Domain: Healthcare — Digestive Physiology
Target concept: Effect of pH on Salivary Amylase Activity

Video generation prompt: Add equal amounts of salivary amylase to two test tubes containing starch solution, one with acidic solution (pH=2) and the other with near-neutral solution (pH=7). After incubation at body temperature, add iodine solution to demonstrate the procedure.

Evaluation guide:
(i) Target concept: Enzyme activity is pH-dependent; human salivary amylase has an activity optimum near neutral pH (about 6.7-7.0) and is essentially inactivated under strongly acidic conditions (pH=2). Minimum mechanism: Iodine forms a blue-black complex with intact starch polymer; loss of that blue-black color indicates the starch has been hydrolyzed into dextrins/maltose/glucose, which no longer bind iodine the same way. At pH=2 the enzyme is denatured and cannot hydrolyze starch, so iodine added to that tube produces blue-black. At pH=7, close to the enzyme's natural optimum, the enzyme stays active and hydrolyzes the starch during incubation, so iodine added to that tube produces yellow/brown/buff instead of blue-black (a real, textbook-consistent contrast, unlike a pH=2-vs-pH=10 pairing where both extremes would similarly inactivate the enzyme).
(ii)Storyline: Frame 0-1: Two empty test tubes enter view, side-by-side, stationary. Frame 2-3: Identical starch solution added to both tubes (visible meniscus rise). Frame 4: Acidic solution (red-tinted, pH=2) added to left tube. Frame 5: Near-neutral solution (pH=7, green-tinted) added to right tube. Frame 6-7: Salivary amylase (clear liquid) added to both tubes. Frame 8: Static pause (incubation implied: no motion, no label change, no temperature display). Frame 9: Iodine (reddish-brown) added simultaneously to both tubes. Frame 10-12: Color develops: left tube homogeneously blue-black; right tube homogeneously yellow/buff. Perspective: fixed frontal view, eye-level, centered on tube pair; full tube height visible; no occlusion; labels readable; lighting even with soft directional highlight on tube curvature.

Manual:
Experimental Procedure
1. Place two identical test tubes side-by-side.
2. Add equal volumes of starch solution to both tubes.
3. Add acidic solution (pH=2) to the left tube; add near-neutral solution (pH=7) to the right tube.
4. Add equal volumes of salivary amylase to both tubes.
5. Incubate both tubes at body temperature (implied static pause).
6. Add iodine solution to both tubes simultaneously.
Observation of Experimental Phenomena
Left tube (pH=2): remains deep blue-black after iodine is added, because the strongly acidic pH denatures salivary amylase before it can hydrolyze the starch.
Right tube (pH=7): turns yellow/brown/buff after iodine is added, because this pH is close to the enzyme's natural optimum (about 6.7-7.0), so the amylase stays active and hydrolyzes the starch into iodine-negative sugars during incubation.
Summary of Experimental Principles
Salivary amylase activity is strongly pH-dependent, peaking near neutral pH (about 6.7-7.0) and being essentially abolished under strongly acidic conditions (pH=2). Comparing an inactivating extreme (pH=2) against the enzyme's near-optimal pH (pH=7) produces a genuine, scientifically accurate color contrast in the iodine-starch test, rather than the original pH=2-vs-pH=10 pairing, whose two extremes would not reliably differ.

Prompt Grounding rubric:
level_1: No test tubes are shown, or salivary amylase is never dispensed into either tube; the starch solution is missing from one or both tubes; neither tube is ever identified as the acidic (pH=2) or the near-neutral (pH=7) condition; iodine is never added; the two tubes are never shown together in the same shot.
level_2: One test tube is present, but only one of the two pH conditions is shown; salivary amylase is added to only one of the two tubes; iodine is added to only one tube; the two tubes are never framed side by side; there's no visible way to tell the two conditions apart.
level_3: Two test tubes sit side by side in the same shot; salivary amylase is added to both; starch solution is visible in both; the acidic (pH=2) solution goes into one tube and the near-neutral (pH=7) solution goes into the other, each introduced separately (e.g., via a labeled dropper or a distinctly colored liquid); iodine is added to both tubes after the incubation pause; every required element (tubes, enzyme, starch, the two pH solutions, iodine) is present and matched to the right tube.
level_4: Everything at the 3-point level holds, and in addition both tubes stay in the same spot for the whole clip (no repositioning or removal); the steps happen in order for both tubes: starch first, then the pH solution (acidic to one tube, near-neutral to the other), then enzyme, then a held static pause for incubation, then iodine added to both at essentially the same time; no third tube, stray pipette, or other extra object appears in the shot.
level_5: Everything at the 4-point level holds, and in addition both tubes match in size, shape, and labeling style; the enzyme is added to both using what looks like the same dropper and a similar number of drops; the pH solutions are each delivered with a visibly consistent method (e.g., a red-tinted liquid for the acidic tube, a green-tinted liquid for the near-neutral tube, both added with a similar dropper motion); the iodine is added to both tubes in a similar volume and dispersion; nothing (hand, reflection, stray object) blocks the view of either tube's liquid color changing.

Scientific and Causal Correctness rubric:
level_1: After iodine is added, both tubes end up the same color (either both stay blue-black or both turn yellow/brown), so there's no visible difference between the acidic (pH=2) and near-neutral (pH=7) tubes; or neither tube's color changes from iodine's usual reddish-brown at all.
level_2: The two tubes do end up different colors, but the assignment is backwards or inconsistent, e.g., the pH=2 tube turns yellow/brown while the pH=7 tube stays blue-black, or the color difference shows up before the iodine is even added.
level_3: After iodine is added, the pH=2 tube ends up a clear blue-black (matching an iodine-starch complex, meaning its starch was not digested), while the pH=7 tube ends up a distinctly different yellow/brown/buff (meaning its starch was digested); the two colors are visually distinguishable at the end of the clip, not a blended in-between shade.
level_4: Everything at the 3-point level holds, and in addition neither tube shows this color difference before the iodine is added (the two tubes still look alike right up until the iodine goes in); once each tube's final color appears it stays even throughout that tube's liquid, without one part of the tube looking different from another part.
level_5: Everything at the 4-point level holds, and in addition the pH=2 tube's blue-black is a deep, saturated shade typical of an iodine-starch reaction, and the pH=7 tube's yellow/brown is a warm, light shade typical of iodine with no intact starch left to bind; both of these final colors hold steady, without fading or drifting toward each other, for the last several seconds of the clip.

Spatiotemporal Consistency rubric:
level_1: One of the test tubes changes position or angle between shots with no visible cause; starch, pH solution, enzyme, or iodine appears in a tube with no pour or drop shown leading up to it; the color change from iodine appears to happen before the iodine actually enters the tube; the shot cuts abruptly between a wide view and a close-up with no transition; the lighting abruptly switches direction or intensity with no visible source change (e.g., a top-lit frame suddenly reads as side-lit); a tube's cylindrical shape visibly warps or stretches out of its normal form.
level_2: The two tubes swap left/right position partway through the clip; the iodine dropper is shown fully extended over a tube and then fully withdrawn with no visible dispensing motion in between; the liquid level in either tube changes with no pour visible; the incubation pause is shown as an instant cut rather than a held, static shot.
level_3: Both test tubes stay in the same spot and orientation, side by side, for the entire clip; nothing appears in or vanishes from either tube without a visible pour or drop causing it; each step (adding starch, adding the pH solution, adding enzyme, the incubation pause, adding iodine) is shown as one continuous action or a clearly held static shot, not a jump cut; neither tube's shape or liquid surface warps or distorts.
level_4: Everything at the 3-point level holds, and in addition each addition (starch, pH solution, enzyme, iodine) follows a smooth dropper or pour motion into the tube rather than the liquid appearing already inside; the liquid added forms a normal falling stream or drop rather than floating blobs or scattered droplets; the meniscus in each tube settles naturally right after each addition; neither tube's edges or labels flicker between adjacent frames.
level_5: Everything at the 4-point level holds, and in addition the order of additions is unambiguous and consistent for both tubes: starch before the pH solution, the pH solution before the enzyme, the enzyme before the incubation pause, and iodine only after that pause has visibly held still for at least a second; the final color in each tube (blue-black on the left, yellow/brown on the right) becomes visible gradually as the iodine mixes in, rather than snapping instantly to its final shade; nothing about the motion or timing obscures the moment the iodine actually contacts each tube's liquid.