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Name: Shannon S.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 11/9/2003
Revised 10/24/2004


Question:
In chemistry class we are learning about different states of matter. A few of us asked our teacher what the name is for the physical change from plasma to a solid, but she did not know. What is it?


Replies:
Dear Shannon,

I am not familiar with any experiment which has ever observed a plasma condensing directly into a solid. Consequently, I do not think there is a name for it!

Best, Prof. Topper


Historically, things were simple. Everything was either a solid, a liquid, or a gas. Now however the concept of a "state of matter" needs to be more general. Many, if not most, solids have more than one crystal structure depending upon the temperature (some solids even have multiple melting points). If the pressure is increased most solids have multiple solid phases. Ice for example, has at least 9 and possibly 11 solid phases. A few liquids, liquid helium is the classic case, have more than one liquid phase. In the case of helium the "normal" liquid and the superfluid liquid below a temperature of about 2.2 Kelvins. A plasma is a (usually gas-like phase composed of ions of the material. I don't know of a special name given to the condensation of a plasma into a condensed phase. Certain molecules (called liquid crystals) are ordered in 1 or 2 directions but not the other 2 or 1 respectively, so that makes up another "state of matter" for those substances.

Like many definitions that were "easy" when introduced the definition of a "state of matter" becomes more complicated and less clear cut as knowledge advances.

Vince Calder


At low temperatures, matter exists in the form of atoms. A positive nucleus is surrounded by electrons. Each atom as a whole is neutral.

In some gases and liquids (air, nitrogen, pure water for example) the atoms move around but hang onto their electrons. If a voltage difference is applied, no electric current can flow because there are no free charges to move. The neutral atoms do not move in an electric field. In some solids too, no current can flow because there are also no free charges to move (glass, wood, plastic for example.)

If a gas is heated to a high temperature, some of the electrons come off the atom, resulting in a mixture of free electrons and a positive ions. That is called the process of ionization, and the resulting mixture is called a plasma. ( Going from a plasma to a gas would be called recombination of the electrons and ions to become neutral atoms.) If a voltage difference is applied to the plasma, the electrons can move, and an electric current can flow. If the electric current is very large, it can further heat the gas and ionization becomes self sustaining. That is how a plasma torch works.

Metals consist of positive atoms that are surrounded by a gas of free electrons. Metals conduct electricity because these electrons can move. The positive atoms or ions cannot move. Metals are sort of a solid plasma. The difference between a gas plasma and metal is that even if a metal is cooled to near absolute zero, the electrons stay free, so a metal is not a real plasma.

Making a real solid plasma would be difficult. White dwarf stars contain solid plasmas. The extreme high pressure forces the materials (carbon and oxygen atoms) into a rigid solid. But the pressure and temperature are so high that the atoms remain ionized.

I am not aware of a name for the change from plasma to solid because it is not something that can easily be made to happen.

Bob Erck



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