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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13$\begingroup$ Actually, it does freeze at 58 K which is such a low temperature that you don't encounter it anywhere on Earth. $\endgroup$– gigacyanCommented Dec 9, 2010 at 15:27
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9$\begingroup$ Or at high pressures. Which you also don't encounter that often. $\endgroup$– MarekCommented Dec 9, 2010 at 15:29
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$\begingroup$ To summarize, air doesn't freeze because the temperature is too high, and the pressure is too low. $\endgroup$– user121330Commented Jul 27, 2017 at 17:48
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$\begingroup$ arxiv.org/abs/1309.0841 $\endgroup$– Count IblisCommented Jul 29, 2017 at 13:23
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6 Answers
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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.
answered Dec 9, 2010 at 15:40
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3$\begingroup$ 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. $\endgroup$– MarekCommented Dec 9, 2010 at 15:47
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1$\begingroup$ 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. $\endgroup$ Commented Dec 9, 2010 at 15:51
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2$\begingroup$ 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. $\endgroup$ Commented Dec 9, 2010 at 16:37
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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 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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$\begingroup$ "There is an entire industry around producing and distilling liquid air." - like this one... $\endgroup$– user172Commented Dec 12, 2010 at 2:24
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$\begingroup$ 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. $\endgroup$– DavidCommented Dec 12, 2010 at 22:27
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$\begingroup$ I'm curious about liquid nitrogen freezing in a vacuum chamber. Wouldn't the lack of pressure make it boil faster? $\endgroup$ Commented Mar 2, 2018 at 17:53
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There's a tiny trace of helium in air, about 5 parts in a million. The pressure isn't high enough for it to freeze no matter how cold it gets
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2$\begingroup$ But this tiny trace helium could get locked inside other frozen gases. I don't really see how helium is "the answer" here. $\endgroup$ Commented Oct 20, 2014 at 21:02
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The state (solid, liquid or gas) of a substance depends on the strength of the bonds made between the molecules of the substance and the (thermal) kinetic energy of the molecules.
To break a bond between molecules requires energy and the stronger the bond the more energy is required to break the bond.
For the gases that make up the air the kinetic energy that the molecules have is easily sufficient to supply enough energy to break any bonds between the molecules.
If the temperature is lowered the kinetic energy of the molecules decreases and below a certain temperature the kinetic energy is not sufficient to break enough bonds between the molecules to stop them staying together in the liquid state, or even lower in temperature in the solid state.
So the answer to you question is that temperature of the air is maintained at a sufficiently high temperature by the Sun such that collisions between molecules do not result in a permanent bond between the molecules.
Go further from the Sun where the surface temperature on the plants is lower and gases do become liquids and even solids.
Of all the gases in the atmosphere carbon dioxide has a relatively high freezing (sublimation) temperature at $-78.5\, ^\circ \rm C$ and the lowest recorded temperature is $-78.5\, ^\circ \rm C$ in the Antarctic there is a possibility that carbon dioxide might freeze.
However, as this article points out, that is unlikely to happen because there are other factors involved.
Given that the boiling points and freezing points tend to decrease as the molecular mass decreases an amazing fact is that water exists in its liquid and solid form on the Earth.
So really this an example of the opposite effect to that mentioned in your question.
Hydrogen sulphide $\rm H_2S$ is a gas at room temperature and yet the lighter water molecule $\rm H_2O$ can exist as a liquid at such a temperature.
This illustrates the importance of the strength of the bonds between molecules.
Water molecules have a large dipole moment (asymmetric charge distribution) which results in a relatively much stronger (hydrogen) bond between water molecules as compared to those between hydrogen sulphide molecules.
So more thermal agitation (corresponding to a higher temperature) is needed to break the bonds between water molecules than those between hydrogen sulphide molecules.
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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.
