Phase Diagrams For Water And CO2

Phase Diagram of Water









Matter comes in different forms, we learn at school: solid,
liquid, and as gas. December days in Canada give us plenty of occasions
to experience these different forms of matter - phases, as they are
called in physics and chemistry - in the case of water: ice and snow,
the both annoying and beautiful appearances of solid water, the liquid
form in rain and fog, and if the Sun succeeds to disperse the fog, tiny
water droplets have evaporated, and the water has been transformed into
invisible gas.

Ice melts at a temperature of 0°C (or 273.15
Kelvin), and water boils at 100°C (or 373.15 Kelvin). However, to be
precise, these melting and boiling temperatures are not fixed - they
depend on the ambient pressure. On top of a mountain, say the Puy
de Dôme, air pressure is lower than in the lowlands, and as
consequence, water boils at temperatures below 100°C.

To get a
better overview how the occurrence of the different phases of water -
solid ice, liquid water, gaseous vapour - depends on temperature and
pressure, it's a good idea to plot in a diagram the transition lines
between the different phases as a function of these two parameters. Such
a diagram is called a phase diagram. And a simplified version of
the phase diagram of water looks like this:

The x-axis of the diagram shows the temperature T in units
of Kelvin (K). Keep in mind that 0°C = 273.15 K
and 100°C = 373.15 K - both temperatures are marked by the grey
vertical lines. The y-axis shows the pressure p in units
of Megapascal (MPa), where 0.1 MPa = 1000 hPa
= 1000 mbar and the standard atmospheric pressure is 1013 mbar.
Since pressure covers a huge range of values from the very small to the
very large, a convenient way to represent this is the usage of a
logarithmic scale. Thus, the phase diagram manages to represent pressure
from 1/100.000 of ambient pressure to 1 million times ambient pressure.
Ambient pressure is marked by the horizontal grey line.

The blue
line in the diagram is the melting line - it separates ice from liquid
water - and the light-blue line the boiling line, which divides liquid
and gaseous water. The green line is the so-called sublimation line,
across which ice transforms directly to the gaseous states, without the
intermediate step of liquid water. All three lines meet at one point
(marked by the black dot) which is called the triple point - at
this value of temperature and pressure, all three forms of water can
coexist. At sufficiently high pressure, water solidifies even at
temperatures well above room temperature: these transitions to different
sorts of ice (distinct by the respective crystal structures) are shown
as the red and orange line. Trying to understand these different phases
of ice is a topic still under investigation, both by experiment and by
theory.

One feature of the diagram might seem strange at first
sight: The boiling line separating liquid and gaseous water ends at one
point. This is a very generic feature of all liquid matter: At high
enough pressure, the distinction between liquid and gas gets lost -
essentially, the difference in density between gas and liquid becomes
zero, and the latent heat of condensation/evaporation vanishes. The end
point of the boiling line, marked by the grey dot, is called the critical
point. If temperature and pressure can be chosen such that the
fluid is very close to the critical point, it will develop bubbles of
gas containing small droplets of liquid containing small bubbles of
gas... and as a result of bubbles and droplets of many different sizes,
covering the range of wavelengths of visible light, the system becomes
opaque. This quite spectacular effect is called critical
opalescence.

But of course, we can also recover our mundane
everyday experience with water in the diagram: If we increase
temperature at constant ambient pressure, following the horizontal grey
line, we cross the blue melting line at 0°C, and the light-blue boiling
line at 100°C - that's the melting of ice and the boiling of water as we
know it. And we see that if ambient pressure is reduced, for example
during stormy weather or on top of a mountain, the crossing of the
horizontal line and the boiling line shifts to lower temperature: Water
will boil at temperatures below 100°C. At a height of 2000 m above sea
level, for example, water boils at about 94°C - things to keep in mind
if boiling
an egg on a mountain.

If you look closely, you can note that
the blue melting line is slightly inclined, meaning that with increasing
pressure, the melting temperature drops slightly. This effect is often
invoked as an explanation for the low friction of skates on ice: The
pressure applied by the weight of the skater reduces the melting
temperature of ice, causing a thin film of liquid water, on which the
blade of the skate glides nearly without friction, or so goes the story.
This, however, is not the whole truth: the small, pressure-induced
reduction of the melting temperature is not sufficient to produce this
effect. While it's correct that the reduction of friction is caused by a
slippery film of water on the su***ce of the ice, this film is created
by complicated mechanisms whose details are still under
debate.

So, an elementary plot such as the phase diagram of
water can still hide some surprises
and riddles for us.

Phase Diagram For H2O and CO2+ Some Questions On This Topic:

http://science.uwaterloo.ca/~cchieh/cact/c123/phasesdgm.html