Domain: Natural Science — Chemistry
Target concept: Detection of Tin Ions in Cassiterite (Sodium Hypophosphite Reduction Method)

Video generation prompt: Place stannic oxide powder in a beaker, add concentrated hydrochloric acid to dissolve it, and then add sodium hypophosphite solution after heating.

Evaluation guide:
Target concept: Qualitative detection of Sn⁴⁺ via thermally activated reduction to elemental tin (Sn⁰) using sodium hypophosphite in concentrated HCl. Minimum mechanistic assumptions: (1) SnO₂ is insoluble in water but dissolves in hot concentrated HCl to form [SnCl₆]²⁻ (pale yellow); (2) H₂PO₂⁻ reduces Sn⁴⁺ to Sn⁰ only under acidic, heated conditions; (3) Sn⁰ forms as a colloidal, matte-black precipitate with instantaneous nucleation and high visual contrast. Phased storyline: [Phase 1: Setup] White SnO₂ powder in beaker → [Phase 2: Acid dissolution] HCl added → immediate white fumes + pale yellow solution → [Phase 3: Thermal activation] Gentle boiling sustained ≥5 s → [Phase 4: Reduction trigger] NaH₂PO₂ solution added → [Phase 5: Diagnostic outcome] Instant (≤1 s) formation of dense, matte-black precipitate. Visibility constraints: Must capture fuming (HCl gas), solution color (pale yellow), boiling (micro-bubbles), and precipitate onset (localized black nucleation spreading uniformly). Perspective: Top-down or 45° oblique view ensuring beaker interior, pipette tip, and precipitate formation are simultaneously visible without occlusion.

Manual:
I. Basic Demonstration Information
Demonstration Title: Detection of Tin Ions in Cassiterite (Sodium Hypophosphite Reduction Method)
Subject: Chemistry - Inorganic Analysis - Qualitative Analysis of Metal Ions
Demonstration Purpose: To visually confirm the presence of Sn⁴⁺ ions derived from cassiterite (SnO₂) via reduction to elemental tin (Sn⁰) using sodium hypophosphite under acidic, heated conditions; to establish visual recognition criteria for the diagnostic black precipitate of metallic tin; and to reinforce the principle that tin(IV) is reducible to tin(0) by hypophosphite in HCl medium.

II. Demonstration Carrier/Equipment
Core Carrier: Stannic oxide (SnO₂) powder (white, fine crystalline), concentrated hydrochloric acid (fuming, ~37%), aqueous sodium hypophosphite (NaH₂PO₂) solution (colorless to pale yellow), borosilicate beaker (100 mL), hotplate with visible heating element (red-glowing), dropper/pipette, lab stand with clamp (to stabilize beaker during heating).

III. Demonstration Content Description
Layout: Place beaker on hotplate. Add ~0.5 g white SnO₂ powder. Add 10 mL concentrated HCl — observe immediate white fuming and dissolution to pale yellow solution. Activate hotplate to gentle boil (small, steady bubbles) for ≥5 s. Then add 2 mL NaH₂PO₂ solution dropwise from pipette.
State Control: Maintain heating *throughout* phosphite addition. Ensure solution remains in gentle reflux — no splashing or violent boiling.
Comparison Baseline: Contrast pre-addition (clear pale yellow solution with fuming) vs. post-addition (instantaneous formation of dense, matte-black precipitate settling at bottom).

IV. Observation of Core Phenomena
Pre-Reduction State: White SnO₂ powder dissolves in HCl with vigorous white fumes (HCl gas); resulting solution is transparent pale yellow — characteristic of hexachlorostannate(IV) ion [SnCl₆]²⁻.
Reduction Event: Within 1 second of NaH₂PO₂ addition *under heating*, a dense, jet-black, finely divided precipitate forms instantly and uniformly — elemental tin (Sn⁰), distinct from gray oxides or brown organics.
Characteristic Stability: Precipitate remains matte-black, non-reflective, and insoluble upon standing; no fading, bubbling, or secondary color change — confirming irreversible reduction to Sn⁰.
Auxiliary Verification: No black precipitate forms if phosphite is added *before* heating or to cold solution — proving thermal activation is essential for kinetically viable reduction.

V. Summary of Key Knowledge Points and Principles
Core Concept Definition: This method exploits the strong reducing power of hypophosphite (H₂PO₂⁻) in acidic, heated media to reduce Sn⁴⁺ → Sn⁰, yielding a visually unambiguous black precipitate — a definitive qualitative test for tin(IV) ions.
Chemical Principle: Sn⁴⁺ + 2H₂PO₂⁻ + 2H₂O → Sn⁰↓ + 2H₃PO₃ + 2H⁺. Heating accelerates both SnO₂ dissolution and electron transfer. The black color arises from light absorption by nanoscale metallic tin particles — a signature optical feature distinguishable from MnO₂ (brown-black, granular) or CuO (black but forms slowly, no fuming prerequisite).
Further Knowledge: Cassiterite is the primary ore of tin; its conversion to soluble Sn⁴⁺ via HCl is prerequisite for detection. Sodium hypophosphite is preferred over other reductants (e.g., Fe²⁺) because it gives sharper, more selective tin precipitation without interfering metal co-precipitates.

VI. Demonstration Precautions
Reagent Purity: Use analytical-grade SnO₂ (no Fe/Cu impurities) to avoid false-color precipitates; HCl must be concentrated (≥35%) to ensure full dissolution and fuming.
Heating Control: Overheating causes H₃PO₃ decomposition (gas evolution), masking tin formation; underheating yields slow/no reduction. Target gentle reflux — small, continuous bubbles.
Timing Precision: Phosphite *must* be added while solution is actively boiling; delay >2 s after heat-off eliminates precipitate formation — a critical verification control.
Safety: Conduct in fume hood; wear goggles/gloves — HCl fumes are corrosive; hot beaker surfaces pose burn risk.

Prompt Grounding rubric:
level_1: No beaker appears anywhere in the frame; no white stannic oxide powder is shown; concentrated hydrochloric acid is never added; there is no heating step; sodium hypophosphite solution is never added; the scene shows none of the objects or actions described in the prompt.
level_2: At least one correct element appears (a beaker, or a white powder), but a critical step is missing or out of order: sodium hypophosphite is added before any heating takes place, or the acid is added only after the hypophosphite; the stannic oxide never appears to dissolve; and there is no sign of heating at all (no hot plate, no rising steam, no glow).
level_3: The core objects are all present (beaker, white powder, an acid being poured in, a dropper or pipette for the second reagent), and the general order is close to right: stannic oxide goes into the beaker, acid is added, and heating begins; but sodium hypophosphite is added while the mixture still looks cold or at room temperature, or there is no clear pause showing that heating had time to take effect before the hypophosphite goes in.
level_4: All four steps appear in the right order: stannic oxide powder is placed in the beaker; concentrated HCl is added with visible fuming or steaming on contact; the beaker is heated (hot plate glowing, liquid gently bubbling); and sodium hypophosphite solution is added once heating is clearly under way; the only shortcomings are minor, such as the heating period looking too brief to tell whether it had really taken hold, the powder not reading clearly as white, or the solution not showing the expected pale yellow tint before the hypophosphite is added.
level_5: Every step matches the prompt precisely: white stannic oxide powder is poured into a clean beaker; concentrated HCl is added and immediately produces visible white fumes with the solution turning pale yellow; the beaker sits on a hot plate with a visible glow and a sustained gentle boil; sodium hypophosphite (NaH₂PO₂) solution is then added while the mixture is still actively heating, and a black precipitate begins forming within about 1-2 seconds; every object's appearance, scale, and state look realistic throughout.

Scientific and Causal Correctness rubric:
level_1: Nothing scientifically plausible happens: there is no color change, no precipitate, and no fuming at any point; or something impossible occurs instead, such as an explosion, an instantly formed gold-colored precipitate, or the solution turning blue; there is no visible cause-and-effect connecting the reagents added to any observed change.
level_2: Reagents are present, but what happens contradicts the chemistry: a black precipitate appears before any heating or before the hypophosphite is even added; the solution stays colorless the whole time despite the acid and oxide being combined; fuming only shows up after the hypophosphite goes in rather than when the acid first meets the oxide; or the precipitate that forms is gray or white rather than a deep black, suggesting no real reduction of tin(IV) took place.
level_3: A black precipitate does form after the hypophosphite is added, but the causal picture is incomplete: heating is missing entirely or only starts after the hypophosphite has already been added; the fuming when HCl first meets the stannic oxide is weak or absent, which would imply the oxide never really dissolved into a tin(IV) species in the first place; and nothing in the timing clearly shows heating plus hypophosphite together triggering the color change; the precipitate could just as easily be explained by some other cause.
level_4: The chemistry holds together: HCl dissolves the stannic oxide into a pale yellow, fuming solution; the heating step visibly continues before the hypophosphite is added, consistent with warming being necessary for the reduction; adding hypophosphite while the solution is still hot is followed within about 1-2 seconds by a black, finely divided precipitate, consistent with tin(IV) being reduced to metallic tin; the only shortcoming is something minor, such as the precipitate looking slightly shiny rather than fully matte, or the initial fuming being too faint to read clearly as HCl gas.
level_5: Every stage is scientifically consistent and mutually reinforcing: white fuming appears the moment HCl contacts the stannic oxide, confirming a real acid-oxide reaction rather than simple mixing; the resulting solution is a clear pale yellow, consistent with a dissolved tin(IV) species; the boiling shown is gentle and sustained rather than violent, consistent with controlled heating; adding hypophosphite while the solution is actively hot produces a dense, matte-black precipitate that appears almost immediately at the point of addition and spreads outward, clearly reading as freshly formed metallic tin rather than a carbon residue or an oxide; and nothing in the scene suggests an unwanted side reaction, such as gas bubbling up from the hypophosphite itself, which would only happen if the mixture were overheated or over-acidified.

Spatiotemporal Consistency rubric:
level_1: The scene breaks down physically: the beaker appears to float above the bench; a reagent appears to pour upward into its container; the hot plate glows only after the black precipitate has already formed; the white powder is visible in one frame and has fully disappeared into solution in the very next frame with no dissolving motion shown in between; or multiple objects blink in and out of the frame, with lighting that shifts abruptly from one setup to a completely different one.
level_2: Clear continuity problems remain: a hand holding the pipette enters the frame without ever having entered beforehand; the beaker's position or angle changes between shots with no visible motion connecting them; heating appears to begin only after the hypophosphite is already in the solution; the black precipitate appears fully formed in the very first frame after the drop lands, with no sign of it spreading outward from a starting point; or the lighting shifts drastically partway through, as if the setting changed entirely.
level_3: Mostly consistent, with a few rough edges: the hot plate glows but there's no corresponding heat haze rising above the beaker; the fuming looks static rather than drifting upward; the black precipitate seems to appear everywhere in the solution at once rather than spreading outward from where the drop landed; or the hand's motion looks slightly disconnected from where the pipette tip actually is, though the overall order of steps and the layout of the scene stay stable.
level_4: The scene holds together well: the beaker stays fixed in place on the bench; the hand enters smoothly, pipette first; fuming rises steadily above the liquid; the hot plate's glow keeps illuminating the beaker from below in the same way throughout, without the beaker's shadow on the bench suddenly jumping to a different side; the black precipitate starts right where the drop contacts the liquid and spreads downward within about a second in a natural, uneven way; the only shortcoming is a slight mismatch between how vigorously the liquid appears to be boiling and how much heat haze is shown above it.
level_5: The whole clip reads as one physically coherent scene: the beaker sits stably on the hot plate throughout, its shadow and faint reflection on the glass staying anchored to the same spot on the bench for the whole clip; a hand enters from one side, lowers the pipette, and releases a droplet that visibly lands and ripples the surface; fuming rises continuously and thickens as heating continues; the liquid shows a gentle, steady simmer rather than anything explosive; the black precipitate forms exactly where the droplet lands and spreads outward unevenly, the way a real precipitate would, before settling; and there are no jump cuts, flickers, or sudden changes in object size or camera angle anywhere in the sequence.