There are essentially 5 states of matter:
Plasma is actually the most abundant state of matter in the universe, because it is the state that exists inside stars that are undergoing nuclear fusion. Ball lightning is an example of plasma that manifests on the Earth.
Phase TransitionsSubstances can go through a number of transitions between their states:
- condensation - gas to liquid
- fusion (or freezing) - liquid to solid
- melting - solid to liquid
- sublimation - solid to gas
- vaporization - liquid or solid to gas
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. It is sometimes convenient to create the diagram using Kelvin as the temperature scale, so that the origin will be 0 for both temperature & pressure, but this is obviously not required.
Curves representing the "Fusion curve" (liquid/solid barrier), the "Vaporization curve" (liquid/vapor barrier), 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.
3D Phase DiagramsIn actuality, a phase diagram needs to be three dimensional to be fully complete, because the equations of state for a material are dependent upon temperature, pressure, and also on volume. These three values - temperature, pressure, and volume - are sometimes called the state coordinates of a material.
The standard, 2-dimensional phase diagram assumes that the volume remains relatively constant. You could also assume that, say, temperature remains constant, and would get a very different looking phase diagram.
Obviously, extending the phase diagram to include all three state coordinates can become rather complex and is generally not required for most analyses of state situations, especially in a non-ideal gas.