As we have progressed, we have seen that solids and liquids are not completely incompressible as they may have non-zero values of kT. And we learn that there are a number of finer points to describing...As we have progressed, we have seen that solids and liquids are not completely incompressible as they may have non-zero values of kT. And we learn that there are a number of finer points to describing the nature of the phases about which we all learn in grade school. In this chapter, we will employ some of the tools of thermodynamics to explore the nature of phase boundaries and see what we can conclude about them.