Domain: Natural Science — Chemistry
Target concept: Coagulation of colloids

Video generation prompt: A test tube containing an appropriate amount of ferric hydroxide colloidal solution is treated with an appropriate amount of Na₂SO₄ solution, and the test tube is observed.

Evaluation guide:
(i) Target concept: Electrolyte-induced coagulation of positively charged hydrophobic colloids.
Minimum mechanism: Colloidal particles must carry net positive surface charge (Fe(OH)₃ in slightly acidic/neutral medium); added sulfate anions must exceed critical coagulation concentration to compress double layer and enable van der Waals aggregation; resulting aggregates must be large enough to scatter visible light distinctly from sol state.
(ii)Storyline: [Frame 0–2] Static shot: upright test tube centered, filled with homogeneous brownish-red translucent liquid; no motion. → [Frame 3–4] Dropper enters frame from top-right, tip aligned with tube opening; no contact yet. → [Frame 5–6] Dropper tip contacts or enters tube; one to three discrete colorless droplets visibly detach and fall into liquid. → [Frame 7–10] Liquid remains undisturbed except for gentle meniscus response; within 1–2 seconds post-entry, localized turbidity appears at impact zone and spreads radially as increasing density of irregular brown aggregates; aggregates remain suspended, no settling or layering. Perspective: Fixed frontal view, eye-level with tube midpoint; entire tube visible at all times; no occlusion by hand or tools after delivery.

Manual:
1. Place a test tube containing brownish-red ferric hydroxide colloidal solution on a stable surface.
2. Add Na₂SO₄ solution dropwise into the test tube using a dropper.
3. Observe the immediate change in the liquid: formation of cloudy aggregates and loss of transparency.
Observation: Brownish-red colloidal solution becomes visibly cloudy with suspended dark brown aggregates; no color change or gas evolution.
Principle: Na₂SO₄ provides SO₄²⁻ ions that neutralize the positive charge on Fe(OH)₃ colloidal particles, causing coagulation and precipitation.

Prompt Grounding rubric:
level_1: No test tube visible; no ferric hydroxide colloidal solution present; no Na₂SO₄ solution container or delivery action shown; no observation action depicted.
level_2: A test tube is present and contains a liquid, but the liquid is not visibly brownish-red; Na₂SO₄ solution is absent or not delivered into the tube; no clear action of adding solution or subsequent observation.
level_3: A test tube containing a brownish-red liquid is shown; a second container (e.g., dropper or small vial) holding a colorless liquid is brought near the test tube; the colorless liquid is delivered into the test tube; the camera remains fixed on the test tube after delivery.
level_4: All elements from score 3 are present AND the delivery action is unambiguous (liquid visibly entering the test tube); the test tube is upright and stationary before/during/after addition; no extraneous objects occlude the tube; no unrelated actions (e.g., shaking, stirring, labeling) occur.
level_5: All elements from score 4 are present AND the delivery occurs in a single continuous motion without interruption or reversal; the test tube occupies consistent central framing throughout; the second container is clearly distinct in shape/size from the test tube; no visual artifacts (e.g., cut, zoom, overlay) disrupt continuity.

Scientific and Causal Correctness rubric:
level_1: No coagulation phenomenon observed; liquid remains uniformly transparent or homogeneously colored; no turbidity, cloudiness, or precipitate formation at any point.
level_2: Liquid changes color (e.g., to yellow/green/black) or becomes uniformly opaque without localized aggregation; precipitate forms but is evenly suspended without settling; or coagulation appears *before* addition of Na₂SO₄.
level_3: After addition, the brownish-red liquid develops visible cloudiness or fine particulate suspension that is non-uniform and distinct from initial clarity; some particles appear denser or darker than background; no settling observed within video duration.
level_4: All elements from score 3 are present AND cloudiness appears *only after* the Na₂SO₄ solution enters the test tube; cloudiness intensifies progressively (increasing particle density or size contrast) during observation; no discoloration (e.g., no shift to orange, purple, or gray); no dissolution or re-dispersion of particles.
level_5: All elements from score 4 are present AND cloudiness manifests as discrete, irregular aggregates (not uniform haze) that visibly increase in number and size relative to initial state; aggregates remain suspended without buoyant rise or rapid sedimentation; no secondary phenomena (e.g., layering, film formation, gas bubbles).

Spatiotemporal Consistency rubric:
level_1: Test tube disappears/reappears; sudden jump in viewpoint or lighting; liquid surface jumps or distorts unnaturally; container deforms or changes shape mid-video; abrupt light/shadow shift unrelated to motion.
level_2: Test tube shifts position horizontally/vertically between shots without smooth motion; liquid surface ripples discontinuously (e.g., still → violent splash → still); delivery action shows teleportation of dropper tip; lighting flickers or changes hue abruptly.
level_3: Test tube remains in fixed spatial position; liquid surface moves only during delivery (gentle meniscus displacement) and settles smoothly afterward; delivery motion is continuous and physically plausible (e.g., dropper tip approaches, contacts rim or interior, releases liquid); no structural breakage or warping of objects.
level_4: All elements from score 3 are present AND motion is consistently smooth (no stutter, freeze, or acceleration spike); lighting remains stable in intensity and direction; shadow cast by test tube and hand/dropper remains geometrically consistent with light source; no inter-frame object clipping or edge aliasing.
level_5: All elements from score 4 are present AND parallax between foreground (dropper) and background (test tube) remains physically accurate throughout; liquid meniscus deformation during entry and recovery follows natural surface tension behavior (no rigid flattening or artificial oscillation); no temporal compression/expansion (e.g., slow-mo without intent, skipped frames).