How can phase diagrams be used?
Phase diagrams can be used to determine the conditions under which substances will undergo a change of state (change of phase).
For example, a phase diagram for water will tell you which state water will be in at a given temperature and pressure. You could also use the phase diagram to determine the conditions which would lead to different phase change processes (melting, boiling, sublimation, deposition etc.)
Here is a video which discusses phase diagrams (start at 3:00, first portion of video discusses the heating curve of water).
Hope this helps...