Domain: Engineering — Chemical Engineering
Target concept: Cooling Crystallization

Video generation prompt: Heat a saturated sodium chloride solution to high temperature, pour it into a transparent stirred crystallizer, activate the stirring device (at 50 r/min), cool it to room temperature, and continuously capture images to observe the changes in the solution.

Evaluation guide:
(i) Target concept: Cooling-induced crystallization from a saturated aqueous NaCl solution under controlled agitation.
Minimum mechanism: Solution must be initially saturated (homogeneous liquid, no solids); temperature decrease must drive supersaturation; nucleation must occur *only after* cooling begins; because NaCl's solubility changes only modestly with temperature, the resulting crystal yield is expected to be small and to grow gradually rather than abundantly or abruptly; stirring must be active *during* crystallization to ensure mass transfer and inhibit preferential wall growth; crystal habit must reflect NaCl's cubic symmetry (polyhedral, angular, non-fibrous).
(ii)Storyline: [Frame 0–2] Heated saturated NaCl solution (clear, still, thermal shimmer visible); [Frame 3–4] Solution poured into stationary transparent crystallizer; [Frame 5–6] Stirring device activates: rotation begins, fluid starts moving; [Frame 7–10] Ambient cooling progresses: solution remains transparent until ~Frame 8, then a modest number of discrete white crystals appear and slowly increase in number while suspended and dispersing; viewpoint remains fixed, lighting stable, no cuts or repositioning.

Manual:
Experimental Procedure
1. Heat saturated sodium chloride solution until visibly warm (no boiling).
2. Pour solution into transparent stirred crystallizer.
3. Activate stirring device (visible rotation begins).
4. Cool system to room temperature (no external refrigerant; ambient dissipation only).
Observation of Experimental Phenomena
- Solution remains clear and homogeneous before cooling.
- During cooling with stirring, a modest number of discrete white polyhedral crystals gradually appear, consistent with NaCl's solubility dropping only slightly as it cools.
- Crystals remain suspended and disperse rather than settling or clustering in one place.
Summary of Experimental Principles
Cooling crystallization: NaCl's solubility falls only modestly between hot and room temperature (roughly a 10% decrease), so cooling this saturated solution produces gradual, limited crystal formation rather than the abrupt, heavy crystallization seen in salts whose solubility drops sharply with temperature; mechanical stirring keeps the crystals that do form suspended and evenly distributed rather than settling or clumping in one spot.

Prompt Grounding rubric:
level_1: No saturated salt solution appears; there is no transparent stirred vessel; no stirring device is shown; neither heating, pouring, nor cooling is depicted; no recognizable sequence of steps occurs.
level_2: One correct element shows up (the transparent vessel, or the salt solution, or the stirrer), but none of the four steps (heating, pouring into the vessel, turning on the stirrer, cooling) appear in any order; the container is opaque, or the stirring element never moves.
level_3: A clear, hot salt solution is shown, poured into a transparent vessel that holds a visibly rotating stirrer, and the scene settles into a stable, unheated setting representing cooling, but at least one of these four steps appears out of order, for example the stirrer starting before the pour finishes.
level_4: The hot solution, the pour into the transparent vessel, the stirrer turning on, and the cooling period all appear in the correct order; the stirring keeps turning continuously; no unrelated action (filtering, labeling, manual shaking) is shown; the camera keeps the same framing on the vessel throughout.
level_5: In addition to the above, the solution is clearly a uniform liquid with no solids before cooling starts, the vessel's transparency makes the stirrer's shape and motion clearly visible, the stirrer turns smoothly around a fixed center, the pour finishes fully before the stirrer starts, and the whole sequence plays out from one unmoving camera angle with no cut or zoom.

Scientific and Causal Correctness rubric:
level_1: The solution stays completely clear from start to finish, or crystals show up with no relationship to cooling or stirring at all; the liquid's color never changes; nothing in the clip connects the stirring or cooling to any solid appearing.
level_2: Crystals show up before cooling starts, or the instant the solution is poured; the crystals look stringy, blob-like, or otherwise not angular/cube-like; the solution turns cloudy before any cooling is shown; something is mistaken for a crystal that is really a bubble or a streak on the glass.
level_3: Crystals begin appearing only after cooling starts and while the stirrer keeps turning; they are white or colorless, matching salt, and modest in number given how little NaCl's solubility drops on cooling; the solution stays clear beforehand and gradually shows a few scattered solid specks afterward, but the crystals cluster only near the walls or the bottom rather than showing any sign of being carried by the stirred flow.
level_4: Crystals appear only during the cooling period and only while the stirrer is running; they are small, white, and angular rather than fibrous or rounded; at least some are visibly suspended and moving with the stirred liquid rather than only settling; the solution's clarity decreases gradually as more small crystals appear, consistent with only a modest amount of crystal forming; nothing dissolves back into the liquid once formed.
level_5: The white, angular crystals appear only after cooling begins and continue at a slow, steady pace while the stirrer keeps running (a small, gradually growing amount consistent with how little NaCl's solubility falls when cooled), and they stay dispersed in the moving liquid rather than piling up in one spot; no crystal appears before cooling starts, no crystal redissolves, and nothing else in the shot (a bubble, a scratch on the glass, a reflection) could be mistaken for a crystal.

Spatiotemporal Consistency rubric:
level_1: The vessel disappears and reappears between shots; the stirrer vanishes then comes back; the lighting jumps from day-bright to dark within a single frame; the liquid level changes with no pouring or evaporation shown to explain it; the vessel's shape distorts or twists unnaturally.
level_2: The stirrer's rotation stalls and restarts for no visible reason; the liquid level changes without any pour or evaporation being shown; the background behind the vessel shifts noticeably; the lighting flickers or casts a shadow that doesn't match the rest of the scene; crystals drift in a way that has nothing to do with the stirrer's motion.
level_3: The stirrer turns smoothly enough that its speed only wavers slightly; the vessel and its contents stay in place with no teleporting or warping; the lighting doesn't invert or flip; but one small flaw remains, such as a brief patch of motion blur that momentarily hides a crystal just as it appears.
level_4: The stirrer rotates smoothly and without interruption; the crystals move in a way that clearly follows the stirred flow (drifting, turning, carried along in a loose vortex); the liquid's surface moves gently and plausibly rather than staying perfectly flat or jumping; nothing outside the vessel shifts unexpectedly, and the view through the transparent walls stays consistent.
level_5: The stirrer's rotation stays smooth and centered throughout; the crystals' paths look consistent with a slow, steady stir rather than random tumbling; the liquid's surface and meniscus behave the same way from one moment to the next; the background seen through the vessel walls stays in the same place relative to the vessel; and the whole clip moves smoothly with no visible juddering, flickering, or stuttering in the motion.