Can fire be considered the fourth state of matter?

[Topher925]

While having a discussion with a friend the idea of fire being the fourth state of matter came up, in other words, a hot enough fire or "flame" can be considered plasma being the fourth state of matter. My response to this was, "No, fire or a "flame" is always in the gas phase, fire is the rapid oxidization of gases with oxygen which emits light and heat while plasma is a superheated gas which has it's conduction band electrons removed do to high temperatures. Plasma itself isn't exothermic like a flame and will not oxidize with oxygen."

Then he showed me this link: http://chemistry.about.com/mbiopage.htm
Written by: Anne Helmenstine, Ph.D.

If the flame is hot enough, the gases are ionized and become yet another state of matter: plasma.

This seems very wrong to me. If by some chance a flame is hot enough that its outer electrons are removed, then how can it react with oxygen and therefore produce more heat? Don't most reactions have a limiting maximum temperature at which they can occur? For a simple example, burning H2 in an O2 atmosphere.

2H2 + O2 -> 2H2O @ ~560C

but...

2H2O -> 2H2 + O2 @ +2000 °C

What gives?

 

[Andy Resnick]

'plasma' is sometimes called a fourth state of matter. Plasma, in this sense, is a stable collection of charged particles and can be either hot or cold, dense or sparse. "cold dusty plasmas" are very interesting model systems.

A fire is a collection of chemical reactions with its own rich set of phenomena (smoldering, soot production, etc) but is distinct from a plasma.

 

[Topher925]

So you also would disagree with the description from the link? How exactly does one generate a "cold" plasma?

 

[Andy Resnick]

John Goree (U Iowa) has done a lot of experimental work on cold dusty plasmas (among others), which can be created many different ways. The one I am most familiar with is microsphere suspensions with excess surface charge on each microsphere.

http://www.spaceref.com/news/viewpr.html?pid=8088

I couldn't figure out where on Helmenstine's website you got your blurbs: the posted address comes up with a very generic "about chemistry" page.

 

[mgb_phys]

I never thought that plasma as a state of matter was very convincing.
Solid/liquid/gas is a useful distinction for material properties and thermodynamics.
But if you count plasma then why not a supercritical fluid or a liquid crystal.
Then add in exotic states like a Bose-einstein condensate or a degnerate neutron star.

 

[Topher925]

I couldn't figure out where on Helmenstine's website you got your blurbs: the posted address comes up with a very generic "about chemistry" page.

Sorry, wrong link: http://chemistry.about.com/od/chemistryfaqs/f/firechemistry.htm

Good point mgb_phys, glass comes to mind too.

 

[alxm]

I'll bite..

I never thought that plasma as a state of matter was very convincing.
Solid/liquid/gas is a useful distinction for material properties and thermodynamics.

'Plasma' is also a useful distinction. Plasmas have markedly different properties than the other states.
Besides, most of the stuff in the universe (that we know about) is in plasma form.

But if you count plasma then why not a supercritical fluid or a liquid crystal. Then add in exotic states like a Bose-einstein condensate or a neutron star.

Supercritical fluids wouldn't be a bad idea either. But liquid crystals - obviously not, since relatively few materials have a liquid crystal state. And the exotic stuff is just exotic. I've got a plasma in my kitchen (fluorescent lights). I don't think my cruddy freezer could maintain a BEC though.

 

[alxm]

This seems very wrong to me. If by some chance a flame is hot enough that its outer electrons are removed, then how can it react with oxygen and therefore produce more heat?

By breaking chemical bonds. In a fairly violent fashion. Ions and radicals in the gas phase aren't stable at all, and will bust up almost any molecule the come into contact with.

Don't most reactions have a limiting maximum temperature at which they can occur?

Only if \Delta S < 0 for the reaction. I don't know where you got your numbers from; A hydrogen gas flame in air burns at over 2000 degrees C, in pure oxygen at almost 3000 C.

 

[mgb_phys]

Besides, most of the stuff in the universe (that we know about) is in plasma form.

Most of the stuff in the universe is hydrogen but it's worth studying a couple of the other elements.

Supercritical fluids wouldn't be a bad idea either. But liquid crystals - obviously not, since relatively few materials have a liquid crystal state.

That's not fair - condensed matter physics might be boring but somebody has to do it!

And the exotic stuff is just exotic. I've got a plasma in my kitchen (fluorescent lights). I don't think my cruddy freezer could maintain a BEC though.

Again depends on your field - a lot of my colleagues think anything nearer than a redshift of 4 is annoying foreground clutter ;-)

 

[Andy Resnick]

Sorry, wrong link: http://chemistry.about.com/od/chemistryfaqs/f/firechemistry.htm

Good point mgb_phys, glass comes to mind too.

Thatks- that link worked. As a general statement, fire doesn't produce a plasma, so I guess I disagree. Plasmas can be produced by arc sources (lightning is maybe the most common), but I don't think oxygen is consumed in those situations, so I'm not sure it's fair to call giant electrical sparks 'flames', either.

 

[Andy Resnick]

I never thought that plasma as a state of matter was very convincing.
Solid/liquid/gas is a useful distinction for material properties and thermodynamics.
But if you count plasma then why not a supercritical fluid or a liquid crystal.
Then add in exotic states like a Bose-einstein condensate or a degnerate neutron star.

That's a reasonable point- I think as we learn more about condensed matter, the concept of thermodynamic 'phase' needs to become more subtle. To some degree, we already do- the superconducting phase, or smectic phase, or a host of other configurations.

Even so, calling 'plasma' a state of matter as distinct from a gas has some logic because gas atoms don't have free charge and don't alter their thermodynamic behavior if an electric or magnetic field is applied.