Oznake na faznom dijagramu pokazuju gdje je slobodnu energiju nemoguće odrediti. Otvoreni prostori, gdje je [[slobodna enrgija|slobodnu enrgiju]] moguće [[analitička funkcija|izračunati]], odgovaraju fazama. Faze su odvojene linijama neodređene energije gdje se [[fazni prijelaz]]i odvijaju, koje se zovu '''granice faza'''.
InNa the diagramdijegramu, the phasefazna boundarygranica betweenizmeđu liquidtečnosti andi gasplina doesne notide continueu indefinitelybeskonačnost. InsteadNasuprot, itzavršava terminatesse atu atački pointna onfaznom thedijagramu phasekoja diagramse called thenaziva [[criticalkritična pointtačka (chemistryhemija)|criticalkritična pointtačka]]. ThisOvo reflectsse theodražava factna that,činjenicu atda extremelyse hightečne temperaturesi andplinovite pressuresfaze, thepri liquidekstremno andvisokim gaseoustemperaturama phasesi becomepritiscima, indistinguishablene razlikuju, inte postaju nešto whatšto isje knownpoznato aspod anazivom [[supercriticalsuperkritični fluid]]. InKod watervode, thekritična criticaltačka pointnalazi occursse at aroundoko 647 [[Kelvin (jedinica)|K]] (374 °C orili 705 °F), andte 22.,064 [[PascalPaskal (unitjedinica)|MPa]].
The existence of the liquid-gas critical point reveals a slight ambiguity in our above definitions. When going from the liquid to the gaseous phase, one usually crosses the phase boundary, but it is possible to choose a path that never crosses the boundary by going to the right of the critical point. Thus, the liquid and gaseous phases can blend continuously into each other. However, it is impossible for the solid-liquid phase boundary to end in a critical point in the same way as the liquid-gas boundary, because the solid and liquid phases have different [[symmetry]].
An interesting thing to note is that the solid-liquid phase boundary in the phase diagram of most substances, such as the one shown above, has a positive [[slope]]. This is due to the solid phase having a higher [[density]] than the liquid, so that increasing the pressure increases the melting temperature. However, in the phase diagram for water the solid-liquid phase boundary has a negative slope. This reflects the fact that ice has a lower density than water, which is an unusual property for a material.
