Domain: Healthcare — Pharmaceutical Chemistry
Target concept: Identification of Pethidine Hydrochloride (Formaldehyde-Sulfuric Acid Colorimetric Method)

Video generation prompt: Place a test tube containing a small amount of pethidine hydrochloride sample on the lab bench. Slowly add concentrated sulfuric acid along the wall of the test tube, followed by one drop of formaldehyde solution. Gently swirl to mix. Use only a pipette and test tube. Fix the camera position and shoot a close-up video with smooth and continuous operation.

Evaluation guide:
(i) Target concept: Interface-localized, formaldehyde-triggered, sulfuric acid-mediated condensation yielding a violet-purple chromophore specific to pethidine.
Minimum mechanism: Pethidine must be present as solid salt; sulfuric acid must form a distinct immiscible lower layer; formaldehyde must contact the interface to initiate electrophilic aromatic substitution; color formation requires protonation and dehydration steps enabled only by concentrated H2SO4.
(ii)Storyline: [Frame 0] Static close-up: test tube on bench, white crystalline solid at bottom. [Frame 1–2] Pipette enters frame, delivers colorless viscous liquid (H2SO4) down inner wall → forms lower layer. [Frame 3] Single spherical formaldehyde drop lands precisely at acid–solid interface. [Frame 4] Violet-purple hue appears *only* at contact line within 1–2 s. [Frame 5] Swirling begins → color spreads upward from interface while maintaining coherence. [Frame 6] Final mixed solution is homogeneous violet-purple. Perspective: fixed frontal close-up; all events visible without occlusion; no off-screen actions; interface must remain in focus throughout.

Manual:
Experimental Procedure
1. Place a clean test tube containing a small amount of white crystalline pethidine hydrochloride on the lab bench.
2. Using a pipette, slowly deliver concentrated sulfuric acid along the inner wall of the test tube.
3. Add exactly one drop of formaldehyde solution onto the acid–sample interface.
4. Gently swirl the test tube to mix.
Observation of Experimental Phenomena
A distinct violet-to-purple color develops at the interface between the sulfuric acid and pethidine hydrochloride within seconds after formaldehyde addition, intensifying and spreading upward upon swirling.
Summary of Experimental Principles
Pethidine hydrochloride reacts with formaldehyde in concentrated sulfuric acid (Marquis reagent) to form a conjugated quinoidal structure that absorbs visible light in the violet-purple range — a specific colorimetric identification test.

Prompt Grounding rubric:
level_1: No test tube is shown; no white crystalline pethidine hydrochloride sample is visible; concentrated sulfuric acid is never added; formaldehyde solution is never added; no pipette appears in the shot; the camera is not fixed (visible pans, zooms, or handheld movement); or the shot is not a close-up of the test tube.
level_2: The test tube contains a white solid but no pipette ever appears; sulfuric acid is not delivered along the inner wall of the test tube; formaldehyde is not added as a single drop; no swirling motion occurs; the camera moves or zooms during the shot; or the framing is a wide shot rather than a close-up centered on the test tube.
level_3: The test tube holds a small amount of white crystalline pethidine hydrochloride; a pipette delivers liquid into the tube; concentrated sulfuric acid is added first, appearing colorless and viscous as it runs down the inner wall; one discrete drop of formaldehyde is added second; the tube is gently swirled afterward; the camera stays fixed for the whole shot; the framing is a close-up centered on the test tube.
level_4: Everything described at the 3-point level also holds, and for this pethidine/sulfuric-acid/formaldehyde sequence: the pipette is held roughly vertical while dispensing; the sulfuric acid visibly touches the inner glass wall before running down to the sample; the formaldehyde drop lands on the liquid surface without splashing; the swirl produces an even rotation of the liquid surface; the test tube itself never shifts position on the bench.
level_5: Everything described at the 4-point level also holds, and: no object other than the test tube, pipette, and bench surface enters the frame; the pipette tip never touches the inside of the test tube; the sulfuric acid stays visibly distinct from the solid sample until formaldehyde is added; the formaldehyde drop keeps a round shape on contact before it disperses into the acid layer; swirling starts only once the formaldehyde drop has been added and continues smoothly as the reaction proceeds; and the whole sequence plays as one continuous shot with no cuts or jumps.

Scientific and Causal Correctness rubric:
level_1: The sulfuric acid–pethidine mixture stays colorless, white, or brown for the entire clip, with no violet or purple hue appearing at the acid–sample interface at any point; or a color other than violet/purple appears in its place (e.g., blue, green, yellow, black); or there is no visible interface between the acid layer and the solid sample before the color reaction is supposed to occur.
level_2: A color does appear but is wrong for this reaction (blue, green, yellow, or black instead of violet-to-purple); or the color appears immediately upon sulfuric acid contact, before the formaldehyde drop is added; or the color spreads through the whole liquid at once rather than starting at the acid–sample interface; or the color fades away within about a second of first appearing.
level_3: A violet-to-purple color develops at the interface between the sulfuric acid layer and the pethidine hydrochloride sample within roughly 1–3 seconds after the formaldehyde drop is added; the color stays localized to the contact zone when it first appears; no other hue appears; the violet-purple color is still visible more than 2 seconds after swirling stops.
level_4: Everything described at the 3-point level also holds, and for this Marquis-type reaction: the color originates only at the sulfuric acid–sample interface, not at the air-liquid surface or where the pipette tip touched the liquid; the color visibly deepens over at least 2 seconds after the formaldehyde drop lands; the color stays mostly confined to the lower part of the liquid column while swirling is happening; no cloudiness, precipitate, or bubbling appears.
level_5: Everything described at the 4-point level also holds, and: no violet hue appears anywhere before the formaldehyde drop touches the acid–sample interface; once swirling begins, the violet-purple color visibly spreads upward from the interface through the liquid; the final mixed solution shows a uniform violet-purple color with no streaks, fading, or separation into layers; and the color's origin and shade are specific to a positive pethidine identification, not a generic acid-charring color or unrelated side reaction.

Spatiotemporal Consistency rubric:
level_1: The test tube or pipette appears or vanishes from the shot with no motion connecting the two states; the liquid inside the tube jumps from one level or position to another between frames with no continuous flow shown; the tube tips or rotates by more than about 30 degrees with no hand or support visible causing it; the overhead lab lighting suddenly brightens or dims between frames with no visible cause; or the background behind the tube visibly warps or stretches.
level_2: The test tube shakes or wobbles hard while the acid is being added; the pipette jerks or trembles noticeably as it moves; the swirling motion starts or stops abruptly rather than speeding up or slowing down gradually; the liquid surface moves unnaturally or breaks apart instead of flowing smoothly; or the highlight/reflection on the glass jumps to a different spot between one frame and the next without the tube or light moving.
level_3: The test tube stays steady on the bench throughout; the pipette approaches and withdraws smoothly; the sulfuric acid runs continuously down the inner wall rather than in stops and starts; the formaldehyde drop forms and detaches cleanly from the pipette tip; the swirl turns in one consistent rotational direction; the liquid surface changes shape gradually as it is mixed; there are no visible cuts, flashes, or dropped frames.
level_4: Everything described at the 3-point level also holds, and for this close-up shot of the reaction: the pipette fully withdraws after each delivery instead of lingering over the tube; the acid stream flows steadily without dripping, splattering, or pulsing; the formaldehyde drop falls straight down without drifting sideways; the swirl keeps a steady rotational speed through a full turn of the liquid without stalling partway; and the test tube keeps the same position and size relative to the edge of the bench throughout.
level_5: Everything described at the 4-point level also holds, and: no blur hides the edges of the test tube, pipette, or liquid surface; the rim of the test tube and the liquid's meniscus line up consistently from one frame to the next; the swirling liquid shows no visible flicker or strobing; the pipette tip is never hidden behind the test tube wall during delivery; and the curve of the liquid surface is not warped or bent by any lens distortion.