The phase boundary for gas-liquid does not continue to the right
for higher and higher temperatures.
Above a certain temperature,
the critical temperature,
a gas cannot be liquefied, no matter how much pressure is applied.
Critical temperature Tc and pressure pc.
Demonstrations of this phenomenon are usually done by sealing a glas tube
which, at room temperature, contains some substance
in the liquid phase filling about one half of the glas tube.
The other half of the glas tube
is filled with the vapor (gas) of the same substance.
One observes easily the surface of the liquid which separates
liquid from gas.
By heating the tube to a temperature above the critical temperature
suddenly the surface of the liquid disappears.
Consequently, only the gas phase exists.
