The compressibility factor chart plots the compressibility factor , equal to , where is the volume per mole, versus the reduced pressure for several values of the reduced temperature . The reduced pressure and temperature are defined by and , respectively, where is the critical pressure and is the critical temperature. Use buttons to select one of five molecules, and move the black dot to display the compressibility factor curve (blue) for any value of between 1.0 and 1.8. The and values for the location of the black dot are displayed on the chart. For an ideal gas, (green line).