Domain: Healthcare — Medicine
Target concept: pH stabilization effect of buffer solutions (formulation pH adjustment)

Video generation prompt: Two transparent beakers are placed side by side on a table, both containing a pink liquid from phenolphthalein indicator. The left beaker holds an ammonia-ammonium chloride buffer solution, pink at its own natural pH; the right beaker holds an equal volume of distilled water that was separately made pink beforehand with a trace of sodium hydroxide. Equal small amounts of vinegar are added to both beakers at the same time, and the beakers are gently shaken.

Evaluation guide:
(i) Target concept: Visual demonstration of pH stabilization via a buffer's reserve capacity, contrasted against a solution with no such reserve, using two liquids engineered to start from the same visible pink phenolphthalein state through different chemistry.
Minimum mechanism: Phenolphthalein is colorless below ~pH 8.2 and pink above it. The left beaker's ammonia-ammonium chloride buffer sits at roughly pH 9.5-10 purely from its own formulation (the ammonium ion's pKa, ~9.25, lies inside this window), giving it a genuine ammonia reserve able to absorb the vinegar's acetic acid and stay above 8.2. The right beaker's distilled water is only transiently pink from a trace of added NaOH and has no such reserve, so the same small vinegar addition drops it back below 8.2.
(ii) Storyline: Frame 0-1: Static wide shot of the two side-by-side beakers, both already pink (left from the ammonia buffer's own pH, right from an earlier trace NaOH addition). Frame 2: A hand enters and dispenses equal small vinegar droplets into each beaker at the same time, with no lag between the two. Frame 3: Both beakers are given a brief, gentle horizontal shake; the liquid sloshes in each. Frame 4: once the shaking stops, the left (ammonia buffer) beaker is still pink and the right (water) beaker has turned colorless. The whole sequence is shown from one fixed frontal viewpoint with both beakers fully visible; no camera movement, no occlusion, no cuts.

Manual:
Experimental Procedure
1. Place two identical transparent beakers side by side on a flat table.
2. Prepare the left beaker with an ammonia (NH3)-ammonium chloride (NH4Cl) buffer solution formulated with ammonia in roughly 5:1 molar excess over ammonium chloride, at a total buffer strength of several hundred millimolar to about 1 mol/L. This composition gives the solution an inherent pH near 9.5-10, comfortably inside phenolphthalein's pink range (~8.2-10) from its own chemistry, with a large ammonia reserve relative to the vinegar added later. Add phenolphthalein; the left beaker is pink without any further pH adjustment.
3. Fill the right beaker with an equal volume of distilled water and add phenolphthalein. Separately from the left beaker's preparation, add dilute sodium hydroxide dropwise only until the water turns pink; because plain water has no acid/base reserve, a mere trace of NaOH is enough to cross the ~8.2 threshold.
4. Simultaneously add equal small measured amounts of vinegar to both beakers.
5. Gently shake both beakers for about 1 second.
Observation of Experimental Phenomena
The left beaker (ammonia buffer) stays pink; the right beaker (NaOH-treated distilled water) turns colorless.
Summary of Experimental Principles
A solution only resists an added acid's effect on pH where it holds a real reserve of conjugate acid and base near its own pKa. The ammonium ion's pKa (~9.25) falls inside phenolphthalein's pink-to-colorless transition window, so the ammonia-ammonium chloride buffer can be formulated to be genuinely pink and, at the same time, to hold a real ammonia reserve able to neutralize the acetic acid in the added vinegar, keeping the pH above ~8.2. Plain distilled water has no such reserve: a trace of NaOH can push its pH into the pink range, but that same trace shift is easily undone by the same small vinegar addition, dropping the pH back below ~8.2 and leaving the indicator colorless.

Prompt Grounding rubric:
level_1: Fewer than two beakers are shown, or the beakers are not placed side by side, or no pink phenolphthalein-tinted liquid is visible in either beaker, or the left beaker is missing the ammonia-ammonium chloride buffer, or the right beaker is missing distilled water, or vinegar is never added, or vinegar is added to only one of the two beakers.
level_2: Two beakers are shown side by side, but at least one setup detail is wrong: the left beaker's liquid is colorless rather than pink (missing its natural ammonia-buffer pink state), or the right beaker's liquid was never made pink beforehand, or vinegar is added to the two beakers at clearly different times instead of together, or no shaking follows the vinegar addition.
level_3: Two transparent beakers stand side by side on the table; the left beaker holds a pink liquid representing the ammonia-ammonium chloride buffer and the right beaker holds an equal-volume pink liquid representing the NaOH-treated distilled water, phenolphthalein's pink coloration visible in both before any vinegar is added; equal small amounts of vinegar are added to both beakers at the same time; both beakers are then given a gentle shake.
level_4: Everything described for level 3 is present, and in addition: the two beakers hold visibly equal liquid volumes throughout, the vinegar is dispensed into both beakers within the same continuous shot with no cut separating the two additions, and the shaking given to each beaker matches the other in duration and vigor.
level_5: Everything described for level 4 is present, and in addition: both beakers stay completely within frame for the whole sequence with no cropping or drifting out of shot, the liquid volumes visibly match before and after the vinegar is added, no rendering glitch (flicker, transparency error, a beaker's contents mismatching its label) breaks either beaker's identity, and the entire sequence plays out as one uninterrupted take.

Scientific and Causal Correctness rubric:
level_1: After the vinegar is added, the two beakers end up showing the same outcome as each other (both stay pink, or both fade to colorless), or a color change happens before any vinegar is added, or the fading happens in the ammonia-buffer (left) beaker while the plain-water (right) beaker keeps its pink, reversing the intended contrast.
level_2: Phenolphthalein's pink coloration is visible in at least one beaker before the vinegar is added, but the post-vinegar result does not follow the buffer chemistry: both beakers fade together, both remain pink together, or the correct beaker keeps its color while the other one shows an inconsistent partial change.
level_3: Both beakers show phenolphthalein's pink coloration before the vinegar is added; once the vinegar is added and the beakers are shaken, the left beaker (ammonia-ammonium chloride buffer) keeps its pink coloration because its ammonia reserve neutralizes the incoming acetic acid near the ammonium ion's own pKa (~9.25), while the right beaker (NaOH-treated distilled water, with no acid/base reserve) loses its pink coloration and turns colorless; the two contrasting end states are clearly visible and hold steady for at least a second after the shaking stops.
level_4: Everything described for level 3 is present, and in addition: the left beaker's pink tone looks the same in hue and depth before and after the vinegar is added, the right beaker's loss of color is complete across its whole liquid volume with no leftover pink patch or gradient, and no intermediate shade (such as pale pink or lavender) appears in the right beaker at any point during the change.
level_5: Everything described for level 4 is present, and in addition: the left beaker shows no detectable drift away from its original pink coloration for the rest of the sequence, the right beaker's color loss happens uniformly across its entire liquid volume rather than as a spreading front, ring, or streak, and both beakers reach their final color state at essentially the same moment relative to when the shaking ends.

Spatiotemporal Consistency rubric:
level_1: A beaker's position, size, or orientation suddenly changes between one frame and the next with no shaking motion to explain it; one of the two beakers disappears or a new one appears partway through the sequence; the room lighting or shadow direction switches abruptly with no visible cause, such as the apparent light source seeming to jump to the opposite side; or the table or background warps or distorts.
level_2: A clear spatial or motion problem is visible: a beaker shifts sideways by more than half its own width between frames outside of the shaking action; the shaking motion stutters, freezes, or jumps rather than moving continuously; the shadow cast by the hand or a beaker jumps to a position inconsistent with the light source; or the liquid surface breaks apart or snaps into a different shape instead of sloshing smoothly.
level_3: Neither beaker jumps position, disappears, or changes shape unexpectedly; the two beakers keep the same left-right arrangement relative to each other for the whole sequence; the shaking motion is continuous and physically plausible, with the liquid sloshing and the meniscus curving naturally rather than freezing rigid or breaking apart; and no beaker or hand pops in or out of the shot.
level_4: Everything described for level 3 is present, and in addition: the sloshing in both beakers starts, peaks, and settles at matching moments rather than one beaker lagging behind the other; the beaker rims stay aligned with the background table edge with no mismatched parallax; the two beakers' round outlines are not warped differently from each other by lens distortion; and the lighting stays steady with no flickering.
level_5: Everything described for level 4 is present, and in addition: the liquid surfaces move smoothly from frame to frame with no skipping or jerky jumps; the beaker outlines stay crisp and in focus throughout the shaking motion; and any reflected highlight on the curved glass moves smoothly across the surface rather than jumping or disappearing partway through.