Domain: Natural Science — Environmental Science
Target concept: Chemical Wastewater Sulfide Determination (Methylene Blue Method)

Video generation prompt: Take 2 mL of sulfide-containing wastewater sample into a test tube, add 2 drops of N,N-dimethyl-p-phenylenediamine solution, then add 1 drop of ammonium ferric sulfate solution.

Evaluation guide:
(i) Target concept: Colorimetric detection of aqueous sulfide via methylene blue formation requires sequential reagent addition to generate a visually unique blue chromophore. Minimum mechanism: Sulfide must be present; N,N-dimethyl-p-phenylenediamine must contact sulfide first to form an intermediate; ammonium ferric sulfate must then oxidize the intermediate to yield methylene blue: no blue forms without both reagents in correct order.
(ii)Storyline: [Frame 0–2] Static shot of upright test tube containing ~2 mL colorless/light-yellow wastewater; [Frame 3–4] Two discrete droplets of N,N-dimethyl-p-phenylenediamine solution descend into the liquid and merge; no color change; [Frame 5–6] One discrete droplet of ammonium ferric sulfate descends; [Frame 7–10] Blue color initiates at the impact site and spreads uniformly through the liquid, with no blue before Frame 7; viewpoint fixed, lighting stable, no cuts or perspective shifts.

Manual:
Experimental Procedure
1. Place 2 mL sulfide-containing wastewater sample in clean test tube.
2. Add exactly two drops of N,N-dimethyl-p-phenylenediamine solution.
3. Add exactly one drop of ammonium ferric sulfate solution.
Observation of Experimental Phenomena
A distinct blue or blue-violet color develops in the liquid phase only after the second reagent is added.
Summary of Experimental Principles
Sulfide ions react with N,N-dimethyl-p-phenylenediamine and are oxidatively coupled by Fe3+ supplied from the ammonium ferric sulfate to form methylene blue dye — a specific colorimetric indicator for sulfide.

Prompt Grounding rubric:
level_1: No test tube present; or no sulfide-containing wastewater sample visible; or N,N-dimethyl-p-phenylenediamine solution absent; or ammonium ferric sulfate solution absent; or none of the three reagents added in any form.
level_2: Test tube present and contains liquid, but only one reagent is visibly added (either the N,N-dimethyl-p-phenylenediamine or the ammonium ferric sulfate), and the 2 mL sample volume is not visually distinguishable as a discrete liquid layer; no evidence of dropwise addition.
level_3: Test tube present with liquid sample; two drops of N,N-dimethyl-p-phenylenediamine solution are added as discrete droplets into the liquid; one drop of ammonium ferric sulfate solution is added as a single discrete droplet into the same liquid; all three components (sample, first reagent, second reagent) are present and co-located in the same test tube; the first reagent is added before the second.
level_4: All conditions for the 3-score level are met; additionally, the two N,N-dimethyl-p-phenylenediamine droplets are clearly distinguishable in count from the single ammonium ferric sulfate droplet; droplets enter the liquid without splashing outside the test tube; no extraneous containers, tools, or reagents appear; the camera stays fixed on the test tube throughout; the reagents are added in the correct order with no reversal.
level_5: All conditions for the 4-score level are met; additionally, the two N,N-dimethyl-p-phenylenediamine drops match each other in size and shape, and the single ammonium ferric sulfate drop is clearly a separate, distinct addition event; the whole sequence plays out in one continuous shot with no cut, zoom, or repositioning; the test tube stays upright and still; nothing occludes the liquid surface or the droplet impact point.

Scientific and Causal Correctness rubric:
level_1: The liquid never changes color throughout the clip; or a deep blue/purple color is already present before either reagent is added; or the final liquid color is red, green, yellow, black, or stays colorless, none matching the blue of methylene blue.
level_2: A color change happens, but the final hue is wrong (e.g., brown, gray, or violet with no blue dominance), or it appears before the ammonium ferric sulfate drop lands, or a color appears in the liquid with no droplet ever visibly entering it.
level_3: Color change occurs only after both reagents have been added, in that order; the final liquid color is blue or blue-violet with a clearly dominant blue hue; no color appears before the second reagent contacts the liquid; the color stays confined to the liquid itself, not the tube wall or the air above it.
level_4: All conditions for the 3-score level are met; additionally, the blue color begins right where the ammonium ferric sulfate droplet enters and spreads outward through the liquid rather than filling instantly; no competing tint (e.g., a lingering yellow or pink cast) remains visible; the blue looks even across the liquid rather than patchy or layered.
level_5: All conditions for the 4-score level are met; additionally, no blue tint appears while the ammonium ferric sulfate droplet is still falling through the air, only after it merges with the liquid; the blue builds steadily to a stable final shade with no fading or flickering for the rest of the clip; the final hue reads as a believable methylene-blue color rather than an artificially saturated or tinted one.

Spatiotemporal Consistency rubric:
level_1: The test tube blinks out of the frame and back in; the liquid level changes with no droplet visibly causing it; a droplet appears already inside the liquid with no visible fall through the air; the screen flashes to black or white for more than an instant; or the test tube visibly cracks, bends, or breaks.
level_2: A droplet drifts upward or accelerates unnaturally instead of falling; the liquid surface dips inward or oscillates rigidly with no damping; the camera cuts between a side view and a top-down view with no transition; or the shadow cast by the test tube swaps sides between consecutive frames.
level_3: The test tube and liquid stay visible throughout; each droplet falls in a plausible downward arc into the liquid; the liquid surface dips briefly at the impact point and settles back smoothly; no object teleports, clips through another, or changes shape abruptly; the light source's direction and brightness stay the same from the first frame to the last.
level_4: All conditions for the 3-score level are met; additionally, each droplet's fall and entry into the liquid is smooth and continuous, without freezing or jumping between positions; the ripple from the droplet's impact spreads outward evenly rather than only on one side; the test tube keeps the same tilt and position in the frame throughout the clip.
level_5: All conditions for the 4-score level are met; additionally, the N,N-dimethyl-p-phenylenediamine droplets produce a brief, localized swirl or cloudiness where they enter the liquid, and the ammonium ferric sulfate droplet's entry is followed by a visible blue front spreading outward from that same point, slowing naturally as it fills the tube rather than snapping to full color.