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.
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.
|