The gas laws are the conclusions of
experiments investigating the relation between the pressure,
volume and temperature of a fixed mass of gas. 



As we have three variables, we will need three experimental
investigations, in each case keeping one of the possible variables
constant. 



The Relation
Between Pressure and Volume, with Temperature Constant 

Consider a quantity of gas in a container of
variable volume (as in Experiment
4TP) 

If the pressure of the gas is measured at different volumes while
the temperature remains constant, the results are as shown by the graph
below. 



Changes of pressure and
volume which take place at constant temperature are called
isothermal changes. 

The curve above is called an isothermal. 



The general shape of this graph should be no
surprise since we know that the pressure exerted by a gas
increases as it is compressed into a smaller volume. 

Try compressing the air in a bicycle pump with your finger
blocking the outlet; it becomes harder to compress as the volume
decreases. 

See also the kinetic theory
in relation to this situation. 



However, if we now plot pressure
against 1/volume, we obtain the following graph. 





As with the pressure law and Charles' law,
these results do not depend on the type of gas. 



The conclusion of this experiment is
expressed in the BoyleMariotte Law (after Robert Boyle and Edme
Mariotte) stated as follows. 

The pressure of a fixed
mass of gas at constant temperature is inversely
proportional to the volume. 



or 





If a fixed mass of gas has initial pressure
p_{1} and initial volume V_{1} and final pressure
and volume p_{2} and V_{2} respectively, then we can
write 


