Domain: Natural Science — Biophysics
Target concept: Protein precipitation, resulting in the formation and sedimentation of visible solids from the solution.

Video generation prompt: On the experimental bench, a transparent beaker containing cell-free microbial culture medium was gently stirred, and then a concentrated salt solution was added. After stopping the stirring, the beaker was left undisturbed.

Evaluation guide:
(i) Target concept: Salt-induced protein precipitation in cell-free biological solution.
Minimum mechanism: Proteins must be present and soluble pre-addition; salt must be added post-stirring to ensure mixing-driven supersaturation; gravitational settling requires sufficient particle size/mass and time for Brownian motion to be overcome.
(ii)Storyline: [Frame 0–2] Beaker with clear medium at rest → [Frame 3–4] Gentle stirring creates transient vortex → [Frame 5] Stir rod fully withdrawn → [Frame 6] Salt solution contacts surface → [Frame 7–8] Localized turbidity appears at interface → [Frame 9–10] Turbidity expands downward, then discrete particles descend → [Frame 11–15] Particles accumulate at base, forming sharp sediment-supernatant boundary; all phases observed from fixed frontal bench-level perspective with unobstructed view of full beaker height.

Manual:
Experimental Procedure
1. Place transparent cylindrical beaker containing clear, colorless-to-pale-amber cell-free microbial culture medium on bench.
2. Gently stir medium with clean stir rod for ≤2 seconds.
3. Remove stir rod, then add concentrated salt solution dropwise to center of liquid surface.
4. Cease all motion; leave beaker undisturbed.
Observation of Experimental Phenomena
White or off-white particulate solids form at liquid-salt interface, then descend continuously under gravity and accumulate as a compact, level sediment layer at the beaker bottom; supernatant above becomes fully transparent.
Summary of Experimental Principles
Protein precipitation: addition of high-concentration salt disrupts hydration shells and electrostatic repulsion, reducing protein solubility and causing aggregation into visible, sedimentable solids.

Prompt Grounding rubric:
level_1: No transparent beaker is shown; the cell-free microbial culture medium is absent or not visibly held in a container; no stirring motion occurs; no concentrated salt solution is added; the beaker is never left undisturbed to rest.
level_2: A transparent beaker holding liquid appears, but the liquid does not read as a clear, colorless-to-pale-amber cell-free culture medium (e.g., it is opaque, an implausible color, or already contains visible particulates); stirring is shown but is vigorous or splashing rather than gentle; the concentrated salt solution is missing, added before stirring, or added while the stirring motion is still ongoing; the beaker never reaches a clearly undisturbed resting state.
level_3: A transparent, liquid-filled beaker sits fully within frame from a fixed bench-level viewpoint; the liquid reads as a clear, colorless-to-pale-amber cell-free culture medium; a stir rod or spoon gently agitates the liquid, is withdrawn, and only then is the concentrated salt solution added; after addition, all motion stops and the beaker is left undisturbed for the remainder of the clip; the beaker's screen position and the camera framing stay fixed throughout.
level_4: Everything required for score 3 is present, and in addition: the beaker is an upright cylindrical vessel; the stir rod is clearly lifted clear of the liquid before the salt solution is introduced; the salt solution's entry into the liquid is a distinct, visible contact event rather than a pour along the beaker wall that obscures where it lands; no incidental disturbances (tapping the bench, tilting or nudging the beaker, a second stir) occur during the resting phase; the beaker and any tools stay in the same relative screen positions from stirring through resting.
level_5: Everything required for score 4 is present, and in addition: the liquid surface stays visually calm during the resting phase, without ripples or a distorted meniscus once addition is complete; the salt solution enters in a single, brief, localized addition rather than a prolonged pour or visible splash; the bench background behind the beaker is plain and uncluttered, so the salt-contact point and the beaker's contents stay clearly visible; no stray objects, on-screen labels, reflections, or occlusions obstruct the beaker at any point in the clip.

Scientific and Causal Correctness rubric:
level_1: No observable state change occurs; the liquid stays uniformly clear and homogeneous throughout; no solid particles ever form or settle; the video shows no causal link between adding the salt solution and any subsequent visual change.
level_2: Some visible change occurs (e.g., clouding, a color shift, or bubbling), but it does not resemble protein precipitation (e.g., it looks like gas evolution, an oily separation, or crystal growth rather than a diffuse particulate solid); or particulate solids appear but never settle (they stay suspended, float, or redisperse); or particulates appear before the salt solution is added, or while stirring is still underway, breaking the salt-triggered causal sequence.
level_3: Only after the salt solution is added and stirring has stopped do visibly white or off-white particulate solids appear within the liquid and then sink under gravity to collect at the beaker's bottom; the solids stay non-transparent and clearly distinguishable from the surrounding liquid; once resting begins, no solids redissolve, disperse back into suspension, or rise upward.
level_4: Everything required for score 3 is present, and in addition: no particulates are visible before the salt solution contacts the medium; the onset of visible particulate formation follows the salt addition within about 1-2 seconds (a clearly visible, prompt onset, not a long delay); the settling solids move continuously downward without drifting sideways, pulsing upward, or reversing direction; the clear liquid layer above the settling solids visibly grows less turbid as sedimentation proceeds.
level_5: Everything required for score 4 is present, and in addition: the particulate solids first appear localized where the salt solution meets the culture medium, visible as a zone of turbidity at that contact region before spreading; the settled solids form a level, compact layer with a sharp upper boundary against the liquid above; the supernatant above the sediment becomes fully clear and visually matches the appearance of the original pre-addition medium, with no lingering haze or streaking.

Spatiotemporal Consistency rubric:
level_1: Severe continuity breaks occur: the beaker's position or apparent size jumps abruptly between frames; the scene's illumination or shadow direction changes abruptly without cause; the liquid surface fractures or jumps to a different level between frames; the stir rod vanishes or appears mid-stroke; the bench or background changes identity partway through the clip.
level_2: Noticeable but less severe breaks occur: the beaker wobbles, rotates, or shifts position without any depicted cause; the stir rod's motion is jerky or skips rather than moving continuously; the salt solution appears to already be inside the beaker before the addition action begins; the settled solids appear instantly as a fully formed layer with no visible descent; the resting phase flickers or drops frames.
level_3: The beaker, stir rod, and the vessel delivering the salt solution keep the same relative screen positions across frames; the stirring motion is smooth and unbroken; the salt solution addition reads as one continuous action rather than a cut; the settling solids advance progressively downward frame-by-frame rather than jumping between positions; the highlight on the beaker's glass and the shadow it casts on the bench hold the same direction and strength from the first frame to the last.
level_4: Everything required for score 3 is present, and in addition: motion blur on the stir rod matches its actual speed rather than appearing blurred while the surrounding liquid stays perfectly sharp (or the reverse); the beaker's rim and the liquid's meniscus line up frame-to-frame without misalignment; the pacing of the stirring and the length of the resting phase both play out smoothly, without sudden speed-ups, freezes, or stutters.
level_5: Everything required for score 4 is present, and in addition: the curvature of the meniscus stays geometrically consistent across every frame; any reflected highlight on the beaker's curved surface glides smoothly and stays coherent with the fixed camera viewpoint; the particulate solids descend in a smooth, continuous downward motion (whether at a steady rate or gradually changing speed) without jerks, freezes, or sudden position jumps; the clip shows no duplicated or obviously interpolated frames.