Chemical Potential Dependence on Temperature and Pressure


Download the CDF file to view the simulation using the free Wolfram CDF player.
Changes in the chemical potential μ of water as a function of pressure P (at constant temperature T) or temperature (at constant pressure P) determine
vapor-liquid, vapor-solid and liquid-solid phase changes. Chemical potential is given by dμ = dG = VdP - SdT. Because the x axes cover narrow ranges of temperature and pressure, the chemical potential plots are linear. Since the objective of this Demonstration is to show the qualitative behavior, the
temperature or pressure is shown on dimensionless relative scales. Also, note
that the y axis scale is not the same for all plots so that the differences in
chemical potential are easier to see. Chemical potential as a function of pressure
is also shown for the solid-liquid phase change for ethanol, which has a different pressure dependence than water. 

Use the drop-down menus to select which two plots are displayed. The more
stable phases (black solid lines) have a lower chemical potential. Use sliders to
set temperature for the μ versus P plots or to set pressure for the μ versus T
plots. Note that the sliders change the temperature and pressure over narrow ranges. Check "add salt" to see the effect of adding salt to liquid water, and set
the salt concentration with a slider. Adding salt leads to boiling point elevation
and freezing point depression. The phase changes that occur depend on the pressure or temperature relative to the triple point; check "show triple point
labels" to show these on the plots.