posted on Nov, 20 2006 @ 05:41 AM
It's been fairly well summed up, but seeing as I actually know a bit about plasma, I'll throw my two cents in anyway. You are correct that a plasma
is much much hotter than a gas, but that is not the only difference.
A gas is composed primarily of atoms. When enough energy (heat) is applied to a gas, the electrons separate from the nuclei and you then have ions
and electrons as the primary entities in the plasma.
Byrd is also correct that plasmas conduct electricity (actually quite well). Gases are typically insulators (I can't think of any conductive gases
offhand) although given enough power, even a gas can conduct. (example: lightning)
Plasmas also give off photons (light). I know this because I was lucky enough to work with plasma in a lab in university. The particular mixture we
had was glowing a brilliant bluish white that hurt the eyes to look at. (that plasma also wrecked two weeks of work, as it ruined my silicon
Plasmas also behave MUCH differently in the presence of magnetic and electric fields than gases do. Typically those fields will do nothing to a gas,
but they will strongly affect the behaviour of a plasma, since the fields will exert forces on the ions and electrons. (the fields don't really do
anything to neutral atoms like those in gases) Since plasma is ionized, the ions will move in the presence of these fields. In fact, putting a
voltage across a plasma actually results in some rather strange behaviour. (look up 'faraday dark space, crooke's dark space, and you will see what
I think the confusion arises from the fact that if you look at a gas or a plasma, it is difficult to tell the difference on a macroscopic scale, at
least by looking at it. Contrast this with the completely obvious differences between solids and liquids and those with respect to gas and plasma.
Also, due to the temperatures required, plasma is not a part of the average persons life, whereas the other three states of matter are everywhere on
earth. (yes, the solar system is mostly plasma due to the sun, but on earth, the opposite is true) Also, at least in my experience, I don't think I
had even heard of plasma as a state of matter until university; certainly, it does not receive the detail of the other three states.