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Q & A: condensed air

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Most recent answer: 10/22/2007
Q:
If you compressed enough of the air around us densely enough would you be able to see it? If so, what would condensed nitrogen, oxygen and carbon dioxide look like? If not, would some nuclear reaction take place before the air was condensed enough to see?
- Drake (age 17)
New Zealand
A:
Actually, if you think about it, you can see air even at normal density. The blue sky just comes from light bouncing off air molecules- so that's air you see. When the setting sun looks reddish, that's because you're seeing it through a thick layer of air, and a larger fraction of the blue light bounced off of air molecules than the red, and so a larger fraction of what's traveling straight toward you is red.

If you want to get air dense enough to see samll amounts of it, the easiest way is to cool it down until it turns liquid. In principle it could be compressed that much without cooling, but that would take a lot nof pressure. Liquid nitrogen (the main component of air) is very common around labs. It's just a clear liquid. Liquid oxygen is too, but since it tends to promote explosions you don't ordinarily come across it. Carbon dioxide (a trace component of air) is also common in condensed form. It's called dry ice, and you've probably seen it (it's white, but that's mainly because a typical block of it is made up of many little crystals oriented in many different directions). At ordinary pressures it CO2 goes straight from gas to solid without forming a liquid. High pressure CO2 forms a fluid which can be used for toxin-free dry cleaning. So far as I know, it's also clear.

None of these materials have any nuclear funny business going on.

Mike W.

You can squeeze gas hard enough to make nuclear reactions happen, but gosh, that's really really hard. It's easier to get the nuclear reactions to happen if you heat up the gas. The reason for this is that nuclei are all positivly charged and repel each other. To get them close enough so that the short-range strong nuclear force is more important than the repulsive electrostatic force, you either have to push really hard or throw the nuclei at each other really fast. Pushing really hard probably means pressures that exist only on neutron stars. And not just the surface (which is made up of a crust of ordinary material), but in the core where it is energetically favorable for nuclei to interact with each other strongly.

The high temperatures are much more easily obtained in laboratories here on earth. Plasmas can be heated to phenomenal temperatures with lasers or electrical discharge in order to try to get light nuclei to fuse together, in big projects which attempt to design a fusion reactor that produces more energy than it uses (so far no luck, but they are slowly improving). Anything that is that hot glows with a characteristic spectrum called the "blackbody" law. The sun is an example of an object which has at its core gases which are at high enough pressure and temperature for nuclear fusion to happen, and it glows bright.

If you ask me, it's far safer to look at a styrofoam cup of liquid nitrogen. It looks a bit like water, although the refractive index is different and the density's different and the viscosity's different, but still it's plenty visible.

If a change in refractive index is enough for you to be hapy that you can see air, just thing of the shimmering, wiggling image distortion you see when looking at something through some air that's been heated (say, over a stove, or over a road on a hot day, or through the exaust of a jet engine).

Tom

(published on 10/22/2007)

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