The coexistence curves of a typical pure *PVT* system are shown in Fig.3.4.

In general, there are 3 possible phases, namely, vapor, liquid, and solid.

All transitions are 1^{st} order
with accompanying latent heat.

Coexisting phases |
Name of curve |
Symmetry change |

vapor-liquid liquid-solid vapor-solid |
vaporization fusion sublimination |
no yes yes |

*A* is the **triple point** at which all 3 phases
coexist.

*C* is the **critical point** at the end of the
vaporization curve.

The vaporization curve *AC* actually denotes a vapor-liquid coexistence region as shown in
the *P-v* diagram in Fig.3.5. Note that the isotherms are also isobars in
the coexistence region.

The 3-D *P-v-T
*diagram is shown in Fig.3.6.