Domain: Natural Science — Chemistry
Target concept: Electrolysis of Saturated Sodium Chloride Solution

Video generation prompt: Insert the graphite anode and iron cathode into a saturated sodium chloride solution, add phenolphthalein indicator, and observe the changes at both electrodes after applying electricity.

Evaluation guide:
(i) Target concept: Electrode-specific alkalization during aqueous NaCl electrolysis due to cathodic water reduction generating OH⁻ ions.
Minimum mechanism: Solution must be aqueous and saturated with NaCl; electrodes must be electrochemically appropriate (graphite anode stable under Cl₂ evolution, iron cathode catalytically inert for H₂ evolution); phenolphthalein must be present and functional (pH-sensitive, colorless <8.2, pink >10); electric current must flow to drive redox reactions.
(ii)Storyline: Frame 0–2s: Static setup: container with clear liquid, graphite and iron rods inserted vertically, wires connected, power supply off. Frame 2–3s: Power activation cue (e.g., switch flip, LED glow). Frame 3–6s: First bubbles emerge at both electrodes, with more frequent/continuous bubbling at the cathode and sparser, more intermittent bubbling at the anode; no color yet. Frame 6–9s: Pink zone appears and expands locally around cathode only; bubbles continue steadily. Frame 9–10s: Stable state: pink confined to cathode periphery, no diffusion into bulk, no anode discoloration; all elements remain fixed in viewpoint and proportion. Perspective: Fixed frontal medium shot, full container and electrodes in frame, no occlusion, top-down visibility of liquid surface and electrode tips.

Manual:
Experimental Procedure
1. Fill a clear container with saturated sodium chloride solution.
2. Add phenolphthalein indicator to solution (colorless).
3. Insert graphite anode and iron cathode side-by-side into solution.
4. Connect electrodes to DC power supply (anode to +, cathode to –).
5. Turn on power supply.
Observation of Experimental Phenomena
• Pink color appears only around iron cathode.
• Gas bubbles form at both electrodes: bubbling at the iron cathode is more frequent and continuous; bubbling at the graphite anode is comparatively sparser and more intermittent, because much of the Cl2 formed there dissolves into or reacts with the brine before it can escape as a bubble, whereas the H2 formed at the cathode is only sparingly soluble and escapes almost entirely as gas.
• No pink color appears at graphite anode or in bulk solution.
Summary of Experimental Principles
Electrolysis of saturated NaCl produces Cl₂(g) at the graphite anode and H₂(g) + OH⁻(aq) at the iron cathode; OH⁻ turns phenolphthalein pink. Iron is an inert cathode; graphite resists oxidation. Cl₂ is considerably more soluble in water than H₂, so even though equal moles of gas are produced at each electrode per Faraday's law, visible bubbling is typically more vigorous and continuous at the cathode.

Prompt Grounding rubric:
level_1: Neither the graphite anode nor the iron cathode is shown inserted in a liquid-filled container; no saturated sodium chloride solution container is visible; phenolphthalein is never added; no wires, power supply, or switch indicating electricity application appear anywhere in the clip.
level_2: Only one of the two electrodes (graphite or iron) is shown inserted into the container, with the other absent; the container holds a liquid that is not identifiable as a saturated NaCl solution (unlabeled, cloudy, or otherwise ambiguous); phenolphthalein is never shown being added; no wires, power supply, or switch appear to indicate that current is applied.
level_3: Both the graphite anode and the iron cathode are inserted into a single container of clear liquid; the container is labeled or otherwise identifiable as saturated NaCl solution; phenolphthalein is shown being added (as a colorless liquid, either dispensed on-screen or already mixed in); at least one clear visual cue (wires connected to the electrodes, a visible power-supply unit, or a switch in the ON position) indicates that electricity is being applied.
level_4: All elements from score 3 are present AND the two electrodes are visually distinguishable by material (graphite: matte black/dull gray rod; iron: metallic silver-gray rod); the liquid level is uniform with no visible sediment; phenolphthalein is added before the power is switched on; wires run from the power-supply terminals to the electrodes without crossing or ambiguity about which electrode each terminal serves.
level_5: All elements from score 4 are present AND the graphite anode and iron cathode sit side by side in the same container, both fully submerged and vertically oriented; the liquid is visibly still and transparent immediately before the power is switched on; phenolphthalein is evenly mixed through the solution with no visible streaks or undissolved droplets; the power supply shows a clear activation cue (e.g., an LED lighting up, a dial turning, or a switch clicking to ON) at or immediately before the first bubbles/color change appear; no unrelated objects, text labels, or on-screen overlays are visible.

Scientific and Causal Correctness rubric:
level_1: No pink color develops at either electrode; both electrodes remain colorless throughout; no gas bubbles form at either electrode; no localized color change is observed anywhere in the solution.
level_2: Pink color appears at the wrong electrode (e.g., pink forms at the graphite anode instead of, or in addition to, the iron cathode), or a non-pink color appears (e.g., yellow, orange); alternatively, gas bubbles appear at only one electrode with no bubbling at all at the other electrode, which is inconsistent with the simultaneous redox reactions occurring at both electrodes during electrolysis.
level_3: Pink color appears exclusively around the iron cathode; no pink appears near the graphite anode; gas bubbles appear at both electrodes, with bubbling at the iron cathode visibly more frequent/continuous than at the graphite anode, consistent with H2 gas (only sparingly soluble in water) escaping the cathode almost entirely as bubbles, while a portion of the Cl2 generated at the anode dissolves into or reacts with the brine before it can bubble out.
level_4: All elements from score 3 are present AND pink color stays confined to the region immediately surrounding the cathode, never spreading to fill the whole container or appearing near the anode; bubbling at the iron cathode is steady and continuous while bubbling at the graphite anode is comparatively sparser and more intermittent; no color change occurs before the power supply is switched on.
level_5: All elements from score 4 are present AND the pink color first appears only after the power supply is activated and then visibly deepens/spreads outward from the cathode over the course of the clip; bubble formation at both electrodes begins at essentially the same moment the power turns on; the solution shows no other color change (e.g., no yellow-green tint at the anode that would indicate Cl2 accumulation/hydrolysis byproducts) and stays clear except for the pink zone and the bubbles.

Spatiotemporal Consistency rubric:
level_1: The graphite/iron electrodes or the container disappear or jump to a new position between frames; the container's shape or size changes abruptly; the liquid surface vanishes or jumps to a different height instantly; the overhead lighting switches from bright to dark with no transition.
level_2: One electrode partially vanishes for part of the clip then reappears; the liquid surface sloshes violently with no apparent cause; a wire connecting an electrode to the power supply detaches and reattaches inconsistently across frames; background objects (e.g., table, power supply) shift position relative to the container between shots.
level_3: The graphite anode and iron cathode remain vertically inserted and stationary relative to the container throughout; the liquid surface stays level and at a constant height; wires stay connected and taut between electrodes and the power supply; no object flickers, warps, or disappears at any point in the clip.
level_4: All elements from score 3 are present AND bubble motion is physically plausible: bubbles at both electrodes rise upward without drifting sideways or sinking; electrode surfaces do not shimmer or flash unnaturally; the container's edges stay sharp and undistorted; the electrodes and container show no visible shaking or vibration from one frame to the next.
level_5: All elements from score 4 are present AND the clip shows no dropped or repeated frames; bubbles are generated continuously at both electrodes without stalling or restarting; the pink color spreads at a steady, gradual pace with no sudden jump in how much of the solution appears pink; there is no visible flicker or double-image ghosting of the electrodes or container at any point.