Crossposted at http://Politicook.net
This diary introduces a new series to follow up the one on the electromagnetic spectrum. Compared to light, phases of matter are extremely complex, so we will break the subjects into small chunks for easier assimilation.
Matter is generally classified as solid, liquid, and gaseous. In addition, a "forth" phase, plasma is generally recognized now. But there are many nuances, like supercritical fluids, that do not fit any of those categories. We start with solids, since, in general, they are the lowest in energy, just as we chose to start with the very long wavelength end of the electromagnetic spectrum.
"Matter" is usually defined as a material that has mass and occupies space. Thus, it is possible to measure mass by one or more methods, pretty much relying on the property of mass referred to as "inertia", which is simply the term that we use to describe the property of matter that causes it to, if unperturbed by an outside force either remains at rest, or, if already in motion, to continue that motion in a straight line forever. We usually use scales or a balance to determine mass due to the force of gravity interacting with the mass, accelerating it. There are other ways though, and scales do not work very well in space. If one knows the charge on a mass, the acceleration of attraction or repulsion from another, known force can be used. There is noting unique about gravity for determining mass, just that it is convenient.
All matter (the so called "dark matter" is not included in this discussion because even the quantum mechanics and astrophysicists have not come to agreement on its nature, or even if it exists) is composed of one or more massive particles, as a first approximation protons, neutrons, and electrons. Protons and neutrons are in turn composed of quarks in the Standard Model, while electrons are elementary. The point is that all matter has mass AND occupies space.
The energetics of a system determine the state of matter. Specifically, the "Gibbs Free Energy" is important, because it includes both an enthalpic (heat) term and an entropic (arrangement, or, and is commonly misrepresented "disorder) one. The two most important components are temperature and pressure. In general, the lower the temperature and/or higher the pressure, the solid state is favored. As temperature increases and/or pressure decreases, the liquid phase is favored, then the gaseous one, and finally, at high energies, plasmas. This is oversimplified but is a good general trending rule for all but a few exceptional materials. A Phase Diagram is useful for describing this behavior for different substances. Here is a generalized one for a well-behaved pure substance that I sketched. Some can become very complex, depending on the substance.
The "fork" is called the "triple point" of a substance, and that is the unique temperature UPDATE: I just reread and saw this defect. It is the temperature AND pressure that solid, liquid, and gas exist in equilibrium. A glass of ice water is not at equilibrium, because even though all three phases are present, the ice is melting at room temperature or refreezing below 0 C.
As I said, this is the simplest of phase diagrams and assumes that there is only one solid phase (water has several), one liquid phase (water has several), and one gaseous phase (water, as you probably have guessed, is more complex). But is does serve to illustrate the general tendencies of matter. This idealized diagram also assumes that the solid phase is of greater density than the liquid (water is anomalous). Thus, there are many layers of detail. I will devote an entire entry in this series to water since it is such a strange substance. But first we will just cover the basics to gain a foundation before we continue.
To recap, phases of matter are determined by energetics as determined by the Gibbs Free Energy equation: Delta E = Delta H - T Delta S, where Delta is "change in", E is free energy, H is enthalpy, T is the absolute temperature, and S is entropy. This may sound a bit intimidating right now, but as we see some examples in real life systems it will make a lot of sense. A phase diagram is a graphical representation of the results of this equation in a real system, but generated by empirical data. While it is possible to calculate phase diagrams, except for all but the simplest substances the results are not very good, and even to do such calculations requires quite a lot of empirical data anyway. But we are getting better.
Next time we will discuss the simplest solids, crystalline ones. And even they are not as simple as one would think at first blush. Any comments, flames, and questions are, as always, welcome. Warmest regards, Doc.