2-dimensional diagrams Phase diagram

a typical phase diagram. solid green line applies substances; dotted green line gives anomalous behavior of water. green lines mark freezing point , blue line boiling point, showing how vary pressure.

the simplest phase diagrams pressure–temperature diagrams of single simple substance, such water. axes correspond pressure , temperature. phase diagram shows, in pressure–temperature space, lines of equilibrium or phase boundaries between 3 phases of solid, liquid, , gas.

the fusion curves water , antimony negative slopes, other liquids have positive slope on p-t diagram.

the curves on phase diagram show points free energy (and other derived properties) becomes non-analytic: derivatives respect coordinates (temperature , pressure in example) change discontinuously (abruptly). example, heat capacity of container filled ice change abruptly container heated past melting point. open spaces, free energy analytic, correspond single phase regions. single phase regions separated lines of non-analytical behavior, phase transitions occur, called phase boundaries.

in diagram on left, phase boundary between liquid , gas not continue indefinitely. instead, terminates @ point on phase diagram called critical point. reflects fact that, @ extremely high temperatures , pressures, liquid , gaseous phases become indistinguishable, in known supercritical fluid. in water, critical point occurs @ around tc = 647.096 k (373.946 °c), pc = 22.064 mpa (217.75 atm) , ρc = 356 kg/m³.

the existence of liquid–gas critical point reveals slight ambiguity in labelling single phase regions. when going liquid gaseous phase, 1 crosses phase boundary, possible choose path never crosses boundary going right of critical point. thus, liquid , gaseous phases can blend continuously each other. solid–liquid phase boundary can end in critical point if solid , liquid phases have same symmetry group.

the solid–liquid phase boundary in phase diagram of substances has positive slope; greater pressure on given substance, closer molecules of substance brought each other, increases effect of substance s intermolecular forces. thus, substance requires higher temperature molecules have enough energy break out of fixed pattern of solid phase , enter liquid phase. similar concept applies liquid–gas phase changes. water, because of particular properties, 1 of several exceptions rule.

other thermodynamic properties

in addition temperature , pressure, other thermodynamic properties may graphed in phase diagrams. examples of such thermodynamic properties include specific volume, specific enthalpy, or specific entropy. example, single-component graphs of temperature vs. specific entropy (t vs. s) water/steam or refrigerant commonly used illustrate thermodynamic cycles such carnot cycle, rankine cycle, or vapor-compression refrigeration cycle.

in two-dimensional graph, 2 of thermodynamic quantities may shown on horizontal , vertical axes. additional thermodynamic quantities may each illustrated in increments series of lines - curved, straight, or combination of curved , straight. each of these iso-lines represents thermodynamic quantity @ constant value.

temperature vs. specific entropy phase diagram water/steam. in area under red dome, liquid water , steam coexist in equilibrium. critical point @ top of dome. liquid water left of dome. steam right of dome. blue lines/curves isobars showing constant pressure. green lines/curves isochors showing constant specific volume. red curves show constant quality.