Domain: Natural Science — Environmental Science
Target concept: Determination of Lead in Water by the Dithizone Spectrophotometric Method

Video generation prompt: Take 2 mL of lead-containing water sample into a test tube, add 2 mL of dithizone chloroform solution, shake thoroughly, then allow to stand and separate into layers.

Evaluation guide:
(i) Target concept: Liquid-liquid extraction of a metal-dithizonate chelate complex via density-driven phase separation, with a visible color shift as the diagnostic signal for lead.
Minimum mechanism: Dithizone must be dissolved in chloroform (not water) and is green in this solvent; chloroform's density is greater than water's; Pb²⁺ present in the aqueous sample reacts with dithizone to form lead(II) dithizonate, a red/violet chelate complex; this complex is hydrophobic and partitions into the chloroform layer, shifting that layer's color from green toward red/violet in proportion to the lead present; no chemical decomposition or emulsification occurs.
(ii)Storyline: [Phase 1: Setup] Test tube centered in frame, containing only a colorless-to-pale liquid (the aqueous sample). [Phase 2: Addition] Green dithizone-chloroform solution is dispensed into the same tube; the color contrast with the aqueous layer is immediately visible. [Phase 3: Mixing] The tube is shaken; the liquids become turbid and temporarily homogenized, during which the green reagent contacts and reacts with the dissolved Pb²⁺. [Phase 4: Separation] The tube is held still; the turbidity clears, the organic phase migrates downward, and a sharp horizontal interface forms and stabilizes; the settled lower layer shows a visible shift from the reagent's original green toward a red/violet hue, reflecting the formation of lead dithizonate. Perspective: fixed frontal close-up, no occlusion, full tube height visible throughout; all transitions are continuous and causally linked (shaking → mixing/reaction → settling → stratification with color shift).

Manual:
Experimental Procedure
1. Place 2 mL of the lead-containing water sample into a clean test tube.
2. Add 2 mL of green dithizone chloroform solution to the same test tube.
3. Cap and shake thoroughly for about 3 seconds so the two phases mix and the dithizone can react with the dissolved Pb²⁺.
4. Place upright and allow to stand until the phases separate completely.
Observation of Experimental Phenomena
Two clear, immiscible layers form: an upper colorless-to-pale aqueous layer, and a lower chloroform layer that shifts from the reagent's original green toward a red/violet tint as the lead-dithizonate complex forms — the more lead present, the stronger the red/violet coloration in the settled organic layer.
Summary of Experimental Principles
Pb²⁺ reacts with dithizone (green in chloroform) to form lead(II) dithizonate, a red/violet chelate complex that partitions into the denser chloroform layer; the resulting green-to-red/violet color shift in the settled lower layer is the diagnostic signal used to spectrophotometrically quantify the lead content of the water sample.

Prompt Grounding rubric:
level_1: No test tube is present; no water sample is visible; no dithizone chloroform solution is visible; no shaking action occurs; no layered separation is ever shown.
level_2: A test tube holds some liquid, but its volume is clearly not close to 2 mL; the dithizone chloroform solution is missing, in the wrong container, or an implausible color for this reagent (e.g., colorless or opaque); no shaking action is performed, or it is performed on the wrong object; no standing/settling phase is shown at all.
level_3: The test tube contains two visibly distinct liquids, one aqueous and one organic; both are added to the same tube; a shaking action is clearly applied to that tube; afterward, the tube is shown held still while the two liquids visibly separate into two layers.
level_4: Everything required for a 3 is present, and the two liquids are added in the correct order: first the water sample, then the green dithizone chloroform solution; shaking happens only after both are present; the standing phase clearly shows two immiscible layers with a sharp horizontal interface.
level_5: Everything required for a 4 is present, and the dithizone chloroform solution is visibly green at the moment it is added, before any reaction takes place; the aqueous layer stays colorless or near-colorless throughout; the organic (chloroform) phase always settles as the lower layer, consistent with its higher density; the interface between layers is flat and fully stabilized within the visible duration; there is no spillage, droplet carryover, or visible mixing artifact once the layers have separated.

Scientific and Causal Correctness rubric:
level_1: No color change occurs in the organic layer at all; the layers never separate; the two liquids look fully miscible or homogeneous throughout; or the lower layer ends up a color inconsistent with any real dithizone chemistry (e.g., blue or black).
level_2: Two layers form, but the lower layer stays purely green with no shift toward red or violet even though lead is present in the sample, showing no evidence of a reaction; or the color shift shows up in the upper (aqueous) layer instead of the lower (chloroform) layer, which is physically implausible given chloroform's higher density; or a red/violet color appears before the two liquids have been mixed by shaking.
level_3: Two immiscible layers form; the lower (chloroform) layer visibly shifts from the reagent's original green toward a red or reddish-violet tint after shaking; the upper aqueous layer stays colorless or near-colorless; the color shift only appears after the shaking/mixing step, never before; however the shift looks faint, patchy, or only partially covers the lower layer.
level_4: Everything true at the 3 level still holds, and the red/violet coloration spreads evenly through the entire lower layer rather than staying patchy or streaked; no green patches remain once the color shift is shown as complete; the color stays stable (it does not fade or revert to green) once settling is finished.
level_5: Everything true at the 4 level still holds, and the lower layer displays a saturated, uniform red-to-violet hue with no lingering green cast and no stray color patches; the upper aqueous layer stays fully transparent and colorless with no red/violet bleed-through; the interface between the two layers is optically sharp, with no haze or distorted meniscus; the relative thickness of the two layers visually matches roughly equal (2 mL : 2 mL) volumes of sample and reagent.

Spatiotemporal Consistency rubric:
level_1: The test tube vanishes from the shot or reappears somewhere else in frame; the liquid volume jumps between levels with no shaking or pouring shown; the shaking motion itself skips or stutters as if frames were missing; the two layers appear fully separated instantly with no shaking or settling shown at all; the overall brightness of the shot flips abruptly between frames.
level_2: The camera angle or framing changes mid-action, for example a cut occurs partway through the shaking; the tube tilts or rotates inconsistently between the setup, shaking, and settling phases; the liquid surface keeps rippling long after the shaking has stopped; the two layers form with visible turbulence that never calms down; the background or viewpoint shifts noticeably between the setup, shaking, and standing phases.
level_3: A single fixed camera viewpoint is kept for the whole clip; the tube stays still during the standing/settling phase; the shaking motion looks like a continuous back-and-forth rather than a jerky or teleporting motion; the liquid surface settles smoothly into stillness after the shaking stops; nothing about the tube or liquid clips through itself or morphs into a different shape.
level_4: Everything true at the 3 level still holds, and the framing (zoom level and tube position in frame) stays identical across the setup, shaking, and standing phases; the tube's position and tilt are the same in every phase; the liquid's motion during shaking shows realistic momentum that gradually dies down rather than stopping instantly; there is no flicker, stutter, or ghost trailing of the tube or liquid at any point.
level_5: Everything true at the 4 level still holds, and the shaking motion accelerates and decelerates naturally rather than moving at a constant mechanical speed; the liquid's sloshing visibly calms down in a smooth, continuously decreasing way rather than stopping abruptly; there is no repeated or duplicated frame anywhere in the sequence; the lighting and the shadow the tube casts stay the same in direction, brightness, and softness from the first frame to the last.