I have not done one of these installments for a while, and the Muse has found me. I hope that readers find it to be interesting.
Everyone is familiar with the three phases (sometimes called states) of matter: solid, liquid, and gas. As Mr. Townshend says on The Punk Meets the Godfather, "...but things ain't quite that simple."
A solid is one of the so-called "condensed" phases of matter, meaning that the atoms or molecules thereof are relatively close together. Solids come is a couple of forms, broadly broken down into crystalline and amorphous.
In crystalline solids, the molecules are arranged in more or less a repeating array with a definite order. Take for example common salt, composed of equal numbers of sodium ions and chloride ions. Due to the sizes of the different ions and their electric charges (sodium is plus 1, chloride minus 1), the ions tend to arrange themselves into an array where each sodium ion is surrounded by six chloride ions, and vice versa.
An extreme example of crystalline solids is diamond, where each carbon molecule is attached to four others in a tetrahedral (109.5 degrees) arrangement. Not only is a diamond a crystal, if it be flawless the entire diamond is ONE huge molecule. Who says that molecules are too small to see?
The other classification for solids is "amorphous", meaning "without form", literally. These materials, while solid, not not have a repeating pattern and so are disordered. A common example is glass, which is essentially a random throwtogether of complex metal silicates. They are sometimes referred to as "supercooled liquids", which, although apt, is misleading. Some are led to believe that glass, being a supercooled liquid, actually flows with time and gravity. This is not correct. Glass is very solid, and does not flow.
This misconception may be in part due to the appearance of glass in old buildings, but that is an artifact of the manufacturing process. In the old process for window glass, large glass globes were blown, then placed on a hot surface and cut open, thus collapsing into a sheet. Because of imperfections in the globes, the glass is wavy and appears to have undergone flow.
Liquids are another condensed phase of matter, the molecules of which are close together, but not as strongly attracted to each other as in a solid. Therefore, liquids are fluids, in that they flow readily and take the shape of whatever container confines them. If in no container, such as rain drops, they tend to assume a spherical shape because that is the lowest energy state. Actual raindrops are tear shaped, but that is because of air drag as they fall. If you watch video from the space shuttle, you can see that water is essentially spherical.
Liquids have more structure than most realize, though, to varying degrees. Water, for example, is a highly structured liquid because of geometry and electrical charge density. As the temperature increases, the organization decreases because of the energy added to the system. Like solids, liquids are "incompressible", meaning that pressure increases do not affect the volume on a macroscopic level. In reality, both solids and liquids are reduced in volume with increasing pressure, but to only a tiny degree.
One interesting class of liquids is the so called liquid crystals, which are extremely organized liquids that, due to their geometry and electrical charge distribution, are affected by external electric fields. Anyone with an LCD display watch, calculator, computer monitor, or TV has seen them.
Gases are, like liquids, fluids in that they flow readily and take the shape of their container. Unlike condensed phases, gases can be compressed or rarefied, depending on external pressure. No one who is living is unfamiliar with gases, since the very air itself is a gas, a mixture of mostly nitrogen and oxygen.
OK, most of you already knew most of this except for some of the little nuggets that I added. Now things get a little strange.
Plasma, the so called "forth state" of matter, is a compressible fluid. Unlike a normal gas, a plasma has a very high energy level, stripping one or more electrons from the nucleus of the atoms. The surface of the sun is a plasma, and many of you have plasma display televisions, which use energy to strip electrons from the phosphors. Plasmas have some interesting properties, because, being electrically charged, can be affected by magnetic and electric fields, and so can be moved about in ways not possible with normal gases. One possibility for power from fusion depends on containing a plasma of hydrogen isotopes in a magnetic container.
OK, still with me I see. Now it really gets weird. There is a phase of matter between liquid and gas. To understand it, first just a little exposure to the models of matter is required. Please bear with me, I will make is fast.
There exists what is called a phase diagram for matter, and those show the relation between temperature, pressure, and the phase present. There is a fairly good phase diagram for water in Wikipedia under "Phase Diagram" but to save my life I have not been able to link it properly.
Anyway, this shows that below certain temperatures, solid and vapor are in equilibrium (this is why freeze drying works), and above a certain temperature, solid and liquid are in equilibrium (why you can have ice in your drink). One point is called the "triple point" and defines the temperature and pressure where ice, water, and water vapor exist at the same time.
The most interesting part is to the extreme right. At the lower right is superheated steam, which is just very hot water vapor under considerable, but not extremely high, pressure. Superheated vapors behave for the most part like normal gases, but because of their energy content deviate a little.
The most interesting is the upper right. See the point labeled "Critical point"? That is the temperature and pressure at or above which a liquid can not be evaporated to a true gas. At the critical point, liquids assume some properties of gases, but not all.
For example, a supercritical fluid has the viscosity (low) of a gas, but the density (high) of a liquid. They often have other bizarre properties as well. For example, supercritical water is an excellent solvent for organic materials, but not salts, while normal water is the opposite. Supercritical water reactors are becoming more common in disposal of toxic organics because one can make, for example, Diesel fuel dissolve in the supercritical water, add the toxics, then blow heated air or oxygen through it, and end up with nontoxic materials.
Well, I hope that I have challenged your thinking about the "three" phases of matter. If I did, then I have done my job this evening. Comments, critisms, and questions are always welcome. And if anyone can get that phase diagram to link, I will update this entry with it and acknowledge you. Warmest regards, Doc.
UPDATE: Please see the comment from beabea with the phase diagram and my annotation to that comment. It makes it much easier to follow. Everyone should tip beabea for the chart, and Ian S for the accurate id of the Mandrake quote. Warmest regards, Doc.
Second UPDATE: Its any one Guess linked to the Wikipedia phase diagram that I referenced. Please tip this contributor well. Wamest regards, Doc.