Saturation curve on P-v and T-v diagrams

The saturation curve (vapour dome) is the boundary of the liquid-vapour two-phase region. It is formed by joining the saturated liquid states and the saturated vapour states for all pressures.

P-v diagram

 P|            CP (critical point)
  |          .-*-.   T > Tc
  |         /     \_______
  | liquid /  wet   \____  T2 (isotherm)
  |  x=0  |  mixture  | x=1   superheated
  |  ||   |-----------|------ T1 (horizontal
  |  ||   |           |        in wet region)
  +------------------------------- v

T-v diagram

 T|                  CP
  |                .--*--.                    / P2
  |               /       \               _.-'
  |          2f  /---------\ 2g ___.--'''       / P1
  |             /           \               _.-'
  |        1f  /-------------\ 1g ___.--'''
  |           /     wet       \
  |          /    mixture      \
  |  compressed               superheated
  |    liquid                   vapour
  +--------------------------------------------- v
      x = 0 line           x = 1 line
  (P2 > P1; in the liquid region the isobars lie almost on the x = 0 line)

Important points, lines and regions

• Critical point (CP): top of the dome (water: 22.09 MPa, 374.1 °C).
• Saturated liquid line (x = 0): left boundary; saturated vapour line (x = 1): right boundary.
• Compressed (sub-cooled) liquid region: left of the x = 0 line.
• Wet (two-phase) region: inside the dome; isotherms on P-v and isobars on T-v are horizontal here.
• Superheated vapour region: right of the x = 1 line.
• On P-v, an isotherm falls steeply in the liquid region, runs horizontally across the dome, then falls more gently in the superheated region. On T-v, an isobar rises steeply in the liquid region, runs horizontally across the dome, then rises again in the superheated region. The critical isotherm (P-v) and critical isobar (T-v) each have a point of inflection at CP.
• The triple line (solid, liquid and vapour together) lies at the bottom of the full diagram.