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The Gas Laws: The Boyle-Marriotte Law
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 Boyle-Mariotte 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 p1 and initial volume V1 and final pressure and volume p2 and V2 respectively, then we can write  
 
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