Domain: Natural Science — Chemistry
Target concept: Iodometric redox titration, using starch indicator, with the endpoint indicated by a color change from colored to colorless.

Video generation prompt: A conical flask containing an iodine solution and starch indicator is shaken while sodium thiosulfate solution is added from a burette.

Evaluation guide:
(i) Target concept: Redox-driven decolorization of the iodine-starch complex via stoichiometric reduction by thiosulfate.
Minimum mechanism: Free iodine complexed with starch produces the characteristic deep blue-black color (distinct from the pale yellow-brown of an iodine/triiodide solution before starch is added); thiosulfate reduces I2 to colorless I-; once I2 is consumed the starch complex can no longer form, so the blue-black color disappears; shaking ensures even mixing and prevents localized over-titration.
(ii)Storyline: [Frame 0-t1] Deep blue-black liquid sits still in the conical flask; the burette above it holds visible liquid with a clear meniscus; [t1-t2] Sodium thiosulfate begins exiting the burette tip and touches the liquid surface, and gentle back-and-forth shaking of the flask starts at the same moment; [t2-t3] The blue-black color fades progressively outward from the point of contact, with the fading spreading evenly rather than in patches; [t3-t4] The entire liquid volume reaches a uniform colorless state while shaking continues until the last drop has entered; [t4-end] The flask stays still, holding the fully colorless liquid; the whole sequence plays out from a single fixed viewpoint, with no cuts, no camera movement, and no obstruction of the flask opening or burette tip.

Manual:
Experimental Procedure
1. Place a conical flask containing iodine solution + starch indicator (deep blue-black) on a stand.
2. Position a burette filled with sodium thiosulfate solution above the flask.
3. While gently shaking the flask, slowly add sodium thiosulfate from the burette until the color disappears.
Observation of Experimental Phenomena
The deep blue-black solution in the conical flask gradually becomes colorless during the addition; the color change is uniform and complete.
Summary of Experimental Principles
Iodine (I2) forms a deep blue-black complex with starch; sodium thiosulfate reduces I2 to colorless iodide (I-), destroying the complex and decolorizing the solution — the titration endpoint is reached when the blue-black color disappears completely.

Prompt Grounding rubric:
level_1: No conical flask is shown, no burette is shown, the flask liquid never appears deep blue-black (the iodine-starch color), no sodium thiosulfate solution is present in the burette, or these objects never appear together in the same shot.
level_2: Only some of the required objects show up (for example a conical flask on its own, or a burette with no visible liquid coming out of it), but the flask, the burette, and the blue-black iodine-starch liquid are never shown working together (say, the burette is empty, or the flask holds a liquid that clearly is not the blue-black complex).
level_3: A conical flask holding deep blue-black liquid (the iodine-starch complex) sits below a burette that is dispensing sodium thiosulfate solution down into it, all in the same shot, with the burette positioned above or beside the flask as in a real titration; whether the flask is being shaken is not yet clear or not shown.
level_4: Everything at level 3 holds, and the flask is clearly being shaken back and forth while the thiosulfate is added (the motion is continuous and centered on the flask itself rather than jerky or frozen), while the burette keeps releasing liquid steadily down into the flask opening.
level_5: Everything at level 4 holds, and the shaking starts at or before the first drop of thiosulfate lands in the flask and continues without pausing until after the last drop enters; the burette tip stays aimed steadily at the flask's mouth the whole time, with no stray object blocking the view and no moment where the stream misses the flask opening.

Scientific and Causal Correctness rubric:
level_1: The liquid never changes color and stays deep blue-black the whole clip, it changes to a color other than colorless (turning green, purple, or black instead), the color change happens before any thiosulfate is added, or no thiosulfate is ever shown entering the flask.
level_2: A color change happens, but it starts from the wrong color (something other than deep blue-black) or ends somewhere other than fully colorless (for example, it only fades to a pale blue); the fading happens with no thiosulfate visibly entering the flask; or the color vanishes all at once between two frames rather than fading.
level_3: The flask starts out deep blue-black (the iodine-starch complex), and it fades to colorless while the thiosulfate is being added; the fading happens gradually rather than in a sudden jump, and it clears evenly through the liquid rather than leaving isolated blue patches once the addition is finished.
level_4: Everything at level 3 holds, and the fading only starts once the first visible drop of thiosulfate touches the liquid, then continues smoothly until the whole flask is colorless, with no patch of blue-black color left anywhere in the liquid by the end of the clip.
level_5: Everything at level 4 holds, and the timing lines up exactly with the addition: no fading before contact, no stalling partway through while thiosulfate keeps being added, and no blue color returning or streaking after the flask has gone clear; the final liquid looks fully clear, with no cloudiness, floating particles, or separated layers.

Spatiotemporal Consistency rubric:
level_1: The flask or burette blinks out of the shot and then reappears somewhere else partway through the clip, the flask visibly cracks or the burette bends in a way glass cannot bend, the shadows in the scene vanish and come back or the light source seems to jump to a new position, or the liquid appears to flow upward out of the burette tip.
level_2: The burette tip drifts sideways across the frame while still dispensing liquid; the flask shakes as though floating free with no hand or clamp holding it; the liquid's surface behaves inconsistently between frames (level rising and falling with no clear cause); or the lighting flickers hard enough that the objects momentarily become hard to make out.
level_3: The flask, burette, and stand keep the same size, shape, and position relative to each other throughout; the shaking looks like a real hand moving the flask back and forth around a fixed point; the liquid surface ripples and sloshes the way a real liquid would while being shaken; and the stream from the burette falls straight down rather than at an angle.
level_4: Everything at level 3 holds, and the motion runs smoothly from frame to frame with no freezing or stutter; the shaking keeps roughly the same speed and range throughout; the liquid level inside the burette drops steadily as thiosulfate is added; and the burette tip and the flask opening stay lined up with each other even as the flask moves.
level_5: Everything at level 4 holds, and the distance between the burette tip and the flask opening looks the same from the first frame to the last; the flask never tips more than a slight lean away from upright (roughly no more than 15° off vertical); nothing in the scene stretches, shears, or warps between frames; the stream of liquid falls in a straight, unbroken line from tip to surface; and no blur or smear ever hides whether the stream is touching the liquid or whether the color has changed.