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192 PHASE EQUILIBRIA

of the gas is possible. We say that there cannot be any CO 2(l) at temperatures above
T (critical) .
Furthermore, supercritical CO 2 does not behave as merely a mixture of liquid
and gaseous CO 2 , but often exhibits an exceptional ability to solvate molecules in
a specific way. The removal of caffeine from coffee relies on the chromatographic
separation of caffeine and the other organic substances in a coffee bean; supercritical
fluid chromatography is a growing and exciting branch of chemistry.

5.3 Quantitative effects of pressure
and temperature change
for a single-component system

Why is ice so slippery?

Effect of p and T on the position of a solid–liquid equilibrium

We say something is ‘as slippery as an ice rink’ if it is has a tiny
The coefficient of fric- coefficient of friction, and we cannot get a grip underfoot. This is
tion µ (also called
‘friction factor’) is the odd because the coefficient of friction µ for ice is quite high – try
quotient of the fric- dragging a fingernail along the surface of some ice fresh from the
tional force and the ice box. It requires quite a lot of effort (and hence work) for a
normal force. In other body to move over the surface of ice.
words, when we apply a At first sight, these facts appear to represent a contradiction in
force, is there a resis- terms. In fact, the reason why it is so easy to slip on ice is that ice
tance to movement usually has a thin layer of liquid water covering its surface: it is
or not? this water–ice combination that is treacherous and slippery.
But why does any water form on the ice if the weather is
sufficiently cold for water to have frozen to form ice? Consider the ice directly
beneath the blade on a skater’s ice-shoe in Figure 5.9: the edge of the blade is
so sharp that an enormous pressure is exerted on the ice, as indicated by the grey
tints.
We now look at the phase diagram for water in Figure 5.10.
The sign of dp/dT for Ice melts at 0 C if the pressure is p O (as represented by T 1 and
◦
the liquid–solid line p 1 respectively on the figure). If the pressure exerted on the ice
on a phase diagram increases to p 2 , then the freezing temperature decreases to T 2 .(The
is almost always posi- freezing temperature decreases in response to the negative slope
tive. Water is the only
of the liquid–solid phase boundary (see the inset to Figure 5.10),
common exception.
which is most unusual; virtually all other substances show a posi-
tive slope of dp/dT .)
If the temperature T 2 is lower than the freezing temperature of the water – and it
usually is – then some of the ice converts to form liquid water; squeezing decreases
the freezing temperature of the water. The water-on-ice beneath the skater’s blade is
slippery enough to allow effortless skating.