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I am in no way experienced in the Physics field so this question may seem a bit silly but i'd appreciate an answer :)

Why doesn't air freeze?

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Actually, it does freeze at 58 K which is such a low temperature that you don't encounter it anywhere on Earth. –  gigacyan Dec 9 '10 at 15:27
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Or at high pressures. Which you also don't encounter that often. –  Marek Dec 9 '10 at 15:29
    
Dry ice (frozen CO₂) is a familiar example of "frozen air" (although CO₂ comprises only 0.039% of the atmosphere). Below −78.5°C, pure carbon dioxide will sublimate from gas to solid at atmospheric pressure. –  nibot Dec 9 '10 at 19:48
    
(Does this mean that below −78.5°C, dry ice will precipitate out of the air? Or is there freezing point depression due to the atmosphere being a solution/mixture of several gases?) –  nibot Dec 9 '10 at 19:53
    
@nibot: yes, the same happens with water below 0°C. –  gigacyan Dec 10 '10 at 8:28
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4 Answers 4

Air is not a pure substance. Only two gases at one atmosphere can freeze out in extreme cold conditions. The gases are water vapor (rain, snow, and sleet) and Carbon Dioxide. I have not heard of CO2 sublimation ~ -79C. Temperatures have been cold enough (~ -90C) but the altitude is higher than sea level, so I am not sure that this had ever happened in recorded history. The other gases freeze out or rain out at much lower temperatures. These gases include the elements Oxygen, Nitrogen, Argon, Xenon, Hydrogen, Helium, and Neon.

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Air does freeze just at temperatures and pressures we don't often experience.

There is an entire industry around producing and distilling liquid air. You take air and compress it. As a result the air increases in temperature; the air is allowed to cool. Then the air is expanded by venting it into a new chamber. The result is a much colder gas.

Through cycles of compression, cooling and venting we can get air to condense into a liquid.

If liquid nitrogen is then placed in a vacuum chamber, it will freeze.

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"There is an entire industry around producing and distilling liquid air." - like this one... –  user172 Dec 12 '10 at 2:24
    
I really like this questions. I asked a similar question to my chemistry teacher in high-school about 10 years ago. She didn't have an answer at first. Its one of those things most people just don't question or think about. –  David Dec 12 '10 at 22:27
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At the normal pressure, 99.9% of air (nitrogen, oxygen and argon) will solidify in 55K (where the oxygen does). Below about 15K also hydrogen (0.000524% of normalized air) solidifies leaving only helium which will be a liquid up to 0K (yet will change into a superfluid about 2.17K).

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Air is composed of several different gases, but the most important ones are nitrogen and oxygen. Since these components have very low melting points, you won't see air freeze any time soon on Earth, but on the other hand, liquid nitrogen is produced routinely for all kinds of purposes (like for physics students to toy with during boring lab work).

The question is now reduced to why nitrogen and oxygen are present in gaseous forms at temperatures we are accustomed to and why they have such low melting points.

To answer the first question, temperature on Earth is what it is thanks to the Sun's light that warms us, the Earth's own geothermal heat, but also because of the greenhouse properties of a lot of gases in our air. (So air has a "hand" in warming itself if you want. :p)

For the second, the metling points of nitrogen and oxygen are so low because the molecules that make up these gases are so tiny and loosely bound. It takes little energy to excite them, therefore at "room temperatures" they are extremely mobile and are basically moving around in straight lines until they bump on each other or on anything else. The characteristic behaviour of molecules in liquids and solids is much less mobile.

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You beat me to the answer but this is nicer than my mine anyway, so +1 :-) By the way, the reason why the temperatures are so low, or more precisely why they are so high for stuff like water is not just because the molecules are tiny but because stuff like water has a precise molecular structure (i.e H-O-H with nice angles between H and O) that enables it to form standard hexagonal ice lattice (it can also form other lattices in different ice phases but this one is the usual one). Of course the complete explanation is much more complicated and could fill whole books. –  Marek Dec 9 '10 at 15:47
    
I agree that the complete explanation is more complicated. It's true that the exact melting point will depend on the structure that the solid will adopt. But I was more interested in giving a general feeling of why one might expect that small molecules will be more motile over than large ones. Although it is not true in the absolute of course. –  Raskolnikov Dec 9 '10 at 15:51
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I think that the phrase "loosely bound" says it all. The water that Marek mentioned is extremely exceptional since it also forms hydrogen bonds and that is why it has such a high melting point in contrast to most other liquids, like ammonia for example. –  Vagelford Dec 9 '10 at 16:37
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