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Single-Component P-V and T-V Diagrams

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P-V diagram
T-V diagram
temperature (K)
497
The van der Waals (VDW) equation of state (EOS) is used for water to generate isotherms on a pressure-volume (
P
-
V
) diagram and isobars on a temperature-volume (
T
-
V
) diagram. Sliders can be used to change the isotherm temperature on the
P
-
V
diagram and the isobar pressure on the
T
-
V
diagram. Note that for both diagrams, the
x
axis is on a log scale because the volume covers a wide range. The phase envelope, which contains the two-phase region where vapor and liquid are in equilibrium, was generated from data for water. The isotherms and isobars have three solutions in the two-phase region, but the only physically meaningful conditions are the orange dots, which correspond to saturated liquid and saturated vapor. The saturated liquid volume (
L
V
) and the saturated vapor volume (
V
V
) are displayed. The horizontal, dashed orange line in each diagram represents a mixture of liquid and gas. This line represents the saturation pressure (
sat
P
) on the
P
-
V
diagram and the saturation temperature (
sat
T
) on the
T
-
V
diagram. On the
P
-
V
diagram, the green areas above and below the saturation orange line would be equal when plotted on a linear volume scale.
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