Earth, Air, Fire and Water
The four elements of antiquity -- cold dry earth, cold wet water, hot wet air, and hot dry fire -- dominated natural philosophy for almost two thousand years. The idea of everything being formed of these four elements was first developed by the Greek philosopher Empedocles of Sicily, and continued to be believed until the rise of modern scientific ideas. Even today, earth, water, air, and fire are not bad symbols for the four states of matter -- solid, liquid, gas, and plasma.
An ideal solid maintains both its shape and its volume, regardless of external forces. Real solids actually have some springiness, and may even flow (like glacier ice) if they are under enough pressure, but the flow is generally slow compared with that of a liquid. Most solids are crystalline, that is, they contain at least regions in which their constituent atoms are arranged in a regular pattern in space. Common salt, for instance, is made up of atoms of chlorine and sodium in regular alternation in a pattern like stacked cubes. A few solids, such as glass, are amorphous (or noncrystalline), and such solids are technically liquids.
An ideal liquid changes its shape freely, but not its volume. Its constituent atoms are bound together into molecules, which slide freely past each other. In real liquids, sliding is not completely free, and the resulting resistance of the liquid to changing its shape is called viscosity. The dividing line between very viscous liquids and solids is not obvious, as they behave similarly.
The major difference shows when a solid is heated. A true, crystalline solid, like ice, absorbs heat without any change in temperature while it is melting into the liquid phase, and there is a clear division between the liquid and solid phases of the ice/water mixture during the process of melting. A viscous liquid, such as glass, simply becomes less viscous as it is heated, until at a high enough temperature it flows freely.
While the molecules of a liquid slide freely past each other but remain in contact, gas molecules have lost all continuous contact with each other. A gas not only changes its shape easily, it expands to fill whatever volume is available to it.
Physicists recognize a fourth state of matter, called plasma. Up until now, we've been talking of atoms and to some extent molecules as if they were permanent entities. We now have to enter the structure of the atom. An atom is composed of a nucleus, or core, made up of massive positive and neutrally charged particles, with a cloud of electrons, relatively lightweight but with negative charges just as strong as the positive ones in the nucleus, orbiting around it. The difference between a plasma and a gas is that in a plasma, so much energy has been pumped into the substance that molecules have been knocked apart and electrons have been separated from their parent atoms. A water molecule in a gas, for instance, is made up of two hydrogen atoms and an oxygen atom, all bound together and sharing their electrons. In a plasma, this group would be knocked apart, leaving an oxygen and a hydrogen atom bound together with one more electron than they should have, and a hydrogen with no electron at all. As the energy continued to increase, the remaining oxygen and hydrogen would be separated. A true plasma, such as the sun's atmosphere, is made up entirely of charged particles.
Low-pressure plasmas, mixed with gasses, are formed whenever you turn on a fluorescent light. The surge of energy required to start a neon tube or fluorescent light is the energy needed to split some of the electrons away from the gas that fills the tube.
Solid -- the ground to walk on, a solid roof over our heads. Liquid -- rain to grow crops, the solvents and lubricants that keep our civilization moving. Gas -- oxygen for our lungs, winds to keep us from suffocating in our own wastes. Plasma -- the fiery sun, ultimate source of energy for all mankind. Perhaps we're not so far from Empedocles after all.
