I wanted to learn more about "weird" states of matter the other day, so I decided to start by researching plasma. I was shocked when I found out that plasma is THE most common naturally occurring state of matter in the universe. It is estimated to compose 99% of visible matter. And yet, in school I have learned virtually nothing about it. I heard about it only from when a teacher would ask "What are the 3 states of matter?" and some kid would answer plasma. The teacher would say "Well technically yes, but we're not going to talk about it now." And this has been happening for years.So here is an overview that hopefully helps your understanding.
Plasma is an ionized gas. A gas is a collection of atoms and/or molecules floating around freely. It is a fluid, meaning it flows like a liquid. It is also very compressible. A gas becomes a plasma after being subjected to either high temperatures or other energy. This causes the molecules (or atoms, but for the rest of this post I'm just going to say molecules and just assume I mean whatever type of particle the gas is composed of) to ionize.
Ionizing a molecule means either adding electrons or ripping them off. In this case, it usually means ripping them off. So the plasma is composed of the ions and the electrons that have been removed. Not all the molecules have to be ionized though. There can be varying ratios of ions to molecules. This ratio is called the degree of ionization. Plasmas are (usually) quasi-neutral, meaning they have approximately equal amounts of positive and negative charges.
Plasmas can be thermal or non-thermal. If it is thermal, that means the ions, neutral particles, and electrons are in thermal equilibrium. In other words, all the particles are roughly the same temperature. In a non-thermal plasma, ions and neutral particles will be the same temperature (normally close to that of the surroundings) while the electrons will be MUCH hotter.
Examples of plasma in everyday life include lightning, neon lights, and the sun. Although flames are present on this chart, there is some controversy over whether they are actually a plasma. The majority says no, and even if fire is plasma, it is a very weakly ionized one that doesn't show all of the properties. There has also been some work done in cooling non-neutral plasmas (plasmas composed entirely of one type of charged particle) done to temperatures within a few milli-Kelvins of absolute zero. In these instances the plasma forms a crystal lattice structure.
This is only a brief overview. It was very difficult to find information that was both trustworthy and at a level I could understand, so if there are any errors please notify me immediately (with sources).
To learn more, check out these links:
http://science.energy.gov/~/media/fes/pdf/about/Low_temp_plasma_report_march_2008.pdf
http://sdphca.ucsd.edu/index.html
http://www.plasmacoalition.org/edu.htm (this one is actually a page full of more resources)
