Definition: For a given substance, it is possible to make a phase diagram which outlines the changes in phase (see image to the right). Generally temperature is along the horizontal axis and pressure is along the vertical axis, although three-dimensional phase diagrams can also account for a volume axis.
Curves representing the "Fusion curve" (liquid/solid barrier, also known as freezing/melting), the "Vaporization curve" (liquid/vapor barrier, also known as evaporation/condensation), and the "Sublimation curve" (solid/vapor barrier)) can be seen in the diagram. The area near the origin is the Sublimation curve and it branches off to form the Fusion curve (which goes mostly upward) and the Vaporization curve (when goes mostly to the right). Along the curves, the substance would be in a state of phase equilibrium, balanced precariously between the two states on either side.
The point at which all three curves meet is called the triple point. At this precise temperature and pressure, the substance will be in a state of equilibrium between the three states, and minor variations would cause it to shift between them.
Finally, the point at which the Vaporization curve "ends" is called the critical point. The pressure at this point is called the "critical pressure" and the temperature at this point is the "critical temperature." For pressures or temperatures (or both) above these values, essentially there is a blurry line between the liquid and gaseous states. Phase transitions between them do not take place, although the properties themselves can transition between those of liquids and those of gases. They just do not do so in a clear-cut transition, but metamorph gradually from one to another.
For more on phase diagrams, including three-dimensional phase diagrams, see our article on states of matter.
Also Known As:
state diagram, change of phase diagram, change of state diagram