Boiling Water in Vacuum
Date: November 2008
What is the boiling point of water in space?
The boiling point of water is "fixed" by its temperature, and the vapor
pressure of water at that temperature. So for example, if the water is at
100 C. the boiling point is 100 C. at a pressure of 1 atm (which is equal
to 760 mm of Hg). If the temperature of the water is about 25 C. the vapor
pressure will be about 25 mm of Hg, which is its boiling point at 25 C. We
are accustomed to thinking of the "boiling point" as the boiling point at an
applied pressure of 1 atm (or 760 mm of Hg), but this is just a bias introduced
with our familiarity of those pressure conditions. The boiling point is the
temperature of the water a predetermined pressure.
While we understand 'boiling' at home in the kitchen, it is actually a very
complex balancing act. We say a liquid is boiling when bubbles of gas appear
within the liquid - usually at the hottest part - and they then rise to the
surface and escape, causing bubbling at the surface. In order for that to
happen, the heat must cause the bubbles in the liquid to push out harder than
the air outside can push in.
At sea level, we know that pure water boils at 100C (Celsius) but what happens
when there is less air pushing in?
On the top of Mount Everest, where the air is very thin, the pressure of the
air is so low that water will boil at about 65C - which made it very very hard
for Sir Edmund Hilary to make himself a cup of tea in the afternoon! In Tibet
even today, they add the milk and the sugar before they boil the tea. These
'impurities' increase the boiling point enough to make the tea more enjoyable.
In outer space, with no air at all, it would be impossible to keep the water
together long enough to apply any heat. Any liquid water would begin to boil
immediately you unscrewed the top of the bottle, or opened the tap, not because
of the temperature, but because of the lack of pressure to hold the bubbles in.
You know by now that matter is made up of tiny objects we call molecules. The
molecules of air move around very fast. Water too is made up of tiny molecules.
When these molecules hit each other they bounce, like two regular balls that
are rolled against each other. The boiling temperature of the water will depend
on how many air molecules are above the water that are bouncing against the
water and keeping it from boiling. The more air particles, the more bounces,
the more heat or energy is required to make the water fight those bounces and
If there is no air above the water to bounce the molecules of water back in,
then the water will just boil right away. If there is air above the water (as
when it is inside the space shuttle)the water will act the same way as on Earth
and the boiling temperature will just depend on how much air is above the
Greg (Roberto Gregorius)
There are two answers to this question.
In an perfect vacuum (or as perfect as you can get) water, or any other
liquid, will boil at extremely low temperatures, well below room
temperature. Even the difference in air pressure between sea level (for
example Seattle, Washington) and a higher elevation (for example Denver,
Colorado) results in a difference in the boiling point of water. Water
in Denver boils at temperatures below 100 C, although only slightly
As for water in space, there are other things at work besides the
vacuum. Space is very, very cold, only a few degrees above absolute
zero. Even though there is a vacuum, water will freeze into a solid in
space. If liquid water is exposed to outer space, it boils violently
and then immediately freezes solid.
Water, despite being such a common place substance and so very vital to life
on earth has a remarkable number of outstanding mysteries and questions
surrounding some of its properties. Also, some of the properties that we
know and understand quite well may appear at first to be quite strange. The
answer to your question may be a little different from what you are expecting.
I cannot give you a numerical value for the boiling point of water in the vacuum
of space. That is because it does not have one!
At pressures below about 1/1000 th of atmospheric pressure water does not exist
as a liquid. If it is below 0 degrees C then it is a solid and if it is warmer
than 0C, then it is a gas. The phases of water go directly from solid to vapor
and vice-versa at such low pressures as you find in space. So what happens if
you were to suddenly take a glass of water from our normal conditions and place
it in the vacuum of space? It would boil violently into vapor.
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Update: June 2012