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Chapter 2 Gases and Gas Mixtures 25
LEARNING GOALS
Comprehension Objectives
Learn about the composition of air.
Understand how gases behave.
Understand the relationship between pressure
and gas volume.
Learn about the solubility of gases.
Understand what partial pressure is and how to
determine partial pressure.
Learn how to determine absolute pressure
at depth.
Learn how to determine the partial pressure of a
gas in a mixture at depth.
Matter has three common forms: solid, liquid, and gas.
Solids have definite volume and shape. Liquids have
definite volume but flow and assume the shape of their
container. Gases have neither shape nor volume. A gas
will uniformly fill the container that holds it. It can be
allowed to expand by increasing the size of the container,
or it can be compressed by forcing it into a smaller
container or by making the container smaller.
A gas is composed of molecules (or atoms) that are
in constant motion. Visualize extremely small, widely
separated individual molecules of a gas in constant,
random motion, bouncing off each other and the sides
of their container in a completely elastic way, and you
will have a rough picture of an “ideal gas.” The behavior
of any real gas only approximates the theoretical ideal,
but at the pressures encountered in scuba diving, the
approximation is very close.
A “gas” can be a mixture of individual component
gases. Air is such a mixture; it is predominately nitrogen
and oxygen. Once combined, the component gases will be
evenly distributed in the gas mixture according to their
proportions. Also, the component gases will not normally
separate; they will stay mixed. When you add oxygen to
air to make nitrox, the oxygen will not separate out and
pool at the bottom of the cylinder. The component gases
will generally be molecules, made up of more than one
atom each composed of one or more elements (oxygen
molecules, carbon dioxide molecules, etc.), but they may
also be individual atoms (argon, helium, and neon do not
form molecules).
Each of the individual components in a gas mixture
will exert its chemical or physical effects independently,
according to its proportion in the mixture.
The pressure of the gas is a manifestation of the
force with which its molecules strike the container. You
can increase the pressure of the gas by compressing it
into a smaller container (there are more molecules in
a given volume) or by increasing its temperature (the
molecules increase their energy and speed up). There is
a proportionality to pressure, temperature, and volume
that will be discussed later in this chapter.
Gases (and solids) will dissolve in liquids. Just as
the amount of a solid, such as salt, that will dissolve is
different for different liquids, such as water and gasoline,
the amount of a gas that can dissolve varies from solvent
to solvent. The amount of gas that can be dissolved in a
liquid is directly proportional to the pressure of the gas
on the liquid. The solubility of a gas also varies with the
temperature: more gas will dissolve at lower temperature.
WHAT’S IN AIR?
Our atmosphere is a mixture of gases (Figure 2-1). The
proportions of the gases in air, excluding water vapor, are
nearly uniform around the globe. The composition of dry
air, expressed as fractions is:
• Oxygen (O2) 0.2095
• Nitrogen (N2) 0.7808
• Argon (Ar) 0.00934
• Carbon dioxide (CO2) ~0.00035 (average)
• Others 0.00004
We normally simplify the numbers and say that
oxygen is 0.21 or 21% of atmospheric air. We will also
include argon, as an inert gas, with the nitrogen and say
that nitrogen is 0.79 or 79% of air. The amount of carbon
dioxide varies somewhat around the world (there is more
present in the more industrialized northern hemisphere),
and while carbon dioxide has a significant role in general
human physiology as well as the physiology of diving,
at 405 parts per million (ppm) (2019) of air, it does not
enter into nitrox calculations. The “others” category
includes neon, helium, krypton, sulfur dioxide, methane,
nitrous oxide, etc.
Figure 2-1 Composition of dry air.
Oxygen (20.9%)
Nitrogen (78%) Misc. Gases (1.1%)