Domain: Healthcare — Medicine
Target concept: Acetate buffer resists pH changes upon addition of acetic acid.

Video generation prompt: Two transparent beakers are placed side by side: one contains an acetic acid-sodium acetate buffer solution, and the other contains distilled water. A pH indicator is added to both solutions. Acetic acid is then added dropwise to each beaker while gently swirling.

Evaluation guide:
(i) Target concept: Buffer resistance to pH change via common-ion effect.
Minimum mechanism: Presence of conjugate base (CH₃COO⁻) in buffer must suppress H⁺-driven dissociation shift; distilled water lacks any acid/base pair to absorb added H⁺, so [H⁺] rises immediately causing indicator protonation.
(ii)Storyline: Frame 1–2: Static shot showing two identically filled, labeled (or contextually distinguishable) beakers side-by-side, each already dosed with pH indicator; the distilled water shows the indicator's neutral (~pH 7) color, while the acetic acid–sodium acetate buffer shows the indicator's color at the buffer's own mildly acidic resting pH (near pH 4.7 for an equimolar mixture); the two starting colors need not match, and the buffer's baseline color is expected to differ from the water's unless the buffer is deliberately formulated with excess acetate to sit closer to neutral. Frame 3–4: Dropper approaches first beaker and releases one drop; simultaneously, same action begins at second beaker, no temporal lag. Frame 5–8: Swirling motion initiates in both beakers concurrently; distilled water color shifts stepwise per drop; buffer color remains close to its own starting hue. Frame 9–10: Final still shows stable contrast: the water beaker has shifted color, the buffer beaker looks essentially like it did at the start. Perspective: Fixed frontal medium shot, eye-level, no occlusion, full beaker height visible, lighting even and shadow-consistent.

Manual:
Experimental Procedure
1. Place two transparent beakers side-by-side on a white lab bench.
2. Fill left beaker with acetic acid-sodium acetate buffer; right beaker with distilled water.
3. Add identical drops of universal pH indicator to both beakers. In the distilled water, the indicator settles at its neutral (~pH 7) color (e.g., green); in the acetic acid–sodium acetate buffer, the indicator settles at whatever color corresponds to the buffer's own mildly acidic resting pH (around pH 4.7 for an equimolar acid:acetate mixture) — this starting color need not match the water's and may appear more yellow-green depending on the indicator used.
4. Using a dropper, add acetic acid dropwise to both beakers simultaneously while gently swirling each.
Observation of Experimental Phenomena
Distilled water beaker shows progressive color change (e.g., green → yellow → orange) as its pH drops sharply with each added drop; buffer beaker's color stays close to its own initial acidic-range hue throughout, changing far less than the water because the acetate/acetic acid equilibrium absorbs the added H⁺.
Summary of Experimental Principles
Acetate buffer resists pH changes upon addition of acetic acid due to equilibrium between CH₃COOH and CH₃COO⁻, whereas distilled water has no buffering capacity and undergoes an immediate, larger pH drop.

Prompt Grounding rubric:
level_1: Neither beaker appears, or only a single container is shown; no acetic acid bottle or dropper appears; the pH indicator is never added to either liquid; no drop-by-drop acid addition is shown; or the two beakers are never placed near each other for comparison.
level_2: Two transparent beakers appear near each other, but the setup is incomplete: the acetic acid dropper or bottle never appears, the indicator is added to only one beaker instead of both, acid is poured in all at once rather than a drop at a time, or neither beaker is ever swirled during the addition.
level_3: Two transparent beakers sit side by side, one holding the acetic acid–sodium acetate buffer and the other holding distilled water (distinguishable by label, dialogue, or context); pH indicator is visibly added to both before the acid addition begins; acetic acid is added drop by drop to both beakers with visible swirling in each, though the timing of drops or swirls between the two beakers need not match exactly.
level_4: All score 3 elements hold, and additionally the drops of acid land in both beakers at a matching pace and rhythm; the swirling in each beaker is continuous and happens at the same time as in the other, rather than one beaker being swirled while the other sits still; the two beakers stay in the same side-by-side arrangement throughout, without being moved, swapped, or blocked from view.
level_5: All score 4 elements hold, and additionally the two beakers match each other in size, scale, and lighting for the whole shot; the dropper tip stays aligned over whichever beaker's opening it is currently dosing; there are no cuts, zooms, or camera repositioning: the entire addition sequence plays out in one continuous shot from a fixed viewpoint.

Scientific and Causal Correctness rubric:
level_1: Neither beaker's color changes at all despite repeated drops of acid, giving no evidence of the expected contrast; or both beakers change color by a similar amount, erasing the buffer-vs-water distinction the demonstration is meant to show.
level_2: One beaker changes color, but not in the way the chemistry predicts: for example the buffer beaker's color shifts noticeably with the added drops while the distilled water changes very little or matches the buffer's shift; i.e., the buffering effect shows up in the wrong beaker, or is missing from both.
level_3: As acid is added drop by drop, the distilled water beaker's indicator color visibly and steadily shifts (e.g., moving from its starting hue toward a more acidic-range color); the buffer beaker's indicator color stays close to its own starting hue through the same additions, so a clear color difference between the two beakers is visible by the end of the sequence.
level_4: All score 3 conditions hold, and additionally the distilled water's color shift becomes more pronounced with each successive drop rather than jumping once and then stopping; the buffer's color stays essentially where it started, without drifting or fading partway through; nothing else in either beaker changes unexpectedly, such as sudden clouding, foaming, or precipitate forming.
level_5: All score 4 conditions hold, and additionally the buffer beaker's color holds steady from the first drop to the last, with no partial fading or brief shift at any point; the distilled water's color moves in one consistent direction across the whole sequence (e.g., steadily from green toward yellow), never reversing partway or jumping back toward its starting hue.

Spatiotemporal Consistency rubric:
level_1: A beaker vanishes and reappears in a different spot, or swaps places with the other beaker; the liquid level in a beaker changes abruptly between one frame and the next with no addition or removal shown; the swirling motion starts or stops instantly with no spin-up or slow-down; the lighting in the shot flips between bright and dim, or the shadows jump to a new direction, from one frame to the next.
level_2: A clear break in continuity occurs: a beaker shifts sideways by more than about 10% of its own width from where it started; the swirling in a beaker reverses direction abruptly without slowing first; the dropper tip disappears mid-drop and reappears a moment later; the shadow cast by a hand or beaker changes shape or direction in a way that doesn't match a single fixed light source.
level_3: The beakers stay in place in the same side-by-side arrangement throughout, without shifting or swapping; each beaker's swirl spins in one consistent direction without freezing or reversing; the dropper moves smoothly downward toward each beaker rather than jumping or stuttering; the liquid surface in each beaker moves naturally with the swirl, without warping or freezing mid-motion.
level_4: All score 3 conditions hold, and additionally the spacing between the two beakers stays the same throughout the shot, and the dropper stays roughly level with the tabletop rather than tilting erratically between frames; neither beaker nor its liquid meniscus stretches, squashes, or jitters from one frame to the next.
level_5: All score 4 conditions hold, and additionally the dropper's downward motion and the swirling hand's motion both look like they move at a steady, even pace rather than suddenly speeding up or slowing down; the swirl in each liquid builds up gradually rather than snapping instantly into a full vortex; background objects behind the beakers stay in the same visible relationship to the beakers throughout, with nothing popping in front of or behind them inconsistently.