Water State in Space
Name: Uri Y.
Form of water according to pressure
I was searching about the effect of pressure on the freezing point of
water, and I found in your site under the topic "Altitude Freezing " this
sentence (this article was singe by Grayce): "the freezing point of water
increases with a decrease in the pressure applied to it". If that is
correct, and as far as I know the boiling temperature of water decreases
with the decrease of pressure. So what happen (or what come first) in
Extreme conditions of low pressure: freezing or boiling?
In the outer space, where there is no pressure at all, the form of water
is gas or ice? (The comets are some kind of ice, isn't it?)
What I am looking for is a graph of the behaving of water according to
pressure changes in constant temperature.
What you want to see is called a phase diagram. You can probably find a
phase diagram for water in any physical chemistry textbook; I know there is
one in my Atkins.
It is true that the freezing point of water decreases as the pressure
decreases. This is because water expands when it freezes, so compressing
ice will tend to convert it to more compact liquid water. Thus, increasing
the pressure lowers the freezing point. This is exactly the same thing as
saying that decreasing the pressure raises the freezing point.
However, this effect is very small at pressures below one atmosphere. The
freezing point of water at one atmosphere pressure is 273.15 Kelvin. As the
pressure is lowered, the freezing point increases ever so slightly, to
273.16 Kelvin. This is at a pressure of 0.006 atmospheres. Below this
pressure, liquid water is not stable. Only the gas and solid are stable.
If you subject water to a hard vacuum, it will boil until it loses enough
heat to make the remaining liquid freeze solid. Vapor (gas) can continue to
escape from the solid, at a rate that depends on the temperature. Vapor can
also condense back onto the solid, at a rate that depends on both the
temperature and pressure of the vapor. High temperatures and low vapor
pressures favor the vapor phase; high vapor pressures and low temperatures
favor the solid phase.
In deep space, the temperature is VERY low. That is how chunks of ice known
as comets can persist in the extreme vacuum. When comets approach the sun,
however, energy from the sun causes some of the solids to vaporize, making
the pretty tail. After enough passes, a comet will completely boil away.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
It depends on your starting conditions of water:
There are hundreds of sites on the Internet that have comprehensive
thermodynamic data on water. What you are looking for is a P-V-T
(pressure - volume - temperature) diagram. This 3 dimensional graph will
show you the state of water at a given T or P.
Here is a purely academic answer. Say for example somehow a certain volume
of liquid water @ T = 77 F and @ P = 1 atm were to be magically placed into
space where T ~ 4 Kelvins and P = vacuum. The liquid all of the sudden will
have no pressure surrounding it. With the sudden lack of pressure the
volume of water would explosively boil off into water droplets. Shortly
thereafter, the water droplets will freeze. Why would not the block of water
just instantly freeze? In space, matter does not cool or heat the same as it
does on the ground. The ability of space to transfer heat is limited.
There is no CONDUCTIVE, or CONVECTIVE heat transfer (since these first two
methods require physical contact w/ the cooler matter)...there is only
RADIATIVE heat transfer.
I hope this helps,
The boiling point of water is lowered with decreasing
pressure. Comets melt like crazy because it is in
Starting from liquid water at a given temperature
and from exposing it to extreme conditions of
low pressure, the internal energy will be source
of energy to the boiling. With the consumption
of part of this energy the temperature will decrease
to a point where the next step would consume
latent heat of fusion, and so you get ice crystals.
This is what happens with urine from astronauts
when disposed from the module...
Although the vapor pressure of ice is very small, the ice will
also evaporate if present some kind of energy source.
As you have mentioned, the evolution of cometary
nuclei with its impurities shows us the surface activity
on being struck by radiant energy from the sun, where
the ammonia, water, carbon monoxide
sublimates, producing the characteristic tail.
In the outer space, in a stage so far from the sun,
water ice sublimation is totally negligible and the
presence of ice is stable.
As you know, Ice can assume a large number of different
crystalline structures. At ordinary pressures the stable
phase of ice is called ice I, and the various high-pressure
phases of ice up to ice XIV !!!
About the sentence " the freezing point of water increases
with the decrease of pressure applied to it", you can say it
is correct, and can be confirmed from a phase equilibrium diagram
Pressure x Temperature for water. If you get this diagram,
then you will be able to find out what you want : a graph of
behaving of water according to pressure changes with
constant temperature. Pay attention to the following : The
equilibrium diagram shows the state of equilibrium reached
after a determined time. In your question, when you say :
"what happens under extreme conditions, when water is
- suddenly - under low pressure conditions...", this means
that you have NOT the equilibrium yet. So, there will be a change,
looking forward the equilibrium. The graph shows already
You can find a lot of info on the phase diagram of pure water by searching
that term, say of www.google.com . You will find everything from the very
simple to very sophisticated quantitative thermodynamic analyses. The
following site will give you some visual insight into the behavior of this
most complicated substance.
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Update: June 2012