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Saturation curve with isobars on T-v

Short answer

The T-v saturation curve is a dome with x=0 and x=1 lines meeting at the critical point; inside, isobars are horizontal. Saturation temperature is the boiling point at a given pressure. Quality is the vapor mass fraction in a mixture, x=m_g/m.

Sketch saturation curve of water in T-v with the help of isobar lines. Show all important points, lines and region. Also define saturation temperature and quality.

Saturation curve with isobars on T-v diagram

When water is heated at constant pressure, the process follows an isobar. At low pressure the isobar has a long horizontal portion; at higher pressure the horizontal portion is shorter and at a higher temperature, and at the critical pressure it shrinks to a point of inflection at the critical point.

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

Regions and lines

  • Left of x = 0 (saturated liquid line): compressed liquid.
  • Dome interior: two-phase mixture; isobars are horizontal at T_sat.
  • Right of x = 1: superheated vapour.
  • Critical point: the top of the dome, where the x = 0 and x = 1 lines meet.
  • Along an isobar: the liquid warms up (T rises, v nearly constant), evaporates at constant T_sat (v from v_f to v_g), and then the vapour is superheated (T and v both rise).

Saturation temperature

The temperature at which a pure substance boils (or condenses) at a given pressure. T_sat rises with pressure; for water it is 100 °C at 101.3 kPa and 179.9 °C at 1 MPa.

Quality

The ratio of the mass of vapour to the total mass of the mixture, x = m_g/(m_f + m_g). It varies from 0 (saturated liquid) to 1 (saturated vapour) and is used in v = v_f + x v_fg.

✓ Answer checked by Claude Opus 5.5 (high effort)

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