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Name:   Uri Y.
Status:   other
Age:   40s
Location: N/A
Country: N/A
Date: 1999-2001


Question:
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.


Replies:
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.
Assistant Director
PG Research Foundation, Darien, Illinois


It depends on your starting conditions of water:

Quick answer:

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,


-Darin Wagner


The boiling point of water is lowered with decreasing pressure. Comets melt like crazy because it is in space.

-Wil Lam


Hi, Uri

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 the equilibrium.

Alcir Grohmann


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.

http://www.sbu.ac.uk/water/phase.html

Vince Calder



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