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Pure
materials, such as water or nitrogen, can exist as solids, liquids, or gasses
depending upon their temperature, pressure and specific volume. The diagram
shows the regions of these (p, v, T) parameters for which each of the phases
exists alone, and also regions in which more than one phase is present. The
yellow surface is the locus of all sets of (p,v, T) points for which the material
may be a fluid, i.e. either a liquid or a gas. The blue surface is the
locus of all (p,v,T) points for which the material exists as a solid. The green,
liquid-solid region of the surface describes the equilibrium melting process
and the mauve liquid-vapor surface is that for equilibrium "boiling."
At a constant pressure, these phase transitions (melting or boiling)
take place at constant temperature. The lines shown on the several surfaces
are lines of constant temperature, or Isotherms.
In the fluid phase a
liquid differs from a gas by having a defined interface with its surroundings
known as a "free surface." A gas will fill all of the space in the
container in which it is placed. The diagram also shows that above some "critical"
condition the material may transfer from a gas state to a
liquid state without showing a boiling or condensing behavior or an interphase
boundary.
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