Are we just going through a phase?
by Judah Schwartz
States of matter are sometimes confused with phases. This is likely due to the fact that in many example systems, the familiar phase transitions are also transformations of the state of matter. In the example of water, the phases of ice, liquid water, and water vapor are commonly recognized. The common phase transitions observed in a one component system containing only water are melting/solidification (liquid/solid), vaporisation/condensation (gas/liquid) and sublimation/deposition (gas/solid).
Transitions between different states of matter of the same chemical component are necessarily a phase transformation, but not all phase transformations involve a change in the state of matter. For example, there are 14 different forms of ice, all of which are the solid state of matter. When one form of ice transforms into another, the crystal structure, density, and a number of physical properties change, but it remains a solid.
Similarly, methanol and hexane are completely miscible liquids above approximately 42°C, but when a solution of the two is cooled below this temperature, the mixture separates into two phases, one rich in methanol, the other in hexane, although both resulting phases are the same state of matter: liquid. (see section "The difference between phases and states of matter")
So if phases are not quite the same as states of matter what do we mean by phases? Here is an excerpt from the Wikipedia article on phases (http://en.wikipedia.org/wiki/Phase_%28matter%29):
In general, two different states of a system are in different phases if there is an abrupt change in their physical properties while transforming from one state to the other. Conversely, two states are in the same phase if they can be transformed into one another without any abrupt changes. There are, however, exceptions to this statement…
An important point is that different types of phases are associated with different physical qualities. When discussing the solid, liquid, and gaseous phases, we talked about rigidity and compressibility, and the effects of varying the pressure and volume, because those are the relevant properties that distinguish a solid, a liquid, and a gas… (see section "General definition of phases")
There are, however, situations in which a solid material can exist is several different forms, or a magnetic material that can have different magnetic properties. When there is an “abrupt” change in the properties of a material – there is a quite technical mathematical definition of what is meant by this – we say that there is a change of phase.
As a general rule it is easier to build models of phenomena that change slowly and smoothly. Phase changes, by virtue of their “abruptness”, are quite different sorts of changes. The study of phase changes of all sorts, both experimentally and theoretically, is an active area of condensed matter physics.