Jere Lull wrote:
On 2007-09-22 16:07:27 -0400, Keith Hughes said:

Actually, do a search on "triple point" and look at the phase diagram
for water. That gives a good graphical depiction of the
pressure/temperature/phase relationships.

That just shows the phase vs temperature, not the energy required to get
the substance to pass that critical temperature.

No, it shows the phase for temperature *versus* pressure, not the same
thing at all. And the point was to clarify the temperature/pressure
relationship.

See
http://en.wikipedia.org/wiki/Thermodynamic_databases_for_pure_substances
and search for "Enthalpy change of phase transitions" for some
explanation of the heat energy required to change phases, with a diagram
for zinc. (Thanks, Brian, for reminding me of the term I couldn't
remember.)

And also think a bit: If the difference between 32 and 33 were so easily
crossed, you wouldn't see so much ice in your drink so long.

Saying the difference between "32 and 33" is misleading (or a
misunderstanding of the process). Actually, it's the difference between
Ice at 32° and Water at 32°. All the energy (enthalpy) change is used
in the phase transition (latent heat of fusion, or evaporation in the
case of distillation), and not change in temperature. Additional heat
input will raise the temperature of the water (or steam in the case of
distillation - i.e. 100°C water [at standard pressure] evaporates to
100°C steam, at the moment of phase change, and additional heat input -
and pressure - are required to raise the steam temperature further).

Keith Hughes