# Phase diagram A phase diagram maps which state of matter is stable at each combination of pressure and temperature. Regions are single phases; the lines between them are where two phases coexist with equal [[Chemical_potential]]; and the triple point is where three meet. It is the most information-dense picture in [[Thermodynamics]] — a whole substance's behaviour on one sheet. ## Microsim — p5.js Sidecar: `Phase_diagram.p5.js` · route leaf: `microsim/p5js/Phase_diagram__20260910T0400Z`. Built to the Betterfire Standard v0 (single `ARTICLE` constant, HUD title and Wikitube URL, control hints, parameter readout and equation). | Control | Does | |---------|------| | `mouse drag` | move the state point | | `[w]` | toggle normal substance / water | | `[space]` | recentre the point | *What to watch:* Drag the state point around the p–T plane and the phase label follows. Toggle to the water diagram and the solid–liquid line tilts the other way — press down and you melt ice instead of freezing it. ## Reading the lines Cross a line and the substance changes phase at constant temperature while absorbing or releasing latent heat. Follow the liquid–vapour line upward and it does not continue forever: it ends at the [[Critical_point_(thermodynamics)|critical point]], beyond which liquid and gas become indistinguishable and the substance is a supercritical fluid. Below the triple point, solid goes straight to vapour — sublimation, which is how freeze-drying and dry ice work. ## Water's backwards slope Almost every substance has a solid–liquid line that leans right: squeeze it and it freezes. [[Properties_of_water|Water]] leans left, because ice is less dense than liquid, so pressure *melts* it. That single anomaly is why ice floats, why lakes overturn instead of freezing solid, and why glaciers slide. At the cold end the diagram gets stranger still — [[Helium]] will not solidify at all under its own vapour pressure however cold it gets, and instead crosses into a [[Superfluidity|superfluid]] phase at the [[Lambda_point]]. **Reads with:** *Thermodynamics and Chemistry, 2nd ed. (Howard DeVoe, 2020)* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/thermodynamics-and-chemistry) · [download](https://www2.chem.umd.edu/thermobook/downloads.htm) · CC BY 4.0. Section 6 of the [[PORTAL_Thury_Hydrodynamics_Apex_Spine|Apex Spine]] book shelf. **On the spine:** [[Chemical_potential]] · [[Phase_transition]] · [[Properties_of_water]] · [[Gibbs_free_energy]] · [[WT!Thury_Hydrodynamics_Compendium]]. <!-- SPINEPATH:BEGIN g20 — shortest chain of Wikipedia links between local articles to a Compendium Main article; do not hand-edit inside --> *Connected to the Apex Spine:* Phase diagram → [[Hydrogen_bond|Hydrogen bond]] — [[WT!Thury_Hydrodynamics_Compendium|Compendium]] section 8, *The hydrogen bond*. <!-- SPINEPATH:END --> <!-- MATTERSIM:BEGIN g33 — Matter & Energy Cluster microsim (framework build, specs/sims/Phase_diagram.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Phase diagram* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Phase_diagram.html" data-title="Phase diagram"></div> *Built from `MICROSIM_GUIDE/specs/sims/Phase_diagram.json`; part of the [[PORTAL_Matter|Matter portal]] spine (section sims and See-also variants).* <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Phase_diagram) : [Wikitube](https://en.wikitube.io/wiki/Phase_diagram) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Physics]], [[PORTAL_Energy]]. --- *Book-section wave · 2026-09-10 · article + p5 microsim shipped together.*