Domain: Engineering — Agricultural Engineering
Target concept: Insert the pH meter separately into acidic soil and alkaline soil, to verify that the pH value of the acidic soil is less than 7, the pH value of the alkaline soil is greater than 7, and the values are different.

Video generation prompt: Prepare two soil samples, labeled as acidic and alkaline, respectively. Calibrate a handheld pH meter. Insert the probe into the moist acidic soil, wait for the reading to stabilize, then clean the probe. Insert the probe into the moist alkaline soil and wait for the reading to stabilize. The pH value is displayed as a numerical value on the instrument screen.

Evaluation guide:
Target Concept: Soil pH classification via direct instrumental measurement, defined operationally as the stable numeric output of a calibrated pH meter when its electrode contacts uniformly moist, labeled acidic/alkaline soil matrices. Minimum mechanistic assumptions: (1) pH meter transduction is instantaneous upon stable electrolyte contact; (2) soil moisture enables H⁺ mobility for electrode sensing; (3) calibration nullifies systematic offset; (4) probe cleaning prevents carryover bias. Phased Storyline: [Phase 1: Setup] Two labeled soils + calibrated meter visible. [Phase 2: Acidic Measurement] Probe insertion → 3–5 s stabilization → clear <7 display. [Phase 3: Intervention] Visible rinse + dry → breaks causal chain. [Phase 4: Alkaline Measurement] Probe insertion → 3–5 s stabilization → clear >7 display. Visibility Constraints: Screen must be large/focused enough to read digits unambiguously; labels must be orthographic and front-facing; no hand occlusion during stabilization; lighting must eliminate glare on screen or soil surface.

Manual:
Soil pH Comparison Demonstration Instructions
I. Basic Demonstration Information
Demonstration Title: Acidic vs. Alkaline Soil pH Verification Using Handheld pH Meter
Subject: Engineering — Agricultural Engineering — Soil Science & Testing
Demonstration Purpose: To visually verify that acidic soil exhibits a pH < 7 and alkaline soil exhibits a pH > 7 using a calibrated handheld pH meter; to demonstrate correct procedural sequencing (calibration → acidic soil measurement → probe cleaning → alkaline soil measurement); and to enable visual recognition of numerical pH values on the instrument screen as the core evidence for soil classification.

II. Demonstration Carrier/Equipment
Core Carrier: Two labeled soil samples — “Acidic Soil” (e.g., peat-based, pH ≈ 4.5–6.0) and “Alkaline Soil” (e.g., calcareous loam, pH ≈ 7.5–8.5), both uniformly moistened to field capacity (no standing water, no dry cracks); calibrated handheld digital pH meter with intact glass electrode probe and stable battery.
Auxiliary Equipment: Distilled water rinse bottle; lint-free lab wipe; calibration buffer solutions (pH 4.01 and 7.01); labeled sample containers (clearly marked “ACIDIC” and “ALKALINE” in bold, contrasting font); neutral-colored lab bench background.

III. Demonstration Content Description
Layout: Place both soil samples side-by-side on a clean, level lab bench under even ambient lighting. Position pH meter within easy reach, screen facing forward and fully visible. Labels must be legible and unobstructed.
State Control: Soils must be uniformly moist (squeezing yields slight dampness but no free water); pH meter must display stable calibration confirmation (e.g., “CAL OK” or matching buffer readings) before first insertion.
Procedural Sequence (strictly enforced): (1) Calibrate meter using pH 4.01 and 7.01 buffers per manufacturer instructions; (2) Insert probe vertically into center of moist acidic soil; (3) Wait ≥5 s until screen reading stabilizes (no flickering or drifting digits); (4) Remove probe, rinse thoroughly with distilled water, and blot dry with lab wipe; (5) Insert probe vertically into center of moist alkaline soil; (6) Wait ≥5 s until screen reading stabilizes.

IV. Observation of Core Phenomena
Expected Screen Display: The pH meter screen shows a clear, stable, numeric value (e.g., “5.2” for acidic soil; “7.9” for alkaline soil) with decimal precision, no units omitted, no “ERR”, “---”, or “LO BAT” indicators.
Visual Feature Recognition Comparison:
- Acidic Soil Reading: Numerical value is a single positive number < 7.0 (e.g., 4.8–6.9), displayed in high-contrast digits (e.g., black on white or green on dark), with no leading zeros or scientific notation.
- Alkaline Soil Reading: Numerical value is a single positive number > 7.0 (e.g., 7.1–8.8), displayed in identical digit style and contrast as acidic reading, confirming instrument consistency.
- Difference Confirmation: The two displayed values are numerically distinct (Δ ≥ 0.5), and their relative positions (<7 vs. >7) are unambiguously discriminable at a glance — no ambiguity between “6.9” and “7.1”, no misreading due to glare or low resolution.
- Probe State: Electrode tip is visibly clean and wet before first insertion; visibly rinsed and blotted before second insertion; no soil clinging during second insertion.

V. Summary of Key Knowledge Points and Principles
Core Concept Definition: Soil pH is a logarithmic measure of hydrogen ion activity; values < 7 indicate net excess H⁺ (acidic), > 7 indicate net excess OH⁻ (alkaline); accurate measurement requires probe calibration, moisture saturation, and contamination prevention.
Principle of Visual Verification: Human observers identify soil acidity/alkalinity not by color or texture, but by direct numerical comparison against the universal reference point 7.0 — this makes the digital display itself the definitive visual feature; stability, contrast, legibility, and positional logic (<7 / >7) constitute the recognition signature.

VI. Demonstration Precautions
- Calibration is mandatory and must be verified on-screen before any soil insertion.
- Probe must not touch container walls or bottom during insertion — only soil matrix contact.
- Moisture level must be consistent: overly dry soil gives false high pH; saturated soil dilutes H⁺ and depresses reading.
- No reinsertion into same soil without recalibration — cross-contamination invalidates comparison.
- Video duration must capture full stabilization (≥5 s per reading) — time-lapse acceleration is prohibited for stabilization phase.

Prompt Grounding rubric:
level_1: No recognizable pH meter, no soil samples, no labels, no numerical display; scene is unrelated (e.g., plant growth, water pouring, or blank screen).
level_2: pH meter present but screen blank/unreadable; one soil sample visible but unlabeled; no calibration shown; no probe insertion observed; or probe inserted but no stabilized reading captured.
level_3: Both labeled soil samples and pH meter visible; calibration step implied but not shown; probe inserted into acidic soil and stabilized reading (<7) displayed, but alkaline soil step omitted or unreadable; or probe cleaning missing but both readings appear.
level_4: All steps shown in correct order: calibration confirmed on-screen, probe inserted into acidic soil → stable <7 reading, probe rinsed/blotted, probe inserted into alkaline soil → stable >7 reading; minor flaws: label text slightly blurred, brief screen glare obscuring one digit, or slight motion blur during insertion.
level_5: Perfect procedural fidelity: (i) Calibration status clearly shown (e.g., "CAL OK" or buffer match); (ii) Acidic soil labeled "ACIDIC", probe fully inserted, screen displays stable numeric value < 7.0 (e.g., "5.4") with no flicker; (iii) Probe visibly rinsed with distilled water and blotted dry; (iv) Alkaline soil labeled "ALKALINE", probe inserted, screen displays stable numeric value > 7.0 (e.g., "7.8"); (v) Both readings use identical font, contrast, and decimal format; (vi) No extraneous objects, no occlusion, no timing violation: stabilization duration ≥5 s per reading is visibly evident.

Scientific and Causal Correctness rubric:
level_1: The acid/alkaline science is violated on both samples at once, or the readings are non-diagnostic: e.g., the peat-based acidic soil shows "8.2" while the calcareous alkaline soil simultaneously shows "6.1" (both on the wrong side of 7), or both samples display the identical value (e.g., both "7.0"), or the values are reversed so the acidic sample reads higher than the alkaline sample, or the screen shows "ERR", dashes, or non-numeric symbols instead of a pH value.
level_2: Exactly one of the two samples breaks the acid/alkaline logic while the other is directionally correct, e.g., the peat-based acidic soil correctly reads "5.4" but the calcareous alkaline soil also reads below 7 (such as "6.8") instead of above it; or both values sit on the scientifically correct sides of 7 but are separated by less than 0.3 pH units (e.g., "6.9" vs. "7.1"), so the two soils cannot actually be told apart from the displayed numbers.
level_3: Both readings fall on the correct sides of 7 (acidic <7, alkaline >7) with at least a 0.3-unit separation, but the causal picture is incomplete: either the separation stays below 0.5 units (e.g., "6.7" vs. "7.0"), leaving weak discriminability, or the soil surface for at least one sample looks dusty/dry or waterlogged, so the moisture needed to mobilize H+ ions for electrode sensing cannot be visually confirmed even though the number itself is on the right side of 7.
level_4: Both readings are directionally correct, separated by at least 0.5 pH units, and each value is chemically plausible for the stated composition (roughly 4.5–6.9 for the peat-based acidic sample, roughly 7.1–8.8 for the calcareous alkaline sample); moist soil is visible at both insertion points so the H+-mobility mechanism is credible; one minor causal gap remains, such as no visible safeguard against carryover from the acidic soil into the alkaline reading, even though the alkaline value stays unambiguously above 7.
level_5: The causal chain of soil-pH chemistry is fully coherent and self-consistent: (i) the acidic soil's value falls within the plausible peat-buffering range (~4.5–6.9) and the alkaline soil's value falls within the plausible calcareous-buffering range (~7.1–8.8), matching the stated soil compositions; (ii) the two values differ by ≥0.5 pH units, so which soil is acidic vs. alkaline is unambiguous from the numbers alone; (iii) nothing in the readings suggests cross-contamination: the alkaline value is not depressed toward neutral in a way implying leftover acidic-soil residue on the electrode; (iv) each reading's direction matches the underlying electrochemistry (higher H+ activity driving the acidic value down, OH⁻ dominance driving the alkaline value up); (v) no reading contradicts core soil-pH science: no equal values, no reversed direction, no implausible extremes such as pH 1 or pH 13 for ordinary garden soils.

Spatiotemporal Consistency rubric:
level_1: Severe spatiotemporal breakdown: e.g., probe appears in alkaline soil before acidic; same hand holds meter and wipes probe simultaneously; soil labels swap positions mid-video; screen shows different values in successive frames with no action; or abrupt jump cuts break continuity.
level_2: Major temporal inconsistency: e.g., rinse shown *after* second insertion; calibration shown *after* first reading; or spatial mismatch: probe inserted while meter is off-screen; labels occluded by hand throughout; or inconsistent lighting (e.g., shadow moves opposite sun direction).
level_3: Minor spatiotemporal gaps: e.g., rinse action shown but no water bottle visible; brief hand occlusion of screen during stabilization; or label orientation changes between shots (e.g., "ACIDIC" rotated 90° in second shot); no violation of physics but reduced clarity.
level_4: Spatial layout consistent: both soils and meter fixed on bench; labels remain legible and stationary; hand motion follows natural arc (insert → withdraw → rinse → insert); lighting uniform; probe depth consistent across insertions; only minor flaw: e.g., wipe motion slightly jerky or one frame shows partial glare.
level_5: Perfect spatiotemporal integrity: (i) Fixed camera angle maintains full view of bench, both soil containers, meter screen, and operator's hands; (ii) All actions occur in continuous, unbroken take (no cuts); (iii) Hand enters frame holding probe, inserts, withdraws, reaches for rinse bottle, applies water, wipes, repositions, inserts again (motion is fluid, anatomically plausible, and temporally proportional); (iv) Lighting stable, no moving shadows; (v) Soil surfaces undisturbed except at probe entry points; (vi) Screen remains in focus and unoccluded for ≥5 s per stabilized reading.