Domain: Healthcare — Medicine
Target concept: Effect of pH on Drug Solubility

Video generation prompt: Two transparent test tubes, each containing 0.1 g of aspirin powder, are placed side by side on the bench. Ten milliliters of hydrochloric acid solution with a pH of 2 are added to the left test tube, and 10 mL of phosphate buffer solution with a pH of 7 are added to the right test tube. The solutions are stirred under identical conditions, and the resulting solutions in both test tubes are observed.

Evaluation guide:
Target concept: pH-dependent solubility of weak acids governed by Henderson-Hasselbalch equilibrium (HA ⇌ H⁺ + A⁻), where solubility increases when pH > pKa due to ionized, hydrophilic species formation. Minimum mechanistic assumptions: (i) aspirin is monoprotic weak acid (pKa ~3.5); (ii) solubility correlates directly with ionization fraction; (iii) visual solubility proxy is sediment height/supernatant clarity under standardized conditions. Phased storyline: (1) Setup: two identical tubes with equal aspirin mass placed side-by-side on bench; (2) Intervention: simultaneous addition of 10 mL pH 2 (left) and pH 7 (right) solutions; (3) Perturbation: identical 5-second magnetic stirring; (4) Stabilization: 2-second settling; (5) Observation: static comparison revealing stark contrast — dense sediment + turbid supernatant (pH 2) vs. minimal sediment + clear supernatant (pH 7). Visibility constraints: must capture both tubes fully in frame, with unobstructed view of sediment interface and meniscus; backlight or side-lighting required to differentiate turbidity; no motion blur during final observation phase; time-lapse acceleration permitted only for stirring (not settling or observation).

Manual:
I. Basic Demonstration Information
Demonstration Title: Effect of pH on Aspirin Solubility (pH 2 vs. pH 7) Demonstration
Subject: Healthcare - Medicine - Pharmaceutics
Demonstration Purpose: To visually demonstrate how the ionization state of weakly acidic drugs like aspirin governs aqueous solubility, using side-by-side comparison of identical aspirin doses in acidic (pH 2) and physiologically relevant neutral (pH 7) environments; enables recognition of solubility differences via sedimentation state and solution clarity; reinforces the principle that unionized forms are less water-soluble.
II. Demonstration Carrier/Equipment
Core Carrier: Two identical transparent borosilicate glass test tubes (15×100 mm); pure acetylsalicylic acid (aspirin) powder (0.1 g per tube, white, fine crystalline, free-flowing); hydrochloric acid solution (0.1 M, pH 2); phosphate buffer solution (0.1 M, pH 7); calibrated 10-mL volumetric cylinder or pump dispenser; magnetic stirrer with dual identical stir bars; neutral-toned lab bench with grid-marked surface.
III. Demonstration Content Description
Layout: Place tubes side-by-side on bench, centered and parallel, 5 cm apart, aligned to grid lines. Confirm identical orientation and height.
State Control: Add aspirin first (dry powder visible at bottom), then add 10 mL solution to each tube using same delivery method. Stir both simultaneously for exactly 5 seconds at 150 rpm. Cease stirring and observe immediately.
Comparison Baseline: Use identical tube geometry, lighting, camera angle, and timing to isolate pH as sole variable.
IV. Observation of Core Phenomena
Left Tube (pH 2): Dense white sediment layer ≥3 mm high; supernatant cloudy/milky; minimal dissolution (<10%); powder particles remain discrete and coarse.
Right Tube (pH 7): Thin or absent sediment layer (≤0.5 mm); supernatant clear/translucent; high dissolution (>80%); solution appears homogeneous under transmitted light.
Characteristic Stability: No color change, gas evolution, or precipitation over 10-second observation window. Sediment interfaces remain sharp and stable.
Auxiliary Comparison: Side-by-side luminance contrast confirms differential light transmission — right tube permits significantly more light passage, verifying higher solute dispersion.
V. Summary of Key Knowledge Points and Principles
Core Concept Definition: Aspirin (pKa ≈ 3.5) is a weak acid; in acidic medium (pH < pKa), it remains predominantly unionized (HA), exhibiting low water solubility; in neutral medium (pH > pKa), it ionizes to salicylate (A⁻), increasing polarity and aqueous solubility.
Pharmaceutical Significance:
- Gastric Solubility Limitation: Low solubility in stomach (pH 1.5–3.5) contributes to delayed absorption and local irritation.
- Enteric Coating Rationale: Coatings resist gastric pH but dissolve in intestinal pH (~6–7.4), releasing drug where solubility and absorption are optimal.
- Formulation Design: Salt forms (e.g., sodium aspirin) bypass pH-dependent solubility limitation.
VI. Demonstration Precautions
- Use fresh, anhydrous aspirin to avoid hydrolysis artifacts (no vinegar odor, no yellowing).
- Verify pH of solutions with calibrated pH meter prior to use (pH 2.0±0.1; pH 7.0±0.1).
- Avoid over-stirring (>5 s) to prevent false homogenization or foaming.
- Record under uniform white LED lighting (5000K, CRI >90) with backlight for clarity assessment.
- Video duration: 10 seconds — 2 s setup, 2 s addition, 5 s stirring + stabilization, 1 s final static comparison.

Prompt Grounding rubric:
level_1: No test tubes are visible; or only one tube appears; or aspirin powder is missing from both tubes; or no pH-specific solution is added to either tube; or the acidic and neutral solutions are swapped onto the wrong tubes (pH 2 on the right, pH 7 on the left); or the two tubes aren't placed side by side on a bench with a clear left/right distinction at all.
level_2: Two transparent test tubes appear side by side, but aspirin powder is missing from one or both; or a solution is added without any pH indication (just unlabeled clear liquid); or the pH 2 and pH 7 solutions are swapped between the left and right tubes; or the bench/background is missing or unrecognizable, so there's no clear spatial context.
level_3: Two transparent test tubes sit on a bench, each holding a small amount of white powder consistent with about 0.1 g of aspirin, and the correct solutions are added -- hydrochloric acid (pH 2) to the left tube, phosphate buffer (pH 7) to the right tube -- but there's no label, color cue, or text confirming the pH values; stirring is implied rather than shown; and there's no clear look at how the two tubes compare after stirring.
level_4: All the core objects are present and correctly assigned: two matching transparent test tubes on a plain lab bench, a small amount of white aspirin powder settled at the bottom of each tube before liquid is added, and 10 mL of clear solution added to each from a dropper or graduated cylinder -- the left tube gets the acidic (pH 2) solution and the right gets the neutral buffer (pH 7); both tubes are stirred using the same kind of tool for a similar-looking duration; and the final comparison shows both tubes arranged side by side as described.
level_5: Everything is grounded precisely: (i) the tubes sit side by side, evenly spaced, on a clean bench; (ii) the aspirin powder looks like the same amount in both tubes before any liquid is added (fine, dry, white, not clumped); (iii) 10 mL of solution is added to each tube in a way that looks measured (e.g., from a graduated cylinder with a visible fill line); (iv) the pH identity of each tube is clear, either through an on-screen label ('pH 2' / 'pH 7') or a distinguishing visual cue that fits the chemistry; (v) both tubes are stirred the same way, for what looks like the same amount of time; (vi) the final still shot shows both tubes clearly enough to compare their solutions directly.

Scientific and Causal Correctness rubric:
level_1: There's no visible difference in solubility between the two tubes (both look fully dissolved or both look fully undissolved); or the aspirin dissolves in the pH 2 tube but not the pH 7 tube (the reverse of what should happen); or something happens that has no basis in this chemistry, like fizzing/bubbling or a color change; or neither tube shows any observable change in how much solid is present.
level_2: One tube shows a little dissolving but the two tubes don't look clearly different from each other; or both tubes appear to dissolve about the same amount despite the pH difference; or cloudiness is shown as if it were dissolving (a hazy suspension mistaken for a clear solution); or the aspirin itself looks like it's degrading (yellowing or clumping) in either tube.
level_3: The right direction is shown: the left (pH 2) tube keeps most of its white aspirin powder undissolved at the bottom with only slight cloudiness in the liquid above; the right (pH 7) tube shows noticeably more dissolving -- less powder left, and the liquid above looks clearer; but there's no way to gauge how big the difference is (no size comparison or benchmark), and nothing on screen ties the difference back to pH or ionization.
level_4: The phenomenon matches the chemistry clearly: the left tube keeps more than about 90% of its white solid visibly undissolved, as a sediment layer at least a few millimeters high, while the right tube has less than about 20% of its solid left, no more than a thin trace roughly half a millimeter or less, with most of the liquid above looking clear; there's no fizzing, no color change, and no sign of the aspirin breaking down; and something on screen (a label or brief text) connects the difference to aspirin being a weak acid that stays mostly un-ionized -- and thus less soluble -- in the acidic tube.
level_5: Everything in score 4 holds, and in addition: both tubes are shown at the same moment right after stirring stops, so the comparison is fair; the left tube's undissolved layer and cloudy supernatant are clearly visible against the right tube's much clearer solution -- the right tube visibly transmits on the order of 70% or more additional light than the left, making the contrast obvious at a glance, not something you have to look for; a brief on-screen note ties the difference to the aspirin equilibrium (un-ionized HA vs. ionized A⁻) and the fact that solubility rises once pH climbs above the aspirin's pKa; and there's nothing throwing off the comparison, like one tube dissolving unnaturally fast or the two liquids flowing differently when the tubes are tilted.

Spatiotemporal Consistency rubric:
level_1: The two tubes sit at clearly different depths or heights (one looks closer/farther or floating oddly); the lighting is uneven in an obvious way (e.g., left tube brightly lit, right tube in shadow, or vice versa); a stirring implement appears in only one tube; the aspirin powder vanishes and reappears; the camera angle suddenly shifts partway through as if it jumped or rotated; or there's no stable view of the bench to judge either tube's position against.
level_2: The two tubes differ in an unexplained way -- different size, shape, or material (e.g., one looks like glass, the other plastic); the bench surface itself looks inconsistent between the two sides (different color or texture); the stirring looks different between tubes (e.g., one stirred by hand, the other by a device); the sediment amount jumps up or down without any stirring or settling action to explain it; or a color cast makes one tube look tinted in a way that doesn't match its actual solution.
level_3: The setup looks consistent overall -- same tube size on both sides, same bench, light coming from the same general direction -- but there are small rough edges: the tubes aren't quite lined up on the same plane, the stirrer is only visible in one tube because the other is partly blocked from view, the two additions/stirring don't quite look simultaneous, or the clip cuts away before the settled result is shown side by side.
level_4: Good overall consistency: both tubes sit at the same height, are parallel to each other, and sit the same distance from the camera; the lighting is even enough that neither tube looks noticeably brighter or more shadowed than the other; both tubes are stirred together, starting and stopping at the same visible cue; after stirring stops, both tubes are held still and in view for at least a few seconds with no motion blur; the camera doesn't pan or zoom, and the bench edge stays visible so the tubes can be judged against a level reference.
level_5: Everything in score 4 holds, and in addition: by eye, the two tube centers look equally spaced from the middle of the frame and at matching height; both tube bottoms and liquid surfaces look equally in focus; the lighting looks even enough that neither tube appears noticeably brighter than the other; the solution is added to both tubes at essentially the same moment, and both are stirred for what looks like the same length of time; and the final side-by-side comparison plays smoothly with no repeated, frozen, or skipped frames.