DIYbanter - View Single Post - 500 microns?

wrote on 8/25/2005 8:37 AM:
rick1matthews wrote:

Pour water on the floor, and it will evaporate without addition of heat...

It's more accurate to say that the heat required to evaporate the water
(about 1000 Btu/lb) comes from the surroundings, ie this evaporation
cools the floor, the air, and so on.

Some of the heat comes from the surroundings in
this example. Some comes from within the water itself.

Key to this discussion is that no external source of
heat is needed to evaporate water.

Were a 70 degree F drop of water suspended (perhaps in a weightless
environment) in a hard vacuum, which in turn is completely
surrounded by shiny surfaces cooled to 100 milliKelvin, the drop
would still evaporate.

So where did the heat come to evaporate it?

Statements were made earlier that water only boils when
heat is added and that water only evaporates when heat
is added. Neither statement is true.

Water evaporates whenever the partial pressure of water
in the surrounding atmosphere is so small that the
capture of the water molecules from the air is less
than the escape of water molecules from the surface
of the water. The rate of escape is determined by
the water temperature. The rate of capture is determined
by the partial pressure of water. The water temperature
at which the two are in equilibrium is called the dew point.
If the water is cooler than the dew point, you get net
condensation. If the water is warmer than this dew point,
you get net evaporation.

When air pressure is very low, the partial pressure
of water can only be lower. Therefore the dew point must
be quite low, so that 72 degree F water (or 35 degree F
water, for that matter), is warmer that its dew point.
Water molecules are always escaping, and there are few
water molecules in the environment to balance the process.

Hence, the water evaporates.

The cooler it is, the slower it evaporates, but it does
still evaporate. In a perfect vacuum, even a 1 Kelvin ice
cube would eventually sublime way, though not in billion
years.

--
Rick Matthews