|MadSci Network: Chemistry|
What is the phase diagram?
A single substance phase diagram shows the relationship between the various forms of the substance -- solid, liquid, and gas -- for different conditions of temperature and pressure. The diagram shows the typical features of a phase diagram.
The curve between the red and green areas represents the boundary between
solid and gas, and is associated with the process of sublimation. The curve
between red and blue is the boundary between solid and liquid, and is
associated with melting and freezing. The curve between blue and green is the
boundary between liquid and gas, associated with evaporation and condensation.
There are two special points on the curve:
(1) the "triple point" -- the single set of conditions under which solid, liquid, and gas can co-exist at equilibrium.
(2) the "critical point". At very high temperature and pressure, there is no distinction between a liquid and a gas. The critical point is the particular temperature and pressure above which liquid and gas are no longer separate phases.
What are the major differences between the phase diagrams for water and carbon dioxide?
(1) For water, the curve between solid and liquid states leans to the left.
For carbon dioxide, and most other substances, it is to the right. Water,
unusually, melts at lower temperatures at higher pressures. This is connected
with the fact that ice is less dense than water, whereas most solids are more
dense than their melts. Strictly, this observation only applies to reasonably
low pressures, up to 1000 atmospheres or so.
(2) For water, the triple point occurs at a pressure of 4.5 torr, or about 0.06 atmosphere and 0 deg C. Carbon dioxide has a particularly high pressure associated with its triple point -- about 5.1 atmosphere at -56 deg C. Liquid carbon dioxide cannot occur at ordinary pressures of 1 atm. Carbon dioxide sublimes directly.
(3) Although the triple point of carbon dioxide occurs at such a high pressure, its critical pressure is much lower than that of water: 73 atmosphere, compared with 218 atmosphere.
(4) At various very high pressures (above about 1000 atm), several different phases of solid ice can form, with quite different structures. A much more complicated phase diagram is needed to accommodate this. Carbon dioxide, on the other hand, only makes a single solid form. Only at extreme pressures above 10000 atm do other solid phases of carbon dioxide appear.
Why are there these differences?
There are two main factors that chemists would use to account for these differences. One is the shape of the molecules. The other is the nature of the forces of attraction between the molecules.
Whether a substance is in a condensed phase -- solid or liquid -- or a gas depends on the strength of the forces between the different molecules. Molecules with stronger forces of attraction between them belong to substances with higher boiling or sublimation points. Molecules of water have quite strong forces of attraction (known as hydrogen bonds) between the hydrogen atom of one molecule and the oxygen atom of another. Molecules of carbon dioxide do not have hydrogen bonds. The only force of attraction between them is a rather weak force known as a dispersion force. Water therefore stays in a condensed phase to much higher temperatures, even though it has a lighter and faster-moving molecule.
Whether a substance prefers to be in a solid or liquid form depends on how efficiently its molecules can pack together in a regular array. That is down to both shape and the nature of the forces.
A carbon dioxide molecule has the shape of a bulging cylinder, like a very short cigar, or a rather elongated beer barrel. The molecules can pack together very efficiently. The carbon atom of one molecule prefers to be adjacent to an oxygen atom from another. The beer-barrel shape is a good one for molecules to pack efficiently together.
A water molecule, on the other hand has a shape which has been described as a boomerang (but that is too flat -- it is more like a boomerang shaped balloon that has been blown up). A cashew nut is roughly the right shape. To form the hydrogen bonds, the ends of two molecules (exactly-- no more and no fewer) need to get close to the middle of another. There is no way that cashew nuts can be packed efficiently in this way. The structure of ordinary ice is a very expanded hexagonal cage structure, with lots of empty space in it. That is why ice is less dense than water. It is also why ice does not form till quite a low temperature, even though liquid water forms at a high temperature.
Try the links in the MadSci Library for more information on Chemistry.