The applet below illustrates pressure-volume isotherms for gases. 

• Adjust the temperatures of the isotherms by clicking on the temperature labels and dragging them up or down. 

• The axes limits can be adjusted by clicking on an axis and dragging, right or left for the volume axis and up or down for the pressure axis.

• When "van der Waals gas" or "real gas" is selected, the van der Waals "a" parameter models the attractive forces between gas molecules while the "b" parameter models the repulsive forces.  To see the difference, first set a = 0 and adjust b.  Then set b = 0 and adjust the a parameter.

• For a "real gas" one can see the horizontal regions of the plot which depict the areas where fluid is condensing from the gas phase to liquid.