Domain: Natural Science — Chemistry
Target concept: Precipitate transformation (white to yellow to black)

Video generation prompt: A test tube containing an appropriate amount of sodium chloride solution was first treated with an appropriate amount of silver nitrate solution until a precipitate formed. Subsequently, an appropriate amount of potassium iodide solution was added until the precipitate color changed. Finally, an appropriate amount of sodium sulfide solution was added until the precipitate color changed again.

Evaluation guide:
(i) Target concept: Sequential anion-driven solid-phase transformation in aqueous precipitation reactions.
Minimum mechanism: Presence of mobile Cl⁻, I⁻, S²⁻ ions in solution; stoichiometric displacement of less insoluble silver salt by more insoluble one; solid-phase identity change must manifest as complete, irreversible color replacement (not coexistence or blending).
(ii)Storyline: Frame 0–2: Static shot of upright test tube with clear NaCl solution (meniscus visible, no precipitate). Frame 3–4: Dropper enters top of frame, descends steadily, contacts liquid surface, delivers AgNO₃; immediate localized turbidity → rapid uniform white precipitate formation and settling. Frame 5–6: Second dropper (visually distinct) enters, delivers KI; white layer dissolves at interface and yellow layer grows downward until fully replaced. Frame 7–8: Third dropper enters, delivers Na₂S; yellow layer similarly dissolves at interface and black layer grows downward until fully replaced. Final frame: stable black precipitate occupying bottom ⅔ of tube, clear supernatant above. All events occur from fixed frontal viewpoint; no camera movement; no object enters/exits frame except droppers; lighting constant; tube remains rigid and upright.

Manual:
Experimental Procedure
1. A test tube holds clear sodium chloride solution.
2. Silver nitrate solution is added → white precipitate forms.
3. Potassium iodide solution is added → white precipitate turns yellow.
4. Sodium sulfide solution is added → yellow precipitate turns black.
Observation of Experimental Phenomena
- Initial solution is clear and colorless.
- After AgNO₃: dense, opaque white precipitate appears and settles.
- After KI: white precipitate fully transforms to bright yellow precipitate.
- After Na₂S: yellow precipitate fully transforms to jet-black precipitate.
Summary of Experimental Principles
Precipitate transformation: AgCl (white) → AgI (yellow) → Ag₂S (black) via anion exchange driven by decreasing solubility product (Ksp).

Prompt Grounding rubric:
level_1: No test tube is visible; no sodium chloride solution, silver nitrate solution, potassium iodide solution, or sodium sulfide solution ever appears; no addition action is performed; no precipitate of any kind ever forms.
level_2: The test tube holds a clear liquid, but only one reagent is ever added (e.g., only the silver nitrate); or the three additions happen out of order (e.g., a sodium sulfide dropper appears before the potassium iodide dropper); or the reagents are shown with no distinguishing containers or droppers at all.
level_3: The test tube holds a clear initial liquid; three separate addition events happen in the stated order: silver nitrate first, potassium iodide second, sodium sulfide third; each addition is followed by a visibly different-looking solid in the tube (the appearance changes at least three distinct times); however, the three source containers look identical to one another (no labels, colors, or shapes distinguish them), or the liquid level in the tube looks inconsistent between the three steps.
level_4: The silver nitrate, potassium iodide, and sodium sulfide are dispensed from three visually distinct containers (different bottle shapes, labels, or dropper colors); each is added to the same test tube strictly in that order; the tube shows three sequential, visibly distinct solid states corresponding to the three additions; the tube itself stays fully visible and stationary; no extra hands, tubes, or objects intrude on the scene.
level_5: Building on the 4-score requirements, each addition is staged unambiguously: the silver nitrate, potassium iodide, and sodium sulfide source solutions all appear colorless before being added; each addition event shows the dropper tip entering frame, reaching the tube's mouth, dispensing liquid, then withdrawing; the resulting solid after each step is dense enough to look clearly opaque rather than a faint haze; no motion blur or occlusion hides any of the three addition events.

Scientific and Causal Correctness rubric:
level_1: No solid ever forms after any addition; the contents stay colorless or unchanged throughout; a color appears that contradicts real chemistry (e.g., turning red or blue, or staying white after the sodium sulfide is added); or black appears before yellow, reversing the true white-to-yellow-to-black order.
level_2: A solid appears after the silver nitrate is added but is not white (e.g., grey, cream, or already yellowish); after the potassium iodide is added the solid does not turn yellow (e.g., it stays white, turns orange/brown, or dissolves away); after the sodium sulfide is added the solid does not turn black (e.g., it stays yellow or turns pale grey); or a color shift happens before the corresponding reagent visibly reaches the tube.
level_3: A white solid forms after the silver nitrate is added, turns yellow after the potassium iodide is added, and turns black after the sodium sulfide is added, in that order; however at least one of the three colors looks off from its textbook description: for instance the 'white' solid looks distinctly cream or off-white, the 'yellow' looks pale or beige rather than a clear yellow, or the 'black' looks dark grey or brown rather than opaque black.
level_4: The three solid colors match the standard descriptions for this sequence (white after silver nitrate, yellow after potassium iodide, opaque black after sodium sulfide), appearing strictly in that order; there is no blending of two colors at once (no visible white flecks once the solid has turned yellow, and so on); the solid stays a discrete settled layer rather than a cloudy suspension at any stage.
level_5: Everything true at the 4 level still holds, and each color transition only happens once the newly added reagent has visibly mixed into the tube (the liquid stops swirling before the next color is judged complete); no leftover particles of the previous color remain once a transition is finished (no white specks once the solid is yellow; no yellow specks once the solid is black); the solid stays granular and settled, never gelatinous or floating, at every stage; the final black layer is fully opaque with no yellow or white showing through.

Spatiotemporal Consistency rubric:
level_1: The test tube vanishes from the shot or reappears at a different spot in the frame; the settled solid suddenly relocates within the tube between frames; the video cuts abruptly between reagent additions with no transition; the scene brightness switches between bright and dim from one frame to the next; the tube's tilt or position changes with no dropper or hand shown causing it; the liquid volume changes with nothing visibly added.
level_2: The test tube stays on screen but visibly shakes or the framing is unsteady; the solid blinks in and out between frames instead of settling continuously; a color change happens as an abrupt frame swap rather than a gradual spread from where the new reagent lands; a dropper is shown already touching the liquid with no visible approach from above; the background behind the tube warps or bends.
level_3: The test tube stays in the same place and orientation throughout the clip; all three additions (silver nitrate, potassium iodide, sodium sulfide) show the dropper tip moving continuously downward into the tube's mouth; the solid at each stage settles gradually rather than appearing instantly; minor flaws remain, however: for instance a hand briefly blocks the tube for a couple of frames during one addition, or the liquid's meniscus looks slightly different between the three stages.
level_4: No hand or object blocks the view of the tube at any point; each dropper's downward motion is smooth and fully visible from entry to contact; each solid forms and settles without freezing, reversing, or skipping; the highlight on the glass and the shadow the tube casts stay in the same place and brightness across all three additions; the tube never changes size, shifts sideways, or rotates at any stage.
level_5: Motion throughout the addition-mixing-settling sequence is smooth with no jerks or repeated frames; the solid looks visibly denser and more settled with each passing frame rather than jumping straight to its final state; the position of the settled layer relative to the tube wall never shifts unexpectedly; the bright reflection on the glass stays fixed relative to the light source across all three stages; no flare, glow, or color fringing appears during any of the three transitions.