We learn about solids, liquids, and gases at an early age, both from life
experience and from schoolwork.  It turns out that the phase a substance
is in at any given time depends on the pressure and temperature of the
surrounding environment.  That dependence is different for each substance.
If we make a graph of temperature and pressure, and fill in where on the
graph a substance is solid, liquid, gas, or any other phase, we can learn
a great deal about that substance.  Such a graph is called a phase diagram.


[more text TK. - triple point, critical point, etc.]

A phase diagram for water is at http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/examples/triplpt.html

A phase diagram for carbon dioxide is at http://scifun.chem.wisc.edu/chemweek/CO2/CO2.html

Simpler, side-by-side phase diagrams for both at http://www.chem.neu.edu/Courses/1131Tom/Lecture25/sld007.htm