[johnson039 (OP)]

why plasma is called as the forth state of material?

(Title modified into the form of a question. Mod)

[lyner]

1. We all love to categorise things.
2. There are sufficient differences in the properties of a plasma and the general properties of the other three states to warrant giving it a separate classification.

BUT any taxonomy can lead you into trouble because of borderline cases.

[Soul Surfer]

It is an obvious and logical process.  look at the properties of each state.

A solid is a material where the energies of the molecules are low enough to allow the intermolecular forces to hold the atoms rigidly in place and cause it to be rigid.


A liquid is the case where the molecules have enough energy for the molecules to slide by each other and not have a rigid form but the energy is not enough for the molecules to be entirely free.

A gas is where the molecules have a high enough energy to overcome intermolecular forces and all the molecules are free and individual but there is not enough energy to ionise any of the molecules.  Gases in general do not conduct electricity.  (solids and liquids can conduct electricity if the action of the intermolecular bonds can liberate electrons to flow through the material this is in general true of metals.)

When the molecular energy levels (temperature) are high enough for electrons to be liberated from the molecules the gas becomes a plasma and has fee charges in it and therefore conducts electricity.

Because of the high temperatures (thousands of degrees K ) needed to create and maintain plasmas these are not encountered normally on the earth's surface except briefly during electrical discharges like lightning and were not included when people originally coined the term the three states of matter.

There are further higher energy states. 

neutron star material occurs at temperatures and pressures where individual nuclei are broken down

The quark gluon plasma occurs when particle energies are so high that individual nucleons are broken down into their component quarks.

[lyner]

plasmas these are not encountered normally on the earth's surface except briefly during electrical discharges like lightning

Here is an example of taxonomy problems. The Ionosphere is a region which is ionised for long periods of time. Although its temperature is not high (it would feel cold up there) the presence of cosmic rays gives some of its particles with high KE. It behaves like / is a plasma and reflects radiowaves.

[DoctorBeaver]

What about Fermionic Condensates, Bose-Einstein Condensates and Filaments?  []