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[Phys-l] Triple point pressure



Hi,

I have a question on triple point pressure. One can read many things from a phase diagram: e.g., the boiling point curve shows the total pressure in the boiling temperature of a liquid (say, water). Another way to look at it is to interpret the above mentioned reading as saturated vapour pressure in the given temperature.
Hence there is no need to require that pure substance is involved when reading the phase diagram.

However, the triple point seems to be a special case.
Wikipedia explanation explicitly defines the triple point of water by requiring pure substance (<http://en.wikipedia.org/wiki/Triple_point>):

"The single combination of pressure and temperature at which pure water, pure ice, and pure water vapour can coexist in a stable equilibrium occurs at exactly 273.16 kelvins (0.01 °C) and a pressure of 611.73 pascals..."

My colleague asked me whether it is possible to have a triple point with higher total pressure than indicated above as long as the partial pressure of water is that 611.73 pascals? My answer is no but this seems to make the triple point different: in that point the saturated vapour pressure must be the same as the total pressure which is not the case for other points in the curves of the phase diagram.

Is this intrepretation correct? Any additional insights?

Regards,

Antti

P.S. My website has changed (see below).

Antti Savinainen, Ph.D., B.Ed.
Senior Lecturer in Physics and Mathematics
Kuopio Lyseo High School
Finland
E-mail: <antti.savinainen@kuopio.fi>
Website: <http://kotisivu.dnainternet.net/savant/>


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