The thermometer is only part of the story: sources & model

Give water an energy budget. Watch its temperature and phase masses, catch a melting plateau, compare two starting states and inspect an open ice structure.

What supports the explanation?

What this model assumes

1. Pure ordinary H₂O, nominal fixed pressure and a movable closed-material boundary. No pressure control, rigid-vessel heating or home pressure apparatus is implied.
2. The rounded transition temperatures and constant properties are an educational approximation. Exact real melting/boiling points depend on pressure and the thermodynamic formulation.
3. The energy ledger computes equilibrium temperature and phase masses. It does not compute a melting front, individual molecule trajectories or an apparatus time-to-boil.
4. Representative molecule counts are rounded; phase mass fractions and the total mass ledger remain exact within floating-point arithmetic.
5. The diagram separates molecular views and uses scale breaks. Gas expansion is not constrained to fit the same literal liquid-sized container.
6. Vapor remains part of the retained material inventory. A white visible plume or mist is not a literal image of vapor molecules.
7. The near-melting density comparison is a separate reference card; its two density values are not a full thermal-expansion model.
8. The home activity observes ordinary ice in open cups. Uncontrolled room heat, unknown initial ice temperature and spatial gradients prevent direct validation of the ideal curve.
9. The modeled system and boundary: Ordinary pure H₂O at nominal 101.325 kPa, retaining all mass with an ideal movable boundary. It represents uniform equilibrium states with pressure work only and negligible bulk kinetic/potential changes. It is not a rigid sealed vessel or an uncovered saucepan. At these conditions Q = ΔH, where H = U + pV; expansion work must not be subtracted a second time.
10. A declared constant-property approximation: The model uses rounded transition temperatures 0°C and 100°C, with a domain of −20°C to 120°C. Ice cₚ = 2020, liquid cₚ = 4180 and vapor cₚ = 2040 J/(kg·K). Fusion enthalpy is 333400 J/kg and vaporization enthalpy 2256400 J/kg. These are not a full IAPWS equation-of-state implementation.
11. Reference conditions stay attached: The ice capacity is a midpoint approximation to NIST’s −20, −10 and 0°C values. The vapor capacity comes from a 100–120°C enthalpy secant at 0.10 MPa. Vaporization enthalpy is tabulated at 100°C saturation and 101.42 kPa, slightly above the model’s nominal pressure. The constant-property choices and rounded transition points are explicitly approximate.
12. Decode energy, do not step a thermometer blindly: The enthalpy reference is ice at −20°C. For 100 g, interval boundaries are 4.040, 37.380, 79.180, 304.820 and 308.900 kJ. Within a single phase ΔT = Q/(mcₚ); within coexistence the converted mass is Q/L. A direct piecewise decoder preserves energy across any number of boundaries.
13. A plateau needs a phase fraction: At approximately 0°C, temperature alone does not distinguish 25% from 75% liquid by mass. Changing between those mixtures for 100 g requires 16.670 kJ in the supplied model. At vaporization, the fraction is also by mass—not the fraction of vessel volume occupied by vapor.
14. Mass, volume and water’s open solid structure: Near melting, reference densities are 916.7 kg/m³ for ice and 999.84 kg/m³ for liquid. Equal 100 g masses occupy about 109.09 and 100.02 mL. Freezing therefore increases volume by about 9.07% in this comparison. These two densities are not used to predict thermal expansion throughout the whole heating path.
15. Thermal motion is not the source of these computed numbers: The representative molecule positions never calculate temperature, pressure or phase fractions. Their motion is an illustration, not molecular dynamics. Zero Celsius is not zero thermal motion. A paused drawing does not mean a physically motionless solid.
16. Boiling is not the only route into air: Water can evaporate into surrounding air below its boiling temperature. That open-system mass and heat transfer lies outside this closed, equilibrium enthalpy path. Real heat-transfer rates, gradients, nucleation, supercooling and dissolved substances require additional physics.
17. A model boundary has an honest receipt: The lower and upper energy limits correspond to −20°C and 120°C in the supplied approximation. A requested pulse beyond a limit is split into accepted and unapplied energy. Unapplied energy is not transferred to the sample. Reversal uses accepted heat, not a request the model could not accept.

What has been checked

Analytical reference cases, conservation or transition invariants, finite drawing commands, bounded setup parsing, discovery and route integrity are checked automatically. These checks do not establish anatomical fidelity, learner outcomes or browser/device compatibility. Independent subject review, learner trials, comprehensive accessibility review and browser video encoding checks remain pending.

Each source supports the associated claim. Sources do not certify this implementation or its visuals.

About the cover illustration

Original offline rendering of the source-derived ice Ih oxygen framework, COD 9015208 / Fortes et al. 2004. Data CC0. The diffraction refinement is at 100 K; symmetry normalization and omitted half-occupied hydrogen positions are recorded in the lesson. Links mark geometric neighbors, not covalent O–O bonds.