and as long as we do not reach one of the bordering lines, we have only one
phase: pure ice.”
Finally, he pointed to the region marked ‘liquid.’
“Here, we can also change both T and P, and we will observe pure liquid
water. Going to the right of this phase map means heating the water; going left
means cooling the water.”
“Let us go back to point A, where we have pure vapor. Suppose we keep
the pressure fixed and change only the temperature. If we go to the right, like
going eastward on a real map, we increase the temperature, which heats up
the steam. In principle we can go to the right indefinitely, and of course at
extremely high temperatures the water molecules can break into oxygen and
hydrogen. But this is of no concern to us; these temperatures are outside the
biologically relevant range.”
“What happens when we cool the vapor? For as long as we do not reach
the line denoted SV — short for solid vapor phases — there is only one vapor
phase. Once we reach point X on the line SV, ice crystals start to appear. If we
try to cool the system further, the temperature does not immediately change.”
“What we shall observe is that more and more vapor will crystallize into
ice, but the temperature will not change. We are locked in the two-phase region
until all the vapor has been transformed into ice. Once there is no longer any
vapor, the temperature will start to decrease, and from there on we move from
point X into the solid phase of pure ice. Here, in contrast to what we have
‘seen’ in the ‘vapor’ region, we see only ice, solid ice.”
“Once we are in the region denoted ‘solid,’ we recover again two degrees
of freedom. Remember, this only means that we can go either left or right
(changing temperature), or up and down (changing pressure), and we will still
be in a one-phase region — here the solid phase of ice.”
“If we continue to cool the solid, the temperature will decrease even more
and we will not observe any changes in the phase of ice. But as we shall
learn later on, some changes at the molecular level do occur as we lower the
temperature. Eventually, we shall be approaching the absolute zero temperature,
beyond which the system cannot be cooled further.”
Professor Holmes noticed the students’ uneasiness.