This may sound stupid but I have always wanted to know what exactly

[MACHO-WIMP]

This may sound stupid but I have always wanted to know what exactly fire is. So can anyone tell me what it is?

 

[LostConjugate]

Common question with good explanations already written up.
In short though, it is a hot gas.

 

[SpectraCat]

This may sound stupid but I have always wanted to know what exactly fire is. So can anyone tell me what it is?

It is complicated, and the exact composition depends on what is burning, but the simple answer is that it is a self-sustaining thermal plasma. A plasma is a material where a significant fraction of the atoms and molecules exist in electronically excited states, or as positive ions and free-electrons .. however the overall charge is balanced (the plasma is electrically neutral). The energy to sustain a fire typically (perhaps always, but I am not sure) comes from an exothermic chemical reaction .. for example the combustion of a hydrocarbon like methane. In fact there are many chemical reactions occurring in flames, since the combustion doesn't happen all in one step .. there are generally several intermediate steps involving free radical chemical species (cf. the research of Craig Taatjes and David Osborne at LBNL). These reactions release sufficient energy to keep the plasma hot enough that the reaction continues until either the fuel or the oxygen is consumed .. or it is actively extinguished in some manner.

That is a fairly qualitative description, but I think it covers the main points.

 

[MACHO-WIMP]

Thank you!

 

[Drakkith]

Hrmm. I don't believe your average fire is hot enough to cause a Plasma to form. A match for example only burns at around 1,000 C, which is not hot enough to form a plasma. A blowtorch burns at 1300 C, while a bunsen burner is 1300-1600 C. In comparison, the following are a list of temperatures required to cause something to emit White light.

Whitish: 1,300 °C (2,400 °F)
Bright: 1,400 °C (2,600 °F)
Dazzling: 1,500 °C (2,700 °F)

While the temperature required for ionization and to sustain plasma differes per material, I don't believe these temps will cause much ionization in most materials.

Also, I just found this:
http://www-spof.gsfc.nasa.gov/Education/FAQs7.html#q97

 

[SpectraCat]

Hrmm. I don't believe your average fire is hot enough to cause a Plasma to form. A match for example only burns at around 1,000 C, which is not hot enough to form a plasma. A blowtorch burns at 1300 C, while a bunsen burner is 1300-1600 C. In comparison, the following are a list of temperatures required to cause something to emit White light.

Whitish: 1,300 °C (2,400 °F)
Bright: 1,400 °C (2,600 °F)
Dazzling: 1,500 °C (2,700 °F)

While the temperature required for ionization and to sustain plasma differes per material, I don't believe these temps will cause much ionization in most materials.

Also, I just found this:
http://www-spof.gsfc.nasa.gov/Education/FAQs7.html#q97

Hmmm .. I don't think that is correct. See for example this page:

[link deleted due to accusations that it is "crackpot site"]

As you can see, there is a picture of a yellow candle flame being deflected in an electric field. That is a characteristic of a plasma. You have a point that combustion flames may not be very good plasmas, in that the degree of ionization is low, but they do seem to have plasma characteristics.

However, I also found this link:

https://www.physicsforums.com/showthread.php?t=203289

As you can see .. this is something of a matter of debate. I haven't finished reading that thread, so I don't know if there was a resolution to the question.

 

[Studiot]

Not all burning substances are gaseous, or involve plasmas.

The energy to sustain a fire typically (perhaps always, but I am not sure) comes from an exothermic chemical reaction .

Yes this is true.

Any chemical reaction that is exothermic (meaning gives off heat) will proceed, all other things being equal. And chemical combination with oxygen to form oxides ( which is really what fire is) are generally strongly exothermic chemical reactions.

However all other things are not equal in this world.

Many reactions including burning require some input energy to start them off. This is known as activation energy. Once you have input this energy to one part of the combustible material and it burns and in doing so the exothermic reaction generates enough to supply the activation energy for the nearby particles of combustible material to start burning.

That is how a fire is self sustaining, once lit. In certain circumstances the energy produced by the burning is spread widely enough and quickly enough to more than just actiate the next particle's burning. Once you have started one particle burning, it will supply the energy to activate the burning of two more, which will activate 2 more each inturn and so on.
This process is known as a chain reaction and leads to an explosion - (This does not have to be nuclear to be a chain reaction).

go well

 

[Drakkith]

Hmmm .. I don't think that is correct. See for example this page:

http://www.plasma-universe.com/Flame

As you can see, there is a picture of a yellow candle flame being deflected in an electric field. That is a characteristic of a plasma. You have a point that combustion flames may not be very good plasmas, in that the degree of ionization is low, but they do seem to have plasma characteristics.

However, I also found this link:

https://www.physicsforums.com/showthread.php?t=203289

As you can see .. this is something of a matter of debate. I haven't finished reading that thread, so I don't know if there was a resolution to the question.

I've read it, and from what I can tell a fire can contain cations and anions, which would react to an electric field. However, the fire does not contain ions and free electrons in sufficient quantitiy to qualify as a plasma. Just because it reacts weakly to an electric field doesn't make it a plasma.

 

[DaveC426913]

In certain circumstances the energy produced by the burning is spread widely enough and quickly enough to more than just actiate the next particle's burning. Once you have started one particle burning, it will supply the energy to activate the burning of two more, which will activate 2 more each inturn and so on.
This process is known as a chain reaction and leads to an explosion

It is not so much the burning that makes the explosion (though that provides the heat), but the conversion to a gas. Many conventional explosions (such as dynamite) are the result of the chemistry going from a solid form to a gaseous form (usually CO2). The gaseous form takes up a LOT more room than the gaseous from and expands rapidly. That's the explosion.

Are thereconventional explosions that do not operate on this principle? i.e. the byproduct is a solid rather than a gas? Would that be an explosion at all?

 

[Drakkith]

Are thereconventional explosions that do not operate on this principle? i.e. the byproduct is a solid rather than a gas? Would that be an explosion at all?

I believe those that do aren't "explosive". Such as the stuff they use to burn through metal objects for field disposal. I don't know if it magnesium or potassium or what.

 

[russ_watters]

I've read it, and from what I can tell a fire can contain cations and anions, which would react to an electric field. However, the fire does not contain ions and free electrons in sufficient quantitiy to qualify as a plasma. Just because it reacts weakly to an electric field doesn't make it a plasma.

Moreover, that link is to a site pushing a crackpot view of physics: the "electric universe theory". So it isn't to be trusted.

 

[SpectraCat]

Moreover, that link is to a site pushing a crackpot view of physics: the "electric universe theory". So it isn't to be trusted.

I was not aware of that .. mea culpa for not checking more carefully before linking to it. I have deleted the link from my post.

 

[Vanadium 50]

This process is known as a chain reaction and leads to an explosion - (This does not have to be nuclear to be a chain reaction).

No. What you describe is a deflagration. You have a detonation only if the combustion propagation is supersonic.

If you light one corner of a newspaper, the chain reaction you describe will occur, but it won't explode.

 

[Studiot]

No. What you describe is a deflagration. You have a detonation only if the combustion propagation is supersonic.

Thank you for your comment, I was not correct to imply that a chain reaction always leads to an explosion. It would have been better to say that it may do so.

But you are not completely correct either.

Combustion is often propagated by a chain reaction, which is a reaction where some of the reactions products act to promote the overall reaction. A chain reaction is often a multi-step reaction where the promoting species are intermediate products, as in the combustion of hydrogen.

Chambers Science and Technology Dictionary defines a deflagration as

Sudden combustion, generally accompanied by a flame and a crackling sound.

So this describes your newspaper example to a tee.
But slow, steady, controlled, sustained, combustion say in an AGA cooker or a biological system cannot be described as 'sudden'.

The glowing coals in a aga cooker are a fine example of combustion where there is no flame and definitely no plasma.

Since this is a thread about fire, not explosions, my comment about chain reactions leading to explosions was necessarily superficial.
There is, however, more than one way for a chemical reaction (chain or otherwise) to turn into an explosion. And at least one of these does not involve 'supersonic speed', which is a measure of distance/ time and therefore not directly comparable to the rate of a chemical reaction in mass/time. You do indeed get an explosion if the speed of the flame front is supersonic.