http://www.ck12.org Chapter 20. Gas Laws
FIGURE 20.1
Internal motion of molecules in a solid and gas. The concept of temperature allows us to obtain information about
such motions without keeping precise track of them.However, it was later understood that the results of this chapter are not, in themselves, irreducible, like (for our
purposes) Newton’s laws were. Using other approaches (statistical mechanics and molecular kinetic theory) it was
shown that the results we study can be considered statistical in their nature. To do this, various substances are
represented through models, based on their microscopic constituents, that accurately model their behavior. The
macroscopic parameters defined above are then found by ’averaging’ some quantity over all the particles that make
up a system. As hinted in the figure above, for instance, temperature is related to the kinetic energy of molecules in
a substance.
For the purposes of this chapter, however, we look at thermodynamics as an empirical science based on observation.
Pressure and Volume
Before we go further into the empirical gas laws, let’s consider the parameters described above and how exactly they
relate to gases. Since gases generally occupy all the volume available to them,the volume of a gas is simply the
volume of the container that holds it.
As you may remember, pressure is defined as force per unit area. When talking about gases, we refer to thepressure
that a gas exerts on the walls of its container. You should remember from the pressure chapter that atmospheric
pressure exerts a force of about 100, (^000) mN 2 , or Pascals. Often, when actually measuring gas pressure, we really
measure the difference between the pressure exerted by the gas and the air pressure, since most common pressure
gauges do not take air pressure into account. It is important to keep this fact in mind when performing experiments.
For instance, consider the example below: an inflatable balloon. The gas pressure inside is greater than the air
pressure outside, since the elastic force wants to contract the balloon and essentially pushes in the same direction as