|The solid, liquid, and gaseous forms of a particular
substance (in the small inset diagram, carbon dioxide is shown) can
all exist in mutual contact as a particular temperature and pressure,
called the triple point. The three lines show how the boiling, melting,
and sublimation change with pressure.
Above the critical point the liquid cannot exist at all; only up to
that temperature can the gas be liquefied by simply compressing it.
In the large diagram, the changes of volume are included: here the
solid, liquid, and gas states are symbolized as surfaces, as are the
conditions for melting (1), boiling (3), and sublimation (6). The
pressure/volume behavior (4) of a gas is simple at high temperatures
and more complex near the critical point (2). The triple point is
shown as a line (5).