What is Air Really?

It may seem a bit trivial to ask, because air is all around us, but 'air' itself is actually rather complex.  Physically, we tend to describe air as a gas, that is a substance where molecules move freely with minimal to no interactions with one another outside of collisions.

To further complicate matters, air is also a mixture of multiple gases. In mathematical modeling, air is generally modeled as uniform, meaning that the mixture of different gasses is the same throughout1.

Gas
Chemical Formula
Molecular Mas
(g/mol)
Ratio Compared to Dry Air (%)
by Volume
Nitrogen
N2 28.02
78.09
Oxygen
O2 32.00
23.20
Argon
Ar
39.94
0.933
Carbon Dioxide
CO2
44.01
0.03
Neon
Ne
20.18
0.0018
Hydrogen
H2 2.02
0.0005
Helium
He
4.00
0.00005
Table: Average percent composition of air. Although there are even more gasses in air, the percent composition is minimal.
Air is primarily nitrogen (which is relatively unreactive) and oxygen (which is incredibly important to our continued existence).
(Source: modified from The Engineering Toolbox)


The particular mixture of gasses in the air is a vital factor of life as we know it. Oxygen, the second most common gas in air, is of critical importance in cellular respiration. However, Oxygen is also very chemically reactive (as we will discuss later). Coincidentally, Nitrogen and Argon (two of the three most common gases in the atmosphere) are both very unreactive2.

For the purposes of describing movement we consider air to be a fluid: a substance that flows and takes the shape from its container. Unlike the vernacular usage, fluid and liquid are not interchangeable. Instead fluid is used to describe both liquids and gasses3.

Despite their ability to both flow and take the shape of their containers, cats are not strictly speaking fluids.
[White
                                  Cat in a Bowl]Despite it's ability to
                                  flow and take the shape of it's
                                  container, this cat is not a liquid.
Image Archived at: http://www.boredpanda.com/cats-are-liquids/

The fluid nature of air allows us to use fluid dynamics to make predictions about the behavior of air under specific conditions of volume, temperature, and pressure, as we will see as we carry on in the next few pages.

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Just some pretty air





Just some pretty air












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