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Name: Audrey
Status: student
Grade: 6-8
Location: CT
Country: USA
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.

Vince Calder

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.

Nigel Skelton

Hi Audrey,

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 actually boil.

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 water.

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 below.

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.

Ian Farrell

Hello Audrey,

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.

Michael Pierce

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