heating the water causes an increase in the temperature. For a while, nothing
of significance is observed; we shall be moving horizontally along the line CZ
(Fig. 2.1). Once we reach point Z on the line marked LV, the liquid-vapor
equilibrium line, something exciting takes place: the water starts to boil. The
commonly known boiling temperature of 100◦C is defined as the temperature
at which the vapor-pressure of water is one atmosphere.
“But we can boil the water at a pressure other than 1 atm. If the pressure
is higher than 1 atm, the water will boil at a higher temperature; if the
pressure is lower than 1 atm, the water will boil at the lower temperature.
Once we reach point Z, the temperature suddenly stops changing. We continue
to supply heat to the system, but the temperature stubbornly refuses to
change. At home, when water boils in the pot, you will notice that even when
you turn the knob of the stove to the maximum setting, the water’s temperature
will stay constant. As we continue to supply more and more heat, water
boils and transforms into the vapor phase, but the temperature stays constant.
At this point, we are observing two co-existent phases — liquid and vapor.
We say that the two phases co-exist at point Z on the equilibrium line, LV.
Along the LV line we have only one degree of freedom. We cannot change
both the pressure and the temperature and still maintain the two phases at
equilibrium.”
“What exactly does he mean by that?” Alice mused. “I wonder how the
water molecules feel when they have one or two degrees of freedom?”
Professor Holmes continued.
“Further supply of heat to the system at the point Z will cause more and
more liquid to evaporate. Until we reach the point when all the water has
evaporated and there’s nothing left of the liquid, at this point we have a pure
vapor phase. Once we reach that point, further heating will no longer “boil”
the liquid, simply because there is no more liquid left. Instead, the temperature
starts to rise. Once again we shall be moving into the region of two degrees of
freedom, and this is the region of the vapor phase denoted ‘vapor’ in the phase
diagram. We shall be moving along the line ZD in the diagram.”
At that point, Alice interrupted Professor Holmes with a question.
Alice's