http://www.ck12.org Chapter 20. Gas Laws
Empirical Gas Laws
Early experiments with various gases showed the following relationships to holdfor a fixed amount (mass or
number of molecules) of gas:
Boyle’s Law (at constant temperature)PV=k 1 ifT= Constant [2]Gay-Lussac’s Law (at constant volume)P
T
=k 2 ifV=Constant [3]Charle’s Law (at constant pressure)V
T
=k 3 ifP= Constant [4]It should be first noted that these laws only hold forquasistatic processes, which are defined as processes where the
system is always at (or very near) thermal equilibrium. Informally, we may think of these as ’slow’, in the sense that
at any point in its path (see diagrams below), the system has time to reach thermal equilibrium. An important fact
about such processes is that they arereversible. All the processes we consider in this book are quasistratic.
Boyle’s law states that at constant temperature, the pressure and volume of a given amount of gas are inversely
related. If you squeeze a balloon, for instance, while keeping its temperature constant, its volume will decrease
and the gas will exert a greater amount of pressure than before to counterbalance the force you apply in addition to
atmospheric pressure.
Gay-Lussac’s law states that at constant volume, the pressure exerted by a gas is proportional to its temperature.
For instance, if the balloon pictures above were perfectly rigid (could not stretch beyond its current volume), by
increasing the temperature of the gas, you would increase the pressure inside, and the balloon would eventually
burst.
Charles’ Law states that at constant pressure, the volume of a gas is proportional to its temperature.
Avogadro’s LawFinally, the Italian scientist Amedeo Avogadro determined experimentally that at constant temperature and pressure,
equal volumes of different gases contain equal numbers of molecules; in other words (ifnis the number of
molecules of a gas):
V
n=k 4 ifP= constant andT= constant [5]