Phase boundary line
A phase boundary line is understood to be the totality of the state variables which clearly describe the state of a substance or mixture of substances during a phase transition or at the phase boundary as an interface . It is therefore composed of points (states) of the respective phase diagram (substance), whereby in the state described by these points there is a thermodynamic equilibrium between two or more phases of the substance or mixture of substances.
Parts and important boundary points of the phase boundary lines are distinctive states within the phase diagram such as the triple point , the critical point or the eutectic point . The area between the phase boundary lines, where there is only a single phase, is called the state space .
Individual phase lines with their most common synonyms and the associated phase changes are:
Solid or liquid → gas (collective term):
- Vapor pressure curve (see saturation vapor pressure ); Synonyms: saturation vapor pressure curve , vapor pressure line , saturation vapor pressure line
- Collective term for the state transitions evaporation / condensation and sublimation / Resublimation in solids .
Liquid → gas:
- Boiling curve ; Synonyms: boiling line , boiling point curve , boiling pressure curve and boiling point curve
- Name of the phase transition: evaporation (also boiling)
- Designation of the conditions during the phase transition: boiling / boiling range
- The course of the boiling point curve can be calculated using the Clausius-Clapeyron equation .
Gas → liquid:
- Condensation curve ; Synonyms: condensation line , (for water and dew line , Taukurve , dew point , dew point line )
- Name of the phase transition: condensation
- Designation of the conditions during the phase transition: condensation point / condensation region
Solid → liquid:
- Melting curve ; Synonyms: melt line , melting point curve , melting pressure curve
- Name of the phase transition: melting
- Designation of the conditions during the phase transition: melting point / melting range
Liquid → solid:
- Solidification curve ; Synonyms: freezing line , freezing point curve , freezing point line
- Name of the phase transition: Freezing
- Designation of the conditions during the phase transition: freezing / freezing range
Solid → gas:
- Sublimation curve ; Synonyms: line of sublimation
- Name of the phase transition: sublimation
- Designation of the conditions during the phase transition: sublimation / sublimation
Gas → solid:
- Resublimation curve
- Name of the phase transition: Resublimation
- Designation of the conditions during the phase transition: Resublimationspunkt / Resublimationsbereich
The rule here is that the lines for pure substances are identical to the lines for the opposite phase transitions. However, this does not have to be the case with several substances, as miscibility gaps can develop between the phase boundary lines (see boiling diagram above right). In these cases, one must therefore differentiate according to the direction of the phase transition. It should also be noted that there is a much wider range of phase transitions than just changing between different physical states . This applies in particular to solids, where again ice has the most so-called modifications .