact/disperse behavior. [Frame 7–8] Dropper moves smoothly further right, dispenses two drops into right tube: same behavior. [Frame 9–10] Static hold: all three tubes fully in frame, colors stably displayed (green in the left tube, yellow-green/olive in the center tube, yellow in the right tube), no motion, no occlusion, full visibility of liquid column and color saturation. Perspective: fixed frontal close-up, eye-level, shallow depth of field focusing on liquid columns.

Manual:
Experimental Procedure
1. Place three clean test tubes side-by-side.
2. Add 2 mL unfermented milk to left tube, 2 mL fermenting milk to center tube, 2 mL fermented milk to right tube.
3. Add two drops of bromothymol blue indicator to each tube.
Observation of Experimental Phenomena
Left tube (unfermented milk) turns green; center tube (fermenting milk) turns yellow-green/olive; right tube (fermented milk) turns yellow.
Summary of Experimental Principles
Bromothymol blue is a pH indicator: yellow below pH 6.0, green near its neutral midpoint (roughly pH 6.8-7.0), and blue only above pH 7.6. Fresh milk is mildly acidic (~pH 6.6-6.8), so it reads green rather than blue. As lactic acid bacteria ferment lactose, pH falls steadily: partway through fermentation (~pH 6.0-6.2) the sample sits at the lower edge of the indicator's color-change band and reads yellow-green, while finished yogurt (~pH 4.2-4.6) is well below that band and reads a saturated yellow. The green → yellow-green → yellow progression tracks the drop in pH caused by lactic acid accumulation during fermentation.

Prompt Grounding rubric:
level_1: No test tubes are visible, or fewer than three appear; no milk-like liquid is present in any tube; no dropper/pipette or bromothymol blue drops are shown; none of the three intended samples (unfermented milk, fermenting milk, fermented milk) can be told apart by tube position or labeling.
level_2: Only one or two test tubes appear, or a milk-like liquid is visible in fewer than three tubes; the dropper/pipette is missing, or indicator drops are never dispensed; the three tubes are stacked, overlapping, or otherwise not shown as three distinct, separately visible vessels, so the three intended samples cannot be told apart.
level_3: Three test tubes stand side by side, each holding about 2 mL of liquid at a similar fill height; the left tube holds the unfermented-milk sample, the center tube the fermenting-milk sample, and the right tube the fermented-milk sample, in the left-to-right order the storyline establishes; a dropper delivers exactly two visible drops into each tube in turn, six drops total, and each drop is seen landing on the liquid surface rather than the tube wall.
level_4: Everything required for a 3 also holds, and additionally: the dropper visits the tubes in the correct left-to-center-to-right order with no tube skipped, revisited, or given an extra/missing drop; the three tubes keep their original spacing and upright orientation before and after dosing; no extra tube, spilled milk, or duplicate dropper appears in the shot.
level_5: Everything required for a 4 also holds, and additionally: tube spacing, milk fill level, and dropper approach angle look visually identical across all three dispensing actions, as if the same dosing motion were simply repeated at each tube; nothing in the frame (glare, hand, cord) ever hides a tube during or after dosing; the scene contains no extra props beyond the three tubes, the milk, and the dropper/indicator.

Scientific and Causal Correctness rubric:
level_1: The indicator produces no visible color change in any tube, or the same color appears in all three tubes; or the colors contradict bromothymol blue's real behavior (e.g., the right, most-fermented tube reads blue, or the left, unfermented tube reads yellow); there is no visible link between fermentation stage and hue.
level_2: Only one tube shows a color change while the other two stay colorless or unchanged; or the tube-to-tube color trend runs backward (the right, most-fermented tube looks less yellow / more blue-green than the left, least-fermented tube); or all three tubes converge on the same hue instead of three distinguishable ones.
level_3: After the indicator is added, the three tubes show three visually distinguishable hues that trend from green (or blue-green) on the left, through a yellow-green/olive middle tube, to yellow on the right, matching the order unfermented → fermenting → fermented; no tube shows a color outside this progression (e.g., no blue appears anywhere, since none of the milk samples is alkaline).
level_4: Everything required for a 3 also holds, and additionally: each tube's color is even from top to bottom (not visibly darker or a different shade near the bottom), with no cloudy, undissolved patch of indicator floating in any tube; the boundary between the three hues is a clean tube-to-tube difference rather than a gradual blend smeared across all three at once.
level_5: Everything required for a 4 also holds, and additionally: the left tube's green sits closer to blue-green than to yellow-green (consistent with the mildly acidic, near-neutral pH of fresh milk), the right tube's yellow looks fully saturated rather than pale (consistent with the much more acidic, fully fermented sample), and the center tube's hue sits visibly between the other two rather than matching either one; the three-tube hue order holds steady for the whole time all three are shown together, with no tube's color drifting into another's before the scene ends.

Spatiotemporal Consistency rubric:
level_1: Tubes jump to new positions between shots with no dropper motion explaining the change; a tube's liquid level changes with no visible pour or drop entering it; a drop appears already merged into the liquid with no visible fall; a tube's tilt or orientation flips from one frame to the next with no hand or support touching it; or the background behind the tubes changes abruptly between shots.
level_2: One tube's position shifts sideways between shots while the other two stay put; the dropper's path skips from one tube to another without passing over the tube in between; a drop leaves the dropper tip but never reaches the liquid surface, or vanishes just above it; the dropper's motion speeds up or stops in a way that doesn't match a smooth squeeze-and-release action; or the lighting or shadow on a tube shifts abruptly between frames (e.g., a shadow vanishing then reappearing), or a tube's cross-section visibly warps as if seen through inconsistent lens distortion.
level_3: The three tubes stay in the same positions and stand upright for the whole sequence; each liquid surface stays flat and still except for the brief ripple right after a drop lands; the dropper moves in one continuous path from the left tube to the center tube to the right tube; each drop can be seen falling and landing rather than appearing mid-air already merged with the liquid; no hard cut skips over the dispensing action.
level_4: Everything required for a 3 also holds, and additionally: the three tubes keep the same spacing and stand parallel to each other throughout, with no tube drifting closer to or farther from its neighbors; the dropper is held at roughly the same height and angle above each tube as it moves along the row; reflections and highlights on the glass shift only as much as the dropper's own movement would cause, with no unexplained new glare appearing on a tube the dropper hasn't reached yet.
level_5: Everything required for a 4 also holds, and additionally: the dropper's left-to-center-to-right movement reads as one smooth, evenly paced motion rather than speeding up, freezing, or resetting partway through; each drop's fall and the small ripple it makes on landing look physically ordinary (no oversized splash, no drop hanging motionless mid-fall); when the dropper passes in front of a tube it visibly blocks part of that tube rather than appearing to float in front of or behind the glass inconsistently from one moment to the next.