Domain: Natural Science — Chemistry
Target concept: Ion cross-linking occurring in sodium alginate droplets leads to the formation of gel beads in calcium chloride (CaCl₂).

Video generation prompt: Add sodium alginate droplets into a transparent container containing calcium chloride solution using a pipette; when the droplets enter and sink, gently rotate the container.

Evaluation guide:
(i) Target concept: Ion-induced hydrogelation via divalent cation cross-linking of anionic polysaccharide chains.
Minimum mechanism: Sodium alginate must be in aqueous solution (soluble anionic polymer); CaCl₂ solution must be present as source of Ca²⁺; physical contact between droplet and solution must occur; gelation must initiate at interface and propagate inward within seconds; resulting beads must be mechanically coherent, spherical, and chemically stable in solution.
(ii)Storyline: [Frame 0–1] Pipette poised above container; [Frame 2–4] Droplet egresses from pipette tip and crosses air–liquid interface; [Frame 5–8] Droplet fully submerged and descends under gravity; [Frame 6–10] Surface contact triggers visible opacification and rounding into sphere (bead formation); [Frame 9–10+] Container rotates gently while beads remain suspended mid-liquid; all phases must be visible from fixed frontal/slightly oblique viewpoint with unobstructed line of sight to droplet entry point and mid-volume region; no occlusion by hand, pipette, or container rim during critical transformation window.

Manual:
Experimental Procedure
1. Hold pipette containing sodium alginate solution above a transparent container filled with clear calcium chloride solution.
2. Squeeze pipette gently to release 3–5 discrete droplets into the solution.
3. Observe droplets sink through the liquid, then rotate container gently by hand.
Observation of Experimental Phenomena
Droplets transform into spherical, opaque-to-translucent gel beads upon contact with CaCl₂ solution; beads remain discrete, suspended or slowly settling, retaining round shape without dissolving or merging.
Summary of Experimental Principles
Ion cross-linking: Ca²⁺ ions from CaCl₂ replace Na⁺ in sodium alginate polymer chains, forming bridges between guluronate blocks, leading to rapid hydrogel network formation and bead solidification.

Prompt Grounding rubric:
level_1: No pipette is shown above the container; no sodium alginate droplets are visible entering liquid; no transparent container holding the CaCl₂ solution is present; no rotation of the container is attempted; none of the add/sink/rotate actions from the prompt occur.
level_2: A pipette, the container, or the clear liquid appears alone (e.g., only the container with liquid, no pipette present), but no droplet exits the pipette tip, no downward sinking motion is shown, and no rotation occurs; the pipette tip and the container's opening are never shown together in the same shot.
level_3: The pipette is positioned above a transparent container filled with clear liquid; sodium alginate droplets are seen leaving the pipette tip and entering the liquid; each droplet visibly sinks toward the bottom of the container; the container is not rotated until after the droplets finish sinking, or is not rotated at all.
level_4: All score 3 conditions hold, and the container begins rotating gently while at least one droplet is still visibly below the surface and not yet settled; the rotation proceeds without abrupt stopping or reversing; the pipette has been withdrawn from the frame by the time rotation starts.
level_5: All score 4 conditions hold, and rotation starts only once the first droplet is fully submerged and descending (not at the instant it touches the surface); the rotation continues smoothly for the whole descent of at least one droplet, keeping one consistent turning direction while the liquid surface visibly tilts slightly in response; the pipette stays completely out of frame for the entire rotation segment.

Scientific and Causal Correctness rubric:
level_1: Droplets stay liquid the whole time: no rounded, opaque gel bead ever forms in the CaCl₂ solution; the liquid looks the same before and after the droplets enter it, with no sign that contact with the calcium ions changed the droplets' state.
level_2: Droplets enter the CaCl₂ solution but either dissolve/disperse into the liquid or clump into shapeless masses rather than forming separate round beads; nothing distinguishes a droplet before contact from a bead after contact.
level_3: After each droplet contacts the CaCl₂ solution, a distinct, rounded, opaque-to-translucent bead appears in its place and holds that shape and position for at least a second, visibly different in texture from the surrounding liquid, showing that contact with Ca²⁺ triggered gelation.
level_4: All score 3 conditions hold, and every bead keeps its spherical shape without flattening, splitting apart, or merging with a neighboring bead; no bead sticks to the container wall or the pipette tip; every bead's opacity looks similar to every other bead.
level_5: All score 4 conditions hold, and each released droplet turns into exactly one bead: no droplet fails to gel and no bead appears without a droplet having produced it; every bead's surface looks smooth, without visible cracks or cloudy patches, consistent with a fully cross-linked calcium-alginate gel.

Spatiotemporal Consistency rubric:
level_1: The container's position jumps abruptly between frames; a bead appears in the liquid with no droplet ever having entered from above it; the scene's brightness swings drastically within a couple of frames; the container's wall or edge abruptly vanishes or appears to break apart; the camera cuts or jumps to a different angle mid-action.
level_2: The droplet's downward path skips: it appears at noticeably different depths in consecutive frames rather than sinking continuously; the container's rotation is jerky or stop-and-go rather than a smooth turn; a bead flickers in and out of view; the container wobbles or tips off its rotation axis in a way that distorts the view of its contents.
level_3: The pipette, container, and liquid keep a consistent size and position in the frame apart from the droplet's intended sinking and the container's intended rotation; the droplet's descent looks smooth and continues downward without reversing; once rotation begins it turns steadily around the container's own axis; no bead or container edge flickers, warps, or briefly disappears.
level_4: All score 3 conditions hold, and the sequence from droplet release to sinking to bead formation plays out without freezing, stuttering, or sudden speed changes; the rotating container and its liquid keep a believable rounded shape with no unnatural sloshing or bulging; when the pipette is withdrawn it does not block the view of the sinking droplet or forming bead.
level_5: All score 4 conditions hold, and the droplet's fall visibly speeds up smoothly as it sinks, with no sudden jump in speed; the rotation makes the meniscus or background reflections shift gently and consistently with a slow, rigid turn and no splashing; the video shows no jarring flashes, ghost trails, or blur that break the sense of continuous motion.