Domain: Natural Science — Chemistry
Target concept: Sulfur dioxide causes the fuchsine solution to decolorize.

Video generation prompt: A test tube contains 5 mL of red fuchsine solution, into which sulfur dioxide gas is introduced.

Evaluation guide:
Target concept: SO₂ acts as a selective reducing agent that disrupts the chromophore of fuchsine, causing irreversible (under ambient conditions) loss of red color. Minimum mechanistic assumptions: (i) fuchsine solution is initially red due to conjugated quinoid structure; (ii) SO₂ dissolves to form sulfurous acid; (iii) sulfurous acid reduces the dye’s oxidized form to a colorless leuco compound; (iv) this reaction requires direct molecular contact; hence visual correlation between bubble entry and localized fade onset is essential for verification. Phased storyline: [Phase 1: Stable red baseline] → [Phase 2: First SO₂ bubble contacts liquid → immediate micro-fade at interface] → [Phase 3: Progressive, uniform red attenuation across solution column] → [Phase 4: End-state: transparent, colorless/pale yellow liquid]. Visibility constraints: Must capture (a) unambiguous red-to-colorless transition (no hue shift), (b) spatial co-location of bubbles and initial fade, (c) temporal lock between gas onset and fade onset (≤2 s lag), and (d) absence of confounding factors (no shaking, heating, or added reagents). Perspective: Front-facing, static, macro view of test tube mid-column; white background mandatory for color fidelity.

Manual:
I. Basic Demonstration Information
Demonstration Title: Sulfur Dioxide Decolorizes Fuchsine Solution Demonstration
Subject: Natural Science - Chemistry - Inorganic Reactions & Gaseous Reactivity
Demonstration Purpose: To visually demonstrate the characteristic decolorizing effect of sulfur dioxide (SO₂) on acidic fuchsine solution — a classic test for SO₂; to reinforce recognition of SO₂ as a reducing agent; and to train observation of color-state transitions as diagnostic chemical evidence.
II. Demonstration Carrier/Equipment
Core Carrier: 1 clean, dry 15-mL test tube containing exactly 5 mL of freshly prepared red fuchsine solution (pH ~4–5, standardized concentration for vivid red hue)
Auxiliary Equipment: SO₂ gas generation apparatus (e.g., Na₂SO₃ + H₂SO₄ in dropping funnel flask, with gas outlet tube); rubber stopper with dual-hole adapter; delivery tube (glass or inert plastic) ending 0.5 cm above tube bottom; white tile or paper background for color contrast; timer (optional, for pacing).
III. Demonstration Content Description
Layout: Place test tube upright in test tube rack against white background. Insert delivery tube vertically into solution until tip is 0.5 cm above bottom — ensure full submersion of tip during gas flow.
State Control: Begin SO₂ generation only after setup is complete; maintain steady, gentle gas flow (1–2 bubbles/sec) for 8–10 seconds. Do not shake or heat.
Comparison Baseline: Retain an identical untreated fuchsine tube off-screen as mental reference for ‘fully red’ state.
IV. Observation of Core Phenomena
Initial State: Solution is uniformly, intensely red (RGB ~220,40,60), transparent, no sediment, clear meniscus at ~5 mL mark.
Transition Phase: Within 2–3 s of first bubble contact, localized paleness appears at bubble path; red intensity diminishes gradually and uniformly across entire column.
Final State: After 8–10 s, solution becomes nearly colorless or very pale straw-yellow — transparency fully retained, no precipitate, no cloudiness. Color loss is irreversible under ambient conditions.
Visual Feature Recognition: The *exclusive association* of fading onset with SO₂ bubble entry — not time, light, or motion — confirms causal specificity. Red → colorless transition is the sole diagnostic visual signature; no other color intermediates (e.g., orange, purple) appear.
V. Summary of Key Knowledge Points and Principles
Core Concept Definition: SO₂ selectively decolorizes fuchsine due to reductive cleavage of the quinoid chromophore, forming a colorless leuco compound — a definitive, sensitive, and historically standard test for SO₂ presence.
Chemical Principle: SO₂ + H₂O ⇌ H₂SO₃; H₂SO₃ reduces the oxidized (colored) form of basic fuchsine (triphenylmethane dye) to its reduced (achromatic) leuco base. This is *not* oxidation, bleaching, or dilution — it is a specific redox reaction.
Diagnostic Specificity: Unlike Cl₂ (oxidative bleach, destroys dye), SO₂ action is reversible upon heating/aeration — but reversal is *not required* for 10-s demonstration; absence of reversal in short video does *not* invalidate correctness.
VI. Demonstration Precautions
Solution Freshness: Use fuchsine solution prepared ≤24 h prior — aged solutions show baseline fading.
Gas Purity: Avoid CO₂ or air contamination — use dedicated SO₂ generator; confirm gas identity via odor (pungent, burnt-match) if audio included.
Tube Clarity: Use borosilicate glass; avoid scratches or stains that distort red perception.
Timing: Strict 10-s maximum — start timer at first bubble entry, end at 10 s; decolorization must be *initiated and advanced* within this window.
Safety: Perform in fume hood; SO₂ is toxic — no direct inhalation.

Prompt Grounding rubric:
level_1: No test tube is visible, or there's no red liquid in it; the liquid isn't red (e.g., colorless, purple, or blue) or its volume is clearly nowhere near 5 mL; there's no gas-delivery tube inserted into the liquid; or the scene doesn't resemble a chemistry lab setup at all.
level_2: A test tube with colored liquid is present, but the color is ambiguous (e.g., pinkish, orange, or brown) rather than clearly red; the volume looks clearly off from the expected 5 mL, for example noticeably less than 2 mL or more than 8 mL in the tube; a gas-delivery tube is only vaguely suggested without being visibly inserted into the liquid.
level_3: The test tube holds a visibly red liquid at roughly the expected 5 mL fill level; a gas-delivery tube is inserted into the liquid, but its tip position is unclear (e.g., it looks like it's only touching the surface rather than submerged) or the framing partly cuts off the tube.
level_4: The test tube holds about 5 mL of clearly red liquid; a gas-delivery tube is inserted below the liquid surface and stays in place, with visible bubbles showing gas is being introduced; the shot is framed so the full liquid column is visible against a plain background; only small imperfections remain, such as slight glare or a marginally off-center tube.
level_5: A single test tube holds a clearly measured ~5 mL of vivid red liquid; a clear gas-delivery tube is inserted well below the liquid surface, held steady, and visibly delivers a continuous, steady stream of small bubbles into the liquid for at least 3 seconds without interruption; the tube is fully in frame against a plain, evenly lit background with no glare, matching the prompt's setup exactly.

Scientific and Causal Correctness rubric:
level_1: There's no connection between the gas and the color change: the red fades on its own, or because of shaking, pouring, or added water rather than gas contact; or the solution turns some other color entirely (e.g., blue, green, or black), outcomes that don't match how SO₂ reacts with fuchsine.
level_2: A color change happens, but in the wrong direction (the red gets darker or turns pink instead of fading); or the fading happens without any gas being introduced (e.g., from heating or just time passing); or the bubbling pattern and timing don't look like a mild gas being introduced through the liquid.
level_3: Decolorization happens and is linked to gas entering the tube, but the gas itself doesn't clearly look like SO₂ being introduced (for instance the bubbling is unlabeled, or far too fast or slow to look like a real gas stream), so it isn't clear the fading is specifically caused by that gas rather than just agitation.
level_4: Decolorization is clearly triggered only once gas bubbles enter the red liquid, with fading beginning near where the bubbles enter and spreading through the rest of the tube in a way consistent with SO₂ gradually reacting with the fuchsine; only small issues remain, such as slightly uneven spreading or an overly generic label for the gas.
level_5: The red liquid stays stable until the gas bubbles make contact, then fades steadily starting near the bubbles and spreading evenly toward colorless or pale yellow, with no gray tint, no cloudiness, and no separate precipitate forming, matching how SO₂ reduces fuchsine's colored form to a colorless one without any competing effect such as heating or added chemicals.

Spatiotemporal Consistency rubric:
level_1: The clip breaks basic continuity: the delivery tube's position or angle changes abruptly with no hand visibly moving it, or the liquid's volume changes with no visible cause; gas bubbles appear to travel downward from the surface instead of rising; a hand appears to insert the delivery tube only after the solution has already turned colorless; or the red color comes back after it had already faded partway through the clip.
level_2: Continuity is mostly there but with clear gaps: the delivery tube is shown at the start but its entry into the liquid isn't shown; bubbling begins well after the tube is inserted with no visible reason for the gap; the color starts fading before any bubble reaches the liquid; a hand holding the tube withdraws before any gas appears to flow; or the lighting shifts enough to make the red look like a different color.
level_3: The gas tube stays in the liquid and fading happens afterward in a sensible order, but finer details are missing: bubbling is visible yet the liquid surface never seems to move from it, or the pace of fading doesn't clearly track with how much gas has been introduced.
level_4: The sequence flows clearly: the tube enters the liquid, bubbles form at its tip and rise, the color nearest the bubbles fades a little first, and then the whole tube gradually lightens; lighting stays consistent, the liquid surface stays undisturbed, and nothing blocks the view during the key fading moments.
level_5: The whole sequence is physically coherent from start to finish: gas bubbles begin rising only once the tube tip is submerged, the color nearest the bubbles noticeably fades first, and the fading then spreads outward through the rest of the liquid at a steady, unhurried pace; the liquid surface stays calm with no splashing; lighting, focus, and camera position stay fixed throughout with no abrupt cuts or speed changes.