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Q & A: phase coexistence

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Most recent answer: 10/22/2007
Can water and steam (gas) exist at the same temperature? Can water and ice exist at the same temperature?
- Marie
Bear, De
The answers to both questions are definitely yes.
 Letís start with water-ice. At 0įC (and ordinary pressure) water and ice coexist stably. If the pressure is changed the coexistence temeprature changes a little bit.

For water-vapor, at 100įC, liquid water coexists stably with pure water vapor, at atmospheric pressure. The coexistence temperature is pretty sharply dependent on pressure. In ordinary atmosphere, what happens is that liquid water coexists with an atmosphere which has some water vapor in it, but is not pure water vapor. Thereís an equilibrium vapor pressure which depends on temperature, and gives the pressure contributed by the water vapor alone in stable equilibrium with the liquid. At typical room temperature itís only a few percent of atmospheric pressure.

Mike W.

(published on 10/22/2007)

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