Water Steam - the Critical Point

Where vapor and liquid are indistinguishable

When water and steam reach the level of absolute pressure 3206.2 psia (221.2 bar) and a corresponding saturation temperature 705.40 oF (374.15 oC), the vapor and liquid are indistinguishable.

  • this level is called the Critical Point

At the critical point there is no change of state when pressure is increased or if heat is added. At the critical point the water and steam can't be distinguished and there is no point referring to water or steam.

For states above the critical point the steam is supercritical. Supercritical is not the same as superheated - which is saturated steam at lower pressures and temperatures heated above the saturation temperature.

Note! - the critical point is not the same as the triple point. The triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. For water the triple point is at temperature 273.16 K (0.01 oC, 32.02 oF) and partial vapor pressure 611.657 pascals (6.11657 mbar, 0.00603659 atm).

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