Domain: Natural Science — Chemistry
Target concept: Substitution reaction of ethyl bromide

Video generation prompt: Add ethyl bromide and aqueous sodium hydroxide to a test tube, heat to boiling, cool, acidify with nitric acid, and then add silver nitrate solution.

Evaluation guide:
(i) Target concept: Visual confirmation of Br⁻ ion generation via substitution followed by selective precipitation as AgBr.
Minimum mechanism: Ethyl bromide must react under basic aqueous heat to release Br⁻; OH⁻ must be removed by acid before Ag⁺ addition to prevent Ag₂O formation; AgBr must precipitate as a pale yellow (cream) solid only post-acidification.
(ii)Storyline: [Step 1] Two colorless liquids (ethyl bromide, NaOH) enter test tube → [Step 2] Test tube heated → visible sustained bubbling/steam emerges → [Step 3] Flame removed, test tube sits motionless → steam ceases → [Step 4] Colorless HNO₃ added → no precipitate → [Step 5] Colorless AgNO₃ added → immediate appearance of a pale yellow (cream), curdy precipitate confined to test tube. Fixed frontal viewpoint; all actions occur within same continuous shot or seamless cuts preserving spatial layout; precipitate must not appear prematurely, transiently, or outside target vessel.

Manual:
Experimental Procedure
1. Add ethyl bromide (colorless liquid) and aqueous sodium hydroxide (colorless liquid) to a test tube.
2. Heat the mixture to boiling (visible bubbling/steam).
3. Cool the test tube to room temperature.
4. Acidify with dilute nitric acid (colorless liquid).
5. Add silver nitrate solution (colorless liquid).
Observation of Experimental Phenomena
A pale yellow (cream) precipitate forms immediately after silver nitrate addition, only after acidification; no precipitate forms before acidification or in reagent containers.
Summary of Experimental Principles
Ethyl bromide undergoes nucleophilic substitution with OH⁻ to form ethanol and bromide ions; acidification removes interfering OH⁻/CO₃²⁻; Ag⁺ then reacts with Br⁻ to form insoluble AgBr (a pale yellow, cream-colored precipitate).

Prompt Grounding rubric:
level_1: No test tube visible; no ethyl bromide container visible; no aqueous NaOH container visible; no heating apparatus visible; no nitric acid container visible; no silver nitrate container visible; none of the five steps depicted.
level_2: At least one correct container is present (e.g., the test tube) and at least one reagent container is visible, but no reagent is shown being added to the tube, and heating, cooling, acidification, and silver nitrate addition are all absent or shown out of order.
level_3: The test tube receives ethyl bromide then aqueous NaOH in visible sequence; heating is shown (flame under the tube or rising steam); cooling is implied (flame removed, tube sits undisturbed); acidification with HNO₃ and addition of AgNO₃ both occur; all five steps appear in the correct order, but at least one step lacks a clearly visible liquid transfer into the tube.
level_4: All five steps are clearly shown in order with unambiguous liquid transfer each time: ethyl bromide in, NaOH in, boiling with visible bubbling/steam, cooling with no heat source and no steam, then HNO₃ followed by AgNO₃; the same test tube is used throughout and no extraneous equipment appears.
level_5: All conditions for score 4 are met, and additionally: every reagent container looks chemically plausible (all four reagents appear as colorless liquids before addition); the same physical test tube is used for every step with no swap; no jump-cut hides any transfer; the camera framing stays fixed for the whole sequence; the acid and the silver nitrate are delivered using the same type of dropper/pipette rather than switching tools between steps.

Scientific and Causal Correctness rubric:
level_1: No precipitate forms after silver nitrate addition; the mixture stays colorless and transparent throughout; nothing consistent with bromide displacement or halide precipitation is visible at any stage.
level_2: A precipitate appears, but at the wrong stage (e.g., during heating or right after acidification, before AgNO₃ is added), or its color has nothing to do with the AgCl/AgBr/AgI ladder (e.g., black or deep orange), or no precipitate forms even after AgNO₃ is added.
level_3: A precipitate appears only after both acidification and AgNO₃ addition, confined to the reaction test tube; the precipitate reads as pale yellow to cream rather than pure white or deep yellow; heating shows visible bubbling consistent with boiling; no precipitate appears in any reagent container.
level_4: All conditions for score 3 are met, and additionally: no dark or brown precipitate forms before acidification (ruling out Ag₂O from residual hydroxide); the pale-yellow/cream precipitate appears promptly upon contact with AgNO₃ -- within about a second, not after a visible delay; the precipitate neither dissolves nor changes color afterward; no bubbling occurs during the acidification step (ruling out carbonate interference).
level_5: All conditions for score 4 are met, and additionally: the precipitate reads distinctly pale yellow/cream — visibly deeper than a pure-white AgCl precipitate would look, yet clearly lighter than a deep-yellow AgI precipitate; its texture appears curdy and clotted rather than a fine haze or gel; the precipitate forms only in the reaction tube; the solution stays clear and colorless through heating, cooling, and acidification, turning turbid pale-yellow only at the moment AgNO₃ is added.

Spatiotemporal Consistency rubric:
level_1: The test tube vanishes and reappears; a reagent vial (ethyl bromide, NaOH, HNO₃, or AgNO₃) teleports between frames; the heating flame appears or disappears with no transition; the scene's lighting swings abruptly between bright and dark with no cause (e.g., the bench suddenly goes dark then flashes back bright between the heating and cooling steps); the camera angle switches abruptly mid-addition (e.g., side view to top-down); a visible cut or flash interrupts a step.
level_2: The test tube abruptly changes size or shape between steps; a reagent vial changes its label or liquid color inconsistently between appearances; the flame flickers erratically with no continuity; steam appears in a single frame then vanishes; the cooled tube shows condensation appearing instantly rather than gradually; the acidification and silver-nitrate additions use visibly different droppers with no explanation.
level_3: The test tube keeps the same position, orientation, and appearance across all five steps; the reagent vials keep fixed positions relative to the tube; steam during heating stays anchored at the tube mouth rather than jumping around the frame; cooling is shown by the flame being withdrawn and the tube sitting undisturbed; the acid and AgNO₃ are added from the same side of frame with visually matching droppers.
level_4: All conditions for score 3 are met, and additionally: liquid is visibly flowing during each addition rather than jumping from empty to full; steam rises steadily without dropping out mid-shot; the dropper tip stays aligned with the tube opening across frames with no parallax jump; the dropper never passes through the tube wall.
level_5: All conditions for score 4 are met, and additionally: sustained bubbling from heating clearly precedes the cooling phase rather than the reverse; the tube sits undisturbed noticeably longer during cooling than the brief moment needed for one drop addition; each dropper's motion is a single smooth approach-contact-retract with no reversal or hovering; the AgNO₃ vial never occludes the test tube and then reappears behind it without a visible motion bridging the two positions.