"rick1matthews" wrote in
message ...
Richard J Kinchwrote:
PrecisionMachinisT writes:

A lot of people have been taught the pseudo-scientific myth that
"water
boils in a vacuum" in some magic sense that it doesn't in the
atmosphere. The truth is that water vaporizes in a vacuum or in
the

atmosphere the same way: *only* because you add heat. In the same
sense, water doesn't boil at 212 deg F, it boils because you add
more
heat after it is at 212 deg F.


Nothing pseudo-scientific about it. I have boiled water with
ice.

Here is how: take a glass flask with an indentation in the bottom,
add
a little water. You may want to add boiling chips to make the effect
more gradual. Attach a cork with a rubber hose to the top to vent.

Put it on a bunsen burner and boil, just to get the steam to drive
all
the air out. Remove heat and clamp the rubber hose. Let it cool.

Invert, add ice to the depression, and watch the cool water boil,
not
with the addition of heat, but with the addition of ice. I have
boiled water down to 60 degrees F with this technique.

What is happening is that cooling the glass on top causes most of
the
remaining steam to condense, establishing a decent vacuum. Lowering
the pressure over the water lowers the boiling point, so the water
now boils.

Julius Sumner Miller would be proud : )


You have said repeatedly that one must add heat for sublimation to
occur. That is not true. Sublimation and evaporation both proceed
by the same mechanism. Pour water on the floor, and it will
evaporate without addition of heat, unless the relative humidity is
100%. Sublimation happens, too, just starting from the solid
instead
of liquid phase of water.

What is happening microscopically is related to the the kinetic
energy
of the molecules. Temperature is not heat, as you pointed out. But
they are related. Temperature is defined to be proportional to the
random translational kinetic energy per molecule of the material.
Heat transfer is equal to the change of the random translation
kinetic energy plus rotational and vibrational kinetic energy plus
changes in potential energy. So any substance above absolute zero
contains heat, and its molecules have a distribution of kinetic
energies.

When a molecule on the surface achieves an atypically large kinetic
energy from collisions with its neighbors, it reaches escape
velocity. If the substance is a liquid, we call it evaporation. If
it is a solid, we call it sublimation. There is always a reverse
process of capture from the atmosphere. But the process is usually
slower.

The temperature of the surface determines the escape rate. The
partial pressure of water in the surrounding atmosphere determines
the capture rate. In a perfect vacuum, the capture rate is zero,
and
the net effect is evaporation or sublimation.