PvT Surface with Solid Region
Now we will see what happens when we include the solid region on the PvT surface. The complete PvT surface for a pure substance is shown to the right. As on the previous page, you can click on the hyperlink of a particular aspect to see how it appears on this surface.
On the PvT surface shown here, red lines are isobars, blue lines are lines of constant temperature (isotherms), and green areas are two-phase regions. All yellow areas are single phase regions.
We have already discussed the single phase liquid (L), single phase vapor (V), supercritical fluid (SCF) and two-phase liquid-vapor (L+V) regions. The single phase solid region is shown here in purple. Between it and the single phase liquid region is a two-phase solid-liquid region. It is analogous to the vapor-liquid two-phase region except that there is no evidence (as indicated by experiments at high pressures) that it terminates in a critical point.
Between the single phase solid and single phase vapor regions is the two-phase solid-vapor region. It is separated from the vapor-liquid two-phase region by the triple line. The significance of the triple line is that it represents the only temperature and pressure at which three phases (solid, liquid and vapor at the locations indicated in the drawing) can coexist.
We could recover the temperature-volume (Tv) diagram from the PvT surface by looking straight down onto the surface. The Tv diagram would be the projection that we see. Since this is a three dimensional surface, it turns out that there are two other projections. These will be described on the next page.