phase boundary

(noun)

The line in a phase diagram that indicates the conditions under which two (transitioning) states of matter exist at equilibrium.

Related Terms

  • solidus
  • Triple point

Examples of phase boundary in the following topics:

  • Major Features of a Phase Diagram

    • The major features of a phase diagram are phase boundaries and the triple point.
    • Phase boundaries, or lines of equilibrium, are boundaries that indicate the conditions under which two phases of matter can coexist at equilibrium.
    • Along the blue phase boundary, water exists as both a vapor and a liquid.
    • Along the dotted green phase boundary, we see the anomalous behavior of water: it exists as a solid at low-enough temperatures and high-enough pressures.
    • In this phase diagram, which is typical of most substances, the solid lines represent the phase boundaries.
  • Interpreting Phase Diagrams

    • The lines that separate these single phase regions are known as phase boundaries.
    • Along the phase boundaries, the matter being evaluated exists simultaneously in equilibrium between the two states that border the phase boundary.
    • When evaluating the phase diagram, it is worth noting that the solid-liquid phase boundary in the phase diagram of most substances has a positive slope.
    • However, the solid-liquid phase boundary for water is anomalous, in that it has a negative slope.
    • The dotted green line refers to the solid-liquid phase boundary for water.
  • Supercritical Fluids

    • In addition, there is no surface tension in a supercritical fluid, as there is no liquid to gas phase boundary.
    • In the pressure-temperature phase diagram of CO2, the boiling separates the gas and liquid region and ends in the critical point, where the liquid and gas phases disappear to become a single supercritical phase.
    • At the critical point, (304.1 K and 7.38 MPa) there is no difference in density, and the two phases become one fluid phase.
    • The dry ice melts under high pressure, and forms a liquid and gas phase.
    • When the vessel is heated, the CO2 becomes supercritical -- meaning the liquid and gas phases merge together into a new phase that has properties of a gas, but the density of a liquid.
  • Alloys

    • Complete solid solution alloys give single solid phase microstructure.
    • Partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history.
    • Complete solid solution alloys give single solid phase microstructure.
    • Partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history.
    • Alloys can be further classified as homogeneous (consisting of a single phase), heterogeneous (consisting of two or more phases), or intermetallic (where there is no distinct boundary between phases).
  • Surface Tension

    • Liquids and solids share a common attribute: a clear and discernible phase boundary that gives the sample a simple but definite shape.
  • The Structure and Properties of Water

    • Its liquid phase, the most common phase of water on Earth, is the form that is generally meant by the word "water."
    • When water achieves a specific critical temperature and a specific critical pressure (647 K and 22.064 MPa), the liquid and gas phases merge into one homogeneous fluid phase that shares properties of both gas and liquid.
    • Well-defined boundaries between solid and liquid, solid and gas, and liquid and gas.
    • During the phase transition between two phases (i.e, along these boundaries), the phases are in equilibrium with each other.
    • The three phases of water – liquid, solid, and vapor – are shown in temperature-pressure space.
  • Liquid to Gas Phase Transition

    • Vaporization of a sample of liquid is a phase transition from the liquid phase to the gas phase.
    • Vaporization of a sample of liquid is a phase transition from the liquid phase to the gas phase.
    • Gradually, Pw will rise as molecules escape from the liquid phase and enter the vapor phase.
    • At the same time, some of the vapor molecules will condense back into the liquid phase (step 2).
    • A bubble is a hole in a liquid; molecules at the liquid boundary are curved inward, so they experience stronger nearest-neighbor attractions.
  • Liquid to Solid Phase Transition

    • Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered to its freezing point.
    • Freezing, or solidification, is a phase transition in which a liquid turns into a solid when its temperature is lowered to or below its freezing point.
    • This is a first-order thermodynamic phase transition, which means that as long as solid and liquid coexist, the equilibrium temperature of the system remains constant and equal to the melting point.
    • The creation of a nucleus implies the formation of an interface at the boundaries of the new phase.
    • Some energy is expended to form this interface, based on the surface energy of each phase.
  • Solid to Gas Phase Transition

    • Sublimation is the phase transition from the solid to the gaseous phase, without passing through an intermediate liquid phase.
    • Sublimation is the process of transformation directly from the solid phase to the gaseous phase, without passing through an intermediate liquid phase.
    • It is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point (the temperature and pressure at which all three phases coexist) in its phase diagram.
    • But at temperatures below that of the triple point, a decrease in pressure will result in a phase transition directly from the solid to the gaseous.
    • At temperatures and pressures below those of the triple point, a phase change between the solid and gas phases can take place.
  • The Phase of Orbitals

    • One orbital, based on in-phase mixing of the orbitals, will be lower in energy and termed bonding.
    • Two atomic orbitals can overlap in two ways depending on their phase relationship.
    • The phase of an orbital is a direct consequence of the wave-like properties of electrons.
    • If the phase changes, the bond becomes a pi bond (π-bond).
    • The in-phase combination of the s orbitals from the two hydrogen atoms provides a bonding orbital that is filled, whereas the out-of-phase combination provides an anti-bonding orbital that remains unfilled.
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