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Q & A: There’s something in the air

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Most recent answer: 10/22/2007
Q:
what is in the sir we breath other than nitrogen and oxygen? and is there a experiment that can be done to show what is our air?
- Alysse (age 16)
Coffs Harbour High, Australia
A:
Air is mostly (78% or so) nitrogen, and most of the rest is oxygen, so you have most of air accounted for already. Air has lots of other stuff in it at lower concentrations. There's carbon dioxide, water vapor, argon, and neon. Plants need carbon dioxide for their metabolism. The argon and neon are chemically inert, to a very good approximation.

In addition to the usual constituents of air, just about anything that is in equilibrium with its vapor (or is a vapor) at standard temperatures and pressures can be found in air at some places and times, or perhaps everywhere if you're willing to look hard enough. Methane is produced by bacteria decomposing biological material -- it's often called "swamp gas". Alcohols and hydrocarbons evaporate and can be found in the atmosphere. Manmade and natural pollutants include oxides of nitrogen and sulfur. These latter can combine with water and form nitric and sulfuric acid, which poison lakes (an effect called "acid rain"). Carbon monoxide is a common pollutant produced by burning fuels. Particulates are little bits of stuff that are suspended in the air -- that is, they float and blow around until they settle on the ground. Black, sooty smoke from burning coal, for instance, looks the way it does because of all the little particulates in it. Ozone is also a common pollutant found in urban areas. Aerosol sprays used to use fluourocarbons which have now been banned, but they persist at a low level in the atmosphere. Walk by a kitchen when dinner's cooking and you'll be treated to a vast array of interesting molecules that smell good but are present in the air at low concentrations, and usually only at dinnertime.

The oxygen and nitrogen can be detected with the standard set of experiments. Flammable stuff burns when there is oxygen, and doesn't when all the oxygen is used up. A lot of the rest of the stuff can be found by slowly freezing air and seeing what comes out.

Water vapor freezes first, leaving ordinary frost (or you can condense it as a liquid, forming dew). Carbon dioxide freezes at a colder temperature, but still much warmer than the oxygen or nitrogen, and is stable as a solid at ordinary pressures. When the oxygen liquifies out, the temperature is just over liquid nitrogen temperature. Liquid oxygen, like its gas, also helps stuff burn, and can be quite hazardous to handle. It's also paramagnetic, so you can hold some between the poles of a (cold!) magnet.

Other stuff you can detect with your nose (sometimes), but there are many hazardous gases which can poison you but which also do not have any smell or color, so don't go sniffing unknown gases as a science experiment. Various gas detectors and "sniffers" are commercially available for detecting the concentrations of specific components of air and pollutants. Oxygen meters are quite common, and smog-check stations will put a detector in the tailpipe of a car to detect various pollutants it is emitting.

Tom

(published on 10/22/2007)

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