Domain: Natural Science — Chemistry
Target concept: Reaction between sodium sulfate solution and barium chloride solution

Video generation prompt: A test tube contains 5 mL of colorless sodium sulfate solution, to which barium chloride solution is added dropwise.

Evaluation guide:
(i) Target concept: Precipitation reaction driven by solubility rules, specifically formation of insoluble BaSO₄ upon ion combination.
Minimum mechanism: Presence of aqueous SO₄²⁻ and Ba²⁺ ions must coincide spatially and temporally within solution phase to nucleate solid BaSO₄; no thermal decomposition, redox, or complexation interfering.
(ii)Storyline: [Phase 1: Static Setup] Test tube centered, filled to ~5 mL with still, transparent, colorless liquid; separate dropper with clear liquid held motionless above tube opening. [Phase 2: Initiation] First droplet detaches and falls vertically; impact causes momentary surface depression. [Phase 3: Causal Emergence] White particles nucleate at impact site within 1–2 frames, then disperse upward via convection and settle slowly. [Phase 4: Progression] Each subsequent droplet triggers identical localized nucleation; cumulative particle load increases suspension opacity uniformly. [Perspective constraint] Fixed frontal view showing full test tube height, dropper tip, and solution column; no occlusion; lighting uniform and diffuse to avoid glare masking particles.

Manual:
Experimental Procedure
1. Place a clean test tube containing 5 mL of colorless sodium sulfate solution upright on stand.
2. Hold a dropper filled with barium chloride solution above the test tube opening.
3. Add barium chloride solution dropwise into the sodium sulfate solution.
Observation of Experimental Phenomena
A white precipitate forms immediately upon contact of each droplet with the solution; the solution becomes cloudy and opaque due to suspended white solid; no color change in liquid phase; precipitate does not dissolve.
Summary of Experimental Principles
Sodium sulfate (Na₂SO₄) reacts with barium chloride (BaCl₂) to form insoluble barium sulfate (BaSO₄), a white precipitate, and soluble sodium chloride (NaCl): Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq).

Prompt Grounding rubric:
level_1: No test tube visible; no sodium sulfate solution shown; no barium chloride solution shown; no dropwise addition action present; no 5 mL volume indication or visual proxy (e.g., calibrated tube fill level); demonstration goal completely unrepresented.
level_2: A test tube is present and contains a colorless liquid, but volume is not visually consistent with ~5 mL (e.g., tube is <1/3 or >4/5 full); barium chloride solution is present but not delivered via dropwise action (e.g., poured, pipetted without visible droplets, or absent); at least one core object (test tube, Na₂SO₄ solution, BaCl₂ solution) is missing or mislabeled (e.g., labeled as HCl or water).
level_3: Test tube is present and contains a colorless liquid at approximate mid-height (visually consistent with ~5 mL in standard 15×150 mm test tube); barium chloride solution is visibly delivered as discrete droplets into the tube; all three core objects (test tube, sodium sulfate solution, barium chloride solution) are present and correctly identifiable by shape, container type, and context; no mislabeling or substitution.
level_4: All conditions for score 3 are satisfied; additionally: droplet delivery occurs exclusively *into* the test tube (no splashing outside, no contact with tube rim only); barium chloride solution is held in a clearly distinct vessel (e.g., dropper or small bottle) prior to delivery; the sodium sulfate solution remains stationary and undisturbed *before* first droplet contact; no extraneous objects (e.g., gloves, labels, text overlays) occlude core objects or actions.
level_5: All conditions for score 4 are satisfied; additionally: exactly one dropper (or equivalent drop-delivery tool) is used; the dropper tip is positioned directly above the test tube opening throughout delivery; all droplets fall vertically and enter the solution without touching tube walls; the initial state shows *only* the filled test tube and the separate barium chloride source, with no pre-mixing, no stirring rod, no secondary reagents, no motion blur obscuring droplet formation or entry.

Scientific and Causal Correctness rubric:
level_1: No precipitate forms; solution remains fully transparent and colorless throughout; or precipitate appears but is colored (e.g., yellow, blue, green); or precipitate forms *before* any barium chloride addition; or precipitate dissolves immediately upon formation; scientific phenomenon is entirely absent or physically impossible for this reaction.
level_2: A white solid appears, but it forms *without* barium chloride addition (e.g., present at start); or forms *after* addition but is not suspended (e.g., instantly sinks as single lump without dispersion); or appears only after vigorous shaking/stirring not in prompt; or co-occurs with incorrect color change (e.g., solution turns cloudy white *then* yellow); causal link between addition and precipitation is visually broken.
level_3: A white precipitate appears *only after* the first droplet of barium chloride contacts the sodium sulfate solution; precipitate is visibly suspended (not fully settled) during video duration; solution transitions from fully transparent to partially opaque due to suspended particles; no color change in liquid phase; no dissolution or disappearance of precipitate during video.
level_4: All conditions for score 3 are satisfied; additionally: precipitate forms *continuously* with each successive droplet (i.e., new particles appear per droplet, not just once); suspension remains stable (no rapid sedimentation dominating the frame); no gas evolution, effervescence, temperature-induced shimmer, or light scattering inconsistent with BaSO₄ colloid/suspension; liquid phase stays colorless and clear *except* where obscured by particles.
level_5: All conditions for score 4 are satisfied; additionally: precipitate particles are uniformly fine (no visible aggregates > background grain); suspension density increases monotonically with droplet count (no sudden jump or plateau); no stray particles outside solution column (e.g., on tube wall above liquid); no halo, bloom, or artificial glow around particles; appearance matches natural colloidal suspension under standard lighting.

Spatiotemporal Consistency rubric:
level_1: Test tube disappears and reappears; liquid level jumps discontinuously; droplets teleport from source to solution without trajectory; precipitate appears instantaneously across entire volume without nucleation progression; camera cuts, zooms, or rotates abruptly; lighting changes mid-sequence (e.g., shadow vanishes then returns).
level_2: Droplet release and impact are not temporally aligned (e.g., droplet visible mid-air but no corresponding impact disturbance); precipitate appears before droplet impact is complete; tube wobbles or shifts position between frames; liquid surface ripples persist unrealistically long or vanish instantly; spatial relationship between dropper and tube changes inconsistently (e.g., dropper starts centered, then drifts off-axis without smooth motion).
level_3: Droplet falls along continuous vertical path from dropper tip to liquid surface; impact causes brief, localized surface deformation (meniscus dip or splash ring) followed by particle dispersion; precipitate nucleates at point of impact and spreads upward/downward gradually; tube remains fixed in frame; lighting and shadows remain constant; no object flickering, warping, or interpenetration.
level_4: All conditions for score 3 are satisfied; additionally: droplet size and spacing are visually consistent (no erratic large/small droplets); surface deformation recovers fully before next droplet impact; precipitate dispersion follows fluid convection patterns (subtle upward plume then settling); no motion blur that eliminates droplet boundaries or precipitate edges; tube walls show consistent refraction and meniscus curvature throughout.
level_5: All conditions for score 4 are satisfied; additionally: no temporal aliasing (e.g., strobing, frozen droplets, judder); no geometric distortion (e.g., curved tube walls, stretched droplets); refraction through liquid and glass remains physically coherent (e.g., meniscus shape unchanged by precipitate presence); no parallax shift between tube, dropper, and background elements across frames.