“The absolute zero temperature, written 0 K — read: zero Kelvin — is
defined as −273.15◦C (degree celsius). All you need to know is that there
exists a lowest temperature beyond which no further cooling is possible.”
“Let us venture into another direction on the phase map. Suppose we
reached point B in the solid region. We hold the temperature fixed but we
increase the pressure on the system, say by adding weights on a piston. This
means that we shall move upwards on the ‘map.’ For a while we will not observe
any change; the phase will remain as solid ice. Note that when we moved from
B to Y, we changed only pressure P, while keeping the temperature fixed.”
“Once we reach point Y in the diagram, we shall observe changes. That is,
if we try to increase the pressure further, the solid ice will start melting and
we shall observe two co-existent phases, solid ice with liquid water. We say
that these phases are at equilibrium, and that the point Y is on the solid-liquid
equilibrium line, denoted SL. As we try to increase the pressure we shall see
more and more of the ice melting and converting into liquid water. At some
point, when all the ice has melted, we shall enter into the new region denoted
‘liquid’ in the phase diagram. This liquid is the most fascinating, as well as the
most vital liquid to the whole phenomenon we call life.”
A student raised his hand and asked: “How come only water sustains life?
What about ice or vapor? Why can’t life be sustained in these phases?”
“That is a very good question,” replied the professor. “To answer this
question we shall need to learn many facts about the role of water in biological
systems. Also, we shall have to learn a lot about the outstanding properties of
this liquid, but for now we are only interested in the region where liquid water
exists.”
“From the macroscopic point of view, that is what we actually see in daily
life. We observe liquid water that has a light bluish color. It is so common
a liquid that it is hard to believe that it has so many outstanding properties.
Many scientists often refer to it as an anomalous liquid!”
“But let’s not get carried away by the properties of water at the moment.
We shall have plenty of time to be deliriously excited with this in our indepth
explorations of water. For now we shall continue to explore the different
terrains of the phase diagram.