| From: Matt | 30/07/99
22:27:24 |
| Subject: What is fire? | post id:
27323 |
| Here is a question that has been
bothering me for a while. What is fire? It's not a solid, it's not a liquid and I don't think it's gas. So I was wondering if you could explain to me what it is. | |
| From: Chris W (Plebeian) | 30/07/99
23:03:55 |
| Subject: re: What is fire? | post id:
27326 |
| Fire is the oxidation of
substances (such as carbon in wood) which manifests itself as light, heat
and flame. I think you are really asking what a flame is... I'll have a stab at it and wait for the corrections. What you see as a flame is the hot molecules, like CO2, rising from the fire. These molecules are very active and excited: they're moving around at a fair pace and banging into each other, electrons in the atoms are rising and falling through energy levels, and the molecules may have other internal vibrations. Some of this activity results in the emission of electromagnetic radiation, mainly as visible light or infra-red (radiant heat). As the molecules rise their temperature drops and they cease to emit visible light (the edge of the flame) but they are still emitters in the infrared region. With the light out of the way, you begin to see the particulates that make up the smoke. You may have seen different coloured flames from the fire. Differing colours can be caused by different chemicals in the rising smoke. Copper in the fire gives a green flame. Different molecules give different frequencies of light because of the discrete energy levels that electrons can have within atoms, and limited internal vibrations that a molecule can have. This effect can be used to determine the makeup of an unknown sample by examining, using a spectroscope, the colours emitted when the sample is burnt. Hope this helps. | |
| From: Daryn Voss (Avatar) | 30/07/99
23:05:29 |
| Subject: re: What is fire? | post id:
27327 |
| In some reactions, energy is
taken in. In others, energy is given out. Reactions in which energy is
given out are called exothermic reactions. Many substances react exothermically with oxygen. Such a reaction is called combustion. If this takes place in an environment where free oxygen is abundant, then the reaction will go very well. Some of the energy produced will go towards the kinetic energy of the materials, or to sound energy, or to various forms, but a lot will be lost as electromagnetic radiation (eg heat radiation, visible light). Much of this radiation may be due to ionisation. Depending on the material being burned, the product of the reaction may be release as a gas that is so hot that some of the molecules lose electrons. When they regain the electrons, they give off energy. (Note, not all materials always burn with a flame e.g flickering coals. These still give off heat and light but without a flame.) In any case, fire is this entire process. It is not a material, but an event. According to Concise Oxford English Dictionary for example, says that fire is "the state or process of combustion, in which substances combine chemically with oxygen and usu. give out bright light and heat." | |
| From: Dr. Ed G (Avatar) | 30/07/99
23:13:37 |
| Subject: re: What is fire? | post id:
27329 |
| I'll assume that know know what
fire is and that what is are asking is, "what is the flame?" The blue part of the flame is mostly plasma, that is a partly ionised hot gas. It is in this region that most of the chemical reactions that sustain the fire take place. The blue colour is the result of the de-excitation of electrons within atoms in the plasma, and the wavelengths given off will be characteristic of the atoms in this plasma. As the plasma rises and cools, all its constituent atoms de-ionised, and many condense to form larger molecules. This is the yellow part of the flame which is mostly hot gas, and small solid particulates (it can no longer be called a plasma). The yellow colour is the result of continuous thermal radiation. All things that are hot emit thermal radiation, and the wavelength at which the greater amount of radiation occurs is determined by its temperature. Things which are at, say 35 degrees C radiate mostly in the infra-red (commonly referred to as radiant heat). However, things which are at a thousand to several thousand degrees C tend to emit mostly in the visible. Examples of this are the Sun, incandescent light bulb filaments, and the yellow part of a candle flame. So, the bottom blue part is mostly ionised gas (plasma) and top yellow part is a mixture of hot (non-ionised) gas and solid particulates (if you want to prove this just hold a spoon over the top of a flame and the bottom will become coated with solid carbon particles). Soupie twist, Ed G. 
| |
| From: Trev(TAO) | 31/07/99
1:37:21 |
| Subject: re: What is fire? | post id:
27352 |
| as a side question to this do
things burn at different temperatures and thus have flames of different
temps if yes what is the coolest
flame? :-)> Trev(TAO) | |
| From: Dr. Ed G (Avatar) | 31/07/99
2:13:53 |
| Subject: re: What is fire? | post id:
27353 |
| The temperature will be different
in different parts of the flame, and the maximum temperature will depend
on the conditions (i.e. speed at which fuel is fed into the flame,
temperature of the fuel as it's fed into the flame, oxygen/fuel mixture,
and of course the type of fuel and how much energy it releases per mole).
So theoretically, you could "dial up" whatever temperature you wanted (up
to an optimal maximum, of course) by fiddling with these parameters.
However, you would need a certain minimum temperature/energy within the
flame to sustain combustion, which would also depend on the conditions...
I don't know what lowest temperature possible to do this might be, though
(given a choice of any fuel). Soupie twist, Ed G. | |
| From: Chris W (Plebeian) | 31/07/99
11:05:04 |
| Subject: re: What is fire? | post id:
27373 |
| I know from personal experience
(I set my hand on fire - embarrassing I know, but there you have it :-} )
that ethanol burns at a quite low temperature and with an almost invisible
flame. Took a few seconds before I cottoned on to what was going
on. | |
| From: Terry Frankcombe (Avatar) | 31/07/99
16:41:04 |
| Subject: re: What is fire? | post id:
27388 |
| According to one of the seminars
at the conference I want to the other week, the actual oxidation reaction
occurs in a very thin shell around the outside of the
flame. | |
| From: Terry Frankcombe (Avatar) | 31/07/99
16:44:52 |
| Subject: re: What is fire? | post id:
27390 |
| I suspect something like methanol
under low oxygen conditions would burn with a fairly cool
flame. | |
| From: YODA OZ | 17/06/99
11:36:50 |
| Subject: FIRE | post id:
18212 |
| Okay, so there are five states,
aqueous, solid, liquid, gas and plasma. Out of these states what is fire? Is is a gas? My friends and I have been arguing about this for some time. If it is a gas, how do you put out the fire because you cannot put out a gas. If there is anything else i need to know about fire please let me know. | |
| From: Rene | 17/06/99
11:40:44 |
| Subject: re: FIRE | post id:
18218 |
| It depends what you mean by fire.
It you are talking about the light photons which are released when the
wood releases energy, then it would be light. Is light plasma?
| |
| From: YODA OZ | 17/06/99
11:45:21 |
| Subject: re: FIRE | post id:
18227 |
| When you see fire it has a blue
flame at the bottom, a red flame, then a yellow/orange flame at the top.
What state is the actual flame. Where does the heat come from? Does it
come from the flame or the energy released from, for example, the
wood? | |
| From: Ben | 17/06/99
11:48:22 |
| Subject: re: FIRE | post id:
18234 |
| think about it... fire needs
oxygen, and a fuel.... be it a gas or a solid or a liquid.... and fire is really the only element that has confusion about its state.... | |
| From: Rene | 17/06/99
11:54:19 |
| Subject: re: FIRE | post id:
18238 |
| Basically, wood is full of
energy. For the wood to release it's energy, you must heat it up to a
point where the molecules can move around and react. Because heat is the
movement of molecules. So once the fire starts, the energy released by it heats up the next section of wood which will then react. The energy released by the wood comes out in the form of light, and heat. So it isn't really a substance, unless you'd classify energy as a substance, which is probably what you'd say plasma is. | |
| From: YODA OZ | 17/06/99
11:57:13 |
| Subject: re: FIRE | post id:
18242 |
| If you say fire is just light,
then how does the light stop. ou know there is always a top of the flame.
If there is an end to the flame how can you say it is
light? | |
| From: Rene | 17/06/99
12:05:37 |
| Subject: re: FIRE | post id:
18250 |
| If you can percieve something,
then it must release or reflect light. Can you touch a flame? no therefore, it is not reflecting light. This means that something must be releaseing the light, ie the fire. think of it not as an object, but more as light and heat. | |
| From: Iain | 17/06/99
12:11:03 |
| Subject: re: FIRE | post id:
18255 |
| The fire is the excited electrons
returning to there ground state. In this process they give off energy in
the form of heat and light. | |
| From: Daryn Voss | 18/06/99
0:42:24 |
| Subject: re: FIRE | post id:
18457 |
| Perhaps it is better to view fire
as a process or event, rather than a
material. | |
| From: Dr. Ed G (Avatar) | 18/06/99
0:48:23 |
| Subject: re: FIRE | post id:
18459 |
| The blue bit is mostly plasma
(and the blue colour is characteristic of the atoms/ions in that plasma -
the old de-excitation of electrons between discrete quantum states giving
off the line emission that's been mentioned in other threads), and the
yellow part is mostly hot gas (and the yellow colour is just the continous
"black body" thermal emission spectra of anything at a temperature of
around a few thousand degrees). Of course as mentioned in a different thread, the flame will also contain small particles of things like carbon (soot) which makes smoke. Soupie twist, Ed G. | |
| From: Yoda Oz | 18/06/99
15:43:56 |
| Subject: re: FIRE | post id:
18539 |
| Dr EdG said that the yellow flame
has other types of elements in it. Somebody (who knows what they are
talking about) can they please explain in very great detail what a fire
consists of and anything else that relates to what a fire
is? PLEASE!!!!>>>>>>..... | |
| From: Dr. Ed G (Avatar) | 18/06/99
19:52:01 |
| Subject: re: FIRE | post id:
18575 |
| I'm not sure what more there is
to say. I thought Terry explained fairly thoroughly what fire is phenomenalogically, i.e. it's simply a rapid oxidation reaction that gives off a lot of heat... and I've covered what causes the blue part (de-excitation of electrons in a plasma) and the yellow part (black body thermal radiation of a hot gas)... and that's about all there is to say. If you're concerned about the fact that there are also hot carbon particles in the mix... well, in effect I guess that makes it a fluid composite... but if you want a want word taxonomic category for fire, then that would be... ummm ... "fire". What's the real question, if you can put it into words? Soupie twist, Ed G. | |
| From: Brad Dull | 21/06/99
22:16:53 |
| Subject: re: FIRE | post id:
18819 |
| This is a simple explanation used
to detail the makeup of Fire to probationary firefighters about 11 years
ago. "Fire is a rapid self sustaining oxidization process accompanied by the evolution of heat and light in varying intensities." The Fire triangle is a basic example of what is required for fire to happen. On one side you have a Fuel (Wood Paper) the other side you have Oxygen ( Air)on the last side you have heat ( Naked Flame). Put all of these together in the right proportions and i think you have a chemical chain reaction. Removal of any of the sides of this triangle and you break this reaction thus extinguishing the fire. There was talk back then that there may be another side to this story, so if anyone knows if the triangle has turned into a square... could you let me know! Another novel way of looking at it is that the rust you see eating your car away is a pretty close relation to FIRE!!! How much you have it on the beach dictates how rapid the oxidization process is!! | |
| From: Brad Dull | 21/06/99
23:17:44 | ||||||||||||
| Subject: re: FIRE | post id:
18825 | ||||||||||||
| Keeping in mind the triangle,
when we take a piece of fuel, for example Wood. When it is heated in open
air it starts releasing vapours(Gas). These flammable vapours are taken
upwards by the heat currents caused by the heat created by both the inital
heat source (Naked Flame), and the Chemical reaction of the breaking down
of the matter. When this Flammable vapour is heated to a certain
temperature it will ignite. When heated further and with the right mixture
of air,the flamable vapour flashes over. When sufficent vapour and heat
and oxygen are available free burning takes place. When free burning is
established the inital heat sorce can be removed leaving the wood to
breakdown and turn to flammable vapour as required. The more area of the
wood surface surface that is heated to this point the more flammable
vapour is released thus the fire spreads and becomes more intense. I believe much of the colour difference in the flame is due to the different mixtures of flamable vapour and oxygen plus the different in the substances burning. I believe a flame is an example of the limits of flammability. You will notice when looking at a burning match, the flame has no contact with the wood. At about 2 mm above the match the vapour air mix is suitable to support combustion. At this stage of the flame the fuel mix is still rich and combustion of the vapour is not complete and the flame is a darker colour because of the unburned particles present. As we go up the flame combustion becomes more efficient and the flame become brighter. When we get to the tip of the flame,the air fuel mix is at it's optimum and combustion is as complete as possible given the natural environment. This is the hottest part of the flame and where the flame terminates it's the upper end of vapours Flammability.
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