Domain: Natural Science — Chemistry
Target concept: The reaction of iron(III) chloride and iron(II) sulfate with sodium hydroxide

Video generation prompt: Two test tubes are placed side by side: the left test tube contains ferric chloride solution, into which sodium hydroxide solution is added; the right test tube contains ferrous sulfate solution, into which sodium hydroxide solution is also added.

Evaluation guide:
(i) Target concept: Visual differentiation of Fe(OH)₃ and Fe(OH)₂ precipitation via color contrast under controlled, synchronous base addition.
Minimum mechanism: Iron(III) and iron(II) cations must be present in aqueous solution at sufficient concentration to yield visibly opaque hydroxides upon OH⁻ introduction; NaOH must be added as a discrete, identifiable action; the initially white Fe(OH)₂ precipitate is expected to oxidize rapidly (within seconds) to its grey-green (dirty-green) form within the observation window, without progressing further to red-brown.
(ii)Storyline: Frame opens on static, centered view of two identical upright test tubes aligned horizontally. Left tube contains homogeneous yellow-brown liquid; right tube contains homogeneous pale green liquid. Two identical colorless liquid streams enter each tube from above at same vertical level and synchronously. Within <1 second of contact, a rust-brown solid appears in the left tube and remains distinct; in the right tube a precipitate appears, white at first, then rapidly turning grey-green (dirty-green) within a few seconds, and remains distinct in color from the left tube's precipitate throughout. No motion, lighting shift, or occlusion interrupts visibility of either tube or its precipitate throughout.

Manual:
Experimental Procedure
1. Place two identical clean test tubes side-by-side.
2. Fill left tube with iron(III) chloride solution (yellow-brown).
3. Fill right tube with iron(II) sulfate solution (pale green).
4. Simultaneously add sodium hydroxide solution to both tubes.
Observation of Experimental Phenomena
Left tube: rust-brown precipitate forms immediately upon NaOH addition and persists.
Right tube: a white precipitate forms immediately upon NaOH addition, then rapidly turns grey-green (dirty-green) within a few seconds as dissolved oxygen begins oxidizing it; the grey-green color persists through the remainder of the observation window.
Summary of Experimental Principles
Fe³⁺ + 3OH⁻ → Fe(OH)₃↓ (rust-brown; forms immediately with no color transition)
Fe²⁺ + 2OH⁻ → Fe(OH)₂↓ (white precipitate initially; oxidizes quickly in air via 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃, passing through a grey-green intermediate that is the color visible for the remainder of the observation window)

Prompt Grounding rubric:
level_1: No two test tubes are visible together in frame; no sodium hydroxide addition is shown; the two tubes are not shown positioned next to one another in the same shot; the ferric chloride and ferrous sulfate solutions are not visually distinct from each other or are unlabeled; no reagent-adding action of any kind is depicted.
level_2: Two containers present but not clearly test tubes; side-by-side placement ambiguous; at least one solution added, but no clear distinction between left and right reagents; no labeling or visual differentiation of FeCl₃ vs. FeSO₄ solutions.
level_3: Two transparent cylindrical test tubes placed side-by-side in same frame; left tube contains yellow-brown solution, right tube contains pale green solution; NaOH solution visibly added to both tubes (liquid stream entering each); no mixing or occlusion of either tube during addition.
level_4: All elements of 3-score satisfied, AND both test tubes are identically sized and oriented; NaOH addition occurs synchronously (visually simultaneous start and duration of liquid flow into both tubes); no extraneous objects, labels, or overlays obscuring the tubes or their contents.
level_5: All elements of 4-score satisfied, AND left tube solution is unambiguously yellow-brown *before* addition and remains so until reaction begins; right tube solution is unambiguously pale green *before* addition and remains so until reaction begins; NaOH solution is colorless and visibly identical in appearance when added to both; no camera movement, zoom, or cut during the entire sequence.

Scientific and Causal Correctness rubric:
level_1: No precipitate forms in either tube after NaOH is added; or the left tube's precipitate is not rust-brown (e.g., it is white, blue, or black); or the right tube's precipitate is a color inconsistent with Fe(OH)₂ chemistry (e.g., blue or black, or a color that is never white or grey-green at any point); or the same precipitate color appears in both tubes.
level_2: A precipitate forms in only one tube; or precipitates form in both tubes but end up the same color; or a color change happens without any solid precipitate being visible; or a precipitate is visible before the NaOH stream reaches the tube.
level_3: A rust-brown precipitate forms in the left tube immediately upon NaOH contact and keeps that color; in the right tube, a precipitate forms upon NaOH contact that is either white or grey-green (dirty-green), and if it starts white it visibly shifts to grey-green within the observation window; the two tubes' precipitate colors stay clearly different from one another throughout.
level_4: All elements of 3-score satisfied, AND both precipitates form only after their respective tube receives the NaOH stream (no precipitate visible beforehand); the left precipitate is opaque and rust-brown (not orange, yellow, or black); the right precipitate, after any brief white stage, settles into an opaque grey-green (dirty-green) appearance and holds that color for the remainder of the video; no color from one tube bleeds into or is mistaken for the other.
level_5: All elements of 4-score satisfied, AND the rust-brown precipitate in the left tube looks granular and settles toward the bottom of the tube; the right tube's precipitate, if briefly white, turns grey-green within a few seconds of contact and stays grey-green (rather than darkening further toward red-brown) for the rest of the video; both precipitates hold their described colors without unexplained fading; neither precipitate's color reverts to a lighter shade once established.

Spatiotemporal Consistency rubric:
level_1: One or both test tubes vanish or jump to a new position between frames; the sodium hydroxide stream cuts on and off abruptly or changes thickness without transition; a flash of light or sudden darkening interrupts the shot; motion blur or dropped frames make it impossible to tell whether a precipitate has formed in either tube.
level_2: Both tubes stay visible, but one tube's position shifts noticeably (more than roughly one tube-width) between shots with no panning motion shown; the NaOH stream flickers off for more than an instant while still appearing to pour; a brief lighting flare washes out the color difference between the rust-brown and white/grey-green precipitates for part of the video.
level_3: Both test tubes stay visible and in the same position throughout; the NaOH stream flows continuously into each tube with no gaps; lighting stays even enough that the rust-brown precipitate (left) and the white-to-grey-green precipitate (right) remain visually distinguishable at all times; neither tube warps, stretches, or is cut off by the frame edge.
level_4: All elements of 3-score satisfied, AND the two tubes keep the same side-by-side spacing and tilt for the entire video (no drifting apart, rotating, or changing depth); the NaOH stream keeps a consistent width and path into each tube; the lighting stays even enough that colors in both tubes look equally saturated at every point in the video; no camera angle change causes one tube to appear to shift relative to the other.
level_5: All elements of 4-score satisfied, AND the liquid surfaces in both tubes stay calm with no visible ripples or shimmer; the tube edges stay sharp with no visible flicker or double-edging; the NaOH streams look smooth and unbroken rather than sputtering; the precipitate colors (rust-brown on the left, white transitioning to grey-green on the right) do not flicker or pulse in brightness from frame to frame.