# What exactly is fire? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [November 22, 2002, 11:10am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131 "2002-11-22T11:10:04Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![confluenceofsouls](https://avatars.discourse-cdn.com/v4/letter/c/7feea3/32.png) [@confluenceofsouls](https://boards.straightdope.com/u/confluenceofsouls)\ **Post date:** [November 22, 2002, 4:11pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/21 "2002-11-22T16:11:22Z")
When we were taught about fires in boot camp, they told us it required four things, not just the three Cecil mentioned: fuel, oxygen, heat, and an uninhibited chemical reaction. ---
**Author:** ![mumblefuzz](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@mumblefuzz](https://boards.straightdope.com/u/mumblefuzz)\ **Post date:** [November 22, 2002, 4:21pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/22 "2002-11-22T16:21:35Z")
Cecil, You seem to be more of a physicist than a chemist. One thing that you don’t bring out clearly enough for my taste is where the energy that is released in a fire comes from. In the oxidation process (whether in an oxygen or a florine atmosphere) the oxidation products have a higher binging energy than the oxidizing agent and the fuel combined. The difference in binding energy is what is released in the reaction. The reaction is exothermic. The release of energy makes everthing hotter. Hot things glow. The other issue is that rates of reaction are temperature dependent. So the hotter the fire gets the faster it burns. Thats why you need a match to start the fire in the first place. (Except in the case of a chunk of sodium in a florine atmosphere – haven’t seen it either but I’d like to.) ---
**Author:** ![Kantalooppi](https://avatars.discourse-cdn.com/v4/letter/k/a5b964/32.png) [@Kantalooppi](https://boards.straightdope.com/u/Kantalooppi)\ **Post date:** [November 22, 2002, 4:23pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/23 "2002-11-22T16:23:36Z")
I have nothing to add, except props for explaining something that, miraculously, never seems to be explained clearly and sufficiently, and idle wondering about who else had “The Sun Is A Mass Of Incandescent Gas” by They Might Be Giants yes yes I know it’s a cover playing in their heads for Cecil’s definition. ---
**Author:** ![mumblefuzz](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@mumblefuzz](https://boards.straightdope.com/u/mumblefuzz)\ **Post date:** [November 22, 2002, 4:25pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/24 "2002-11-22T16:25:15Z")
Chemical reaction. Not nuclear. Nothing like what happens in the sun. And not a plasma. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [November 22, 2002, 4:35pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/25 "2002-11-22T16:35:03Z")
> [@](#): > > \*Originally posted by EEMan \* > but were not talking Ideal Gas Behaivior… this is where my confusion comes in to why gravity makes any difference… > > were talking about ‘micro-currents’ and ‘gas eddies’ so even though we are talking one container… we are trying to see what happens in that container… (bear with me I’m an EE not a ME) this would be fluid dynamics… > > I don’t see how the response of the excited atoms could be held in a single spot… in fact without gravity to hold them down… i would assume the would move MORE not LESS… I think your confusion arises because there’s four different things going on in the air around a candle flame at the same time: 1. the air is heating up and getting less dense, 2. there’s diffusion of air molecules around the flame, 3. there is bulk motion of the air, and 4. the candle is consuming oxygen and putting out CO[sub]2[/sub] (I think…maybe water vapor, too?) When I’m talking about the ideal gas law, I’m talking about the first effect: if the candle heats up a blob of air, that blob of air expands and becomes less dense because the PV/T quantity remains constant. What _you’re_ talking about is the diffusion of the air molecules. You rightly recognize that, if I define a “blob” of air at a certain time, individual molecules are going to pass back and forth across the boundary of the blob, making the boundary sort of fuzzy. Now, I think you’re assuming that the diffusion effect happens more quickly than it does. What’s going on around the candle in your house (in a gravitational field, in other words) is that the transfer of heated air is dominated by the bulk convection motion: air around the flame is heated which expands the air, the heated air rises, and cooler air is pulled up from below, setting up vertical bulk motion of the air. This happens _much more quickly_ that the diffusion. You can pretty easily prove this to yourself: light a candle somewhere out of the draft, wait a minute or so for the convection patterns to establish, and hold your hand a foot or so above the flame. Compare that temperature to the temp. a foot or so away from the flame horizontally (or even an inch or so a way horizontally). The temperature above the candle should be much higher. What I think is happening in space (no gravity) is that the air motion around the candle is being dominated by the diffusion. The candle flame heats the air around it, which expands the air, but the heated air _does not rise_ (there’s no “up”, right?), and just stays around the flame. At a slower rate, the more energetic, “hot” air molecules diffuse outward, and cooler air molecules (including some oxygen) diffuse inward toward the flame. **Brave Sir Robin** : Good point. I withdraw my statement. ---
**Author:** ![Mister.Physics](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Mister.Physics](https://boards.straightdope.com/u/Mister.Physics)\ **Post date:** [November 22, 2002, 4:39pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/26 "2002-11-22T16:39:59Z")
Here’s my opinion on this topic: (1) Oxygen is not strictly speaking required for something to burn or produce flame - as discussed above, Fluorine will do the trick nicely. (2) “Incandescent” has been misused throughout the article. Note that incandescent is NOT synonymous with “glowing”. Rather, incandescent means giving off light _due to a high temperature_. Having a high temperature and glowing does not necessarily constitute incandescence. The blue portion of a flame (which may be the entire flame in some cases) is not a region of incandescence. Incandescence is characterized by a full spectrum (ie blackbody radiation). (3) Flames _are_ ionized - at least they are every time I have checked it. They do conduct electricity fairly well (being able to discharge charged objects placed in them). Also, I do a demonstration in which I place a lit candle next to a Van de Graaff generator which is running slowly - the flame of the candle is becomes forked with half attracted to the generator, and the other half repelled from it - clearly indicating the presence of both positive and negative ions. Here is a good definition of “flame” from the Oxford dictionary of science: A hot luminous mixture of gases undergoing combustion. The chemical reactions in a flame are mainly free-radical chain reactions and the light comes from fluorescence of excited molecules or ions or from incandescence of small solid particles (e.g. carbon). - A Dictionary of Science, Oxford University Press, © Market House Books Ltd 1999 Barry ---
**Author:** ![EEMan](https://avatars.discourse-cdn.com/v4/letter/e/a6a055/32.png) [@EEMan](https://boards.straightdope.com/u/EEMan)\ **Post date:** [November 22, 2002, 6:22pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/27 "2002-11-22T18:22:36Z")
Thank you Zut… my ME buddy just tried to explain ‘convection currents’ to me… you did a much better job… (not sure he really knew what I was asking) I now see how the tapered shape would occur… (as I think you have explained it)… the air current is forced in a vertical manner ‘shaping’ the flame into a more conical shape… the diffusion remains nearly the same speed… so the difference would have to be convection… Thank you for your time explaining where we differed in our approach… I still think the word ‘visible’ needs to be added to the def… ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [November 22, 2002, 7:10pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/28 "2002-11-22T19:10:53Z")
**EEMan** : Exactly. Your understanding should explain why there’s a difference in shape between standard and no-gravity flame. And I cautiously agree with you about the word “visible”. I’m glad my explanation was reasonably clear. -Zut, PhD., ME ---
**Author:** ![akrako1](https://avatars.discourse-cdn.com/v4/letter/a/c77e96/32.png) [@akrako1](https://boards.straightdope.com/u/akrako1)\ **Post date:** [November 22, 2002, 8:54pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/29 "2002-11-22T20:54:37Z")
What is fire? Fire is alive. It grows, consumes, excreated and reproduces. Fits most of the criteria I can think of. Prove its not! ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [November 22, 2002, 11:29pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/30 "2002-11-22T23:29:28Z")
> [@](#): > > \*Originally posted by Brave Sir Robin \* > \*\*As far as I know, oxidation doesn’t require oxygen. If you put any substance that is able to be oxidized into a flourine atmosphere and apply heat, you’ll get rapid oxidation. I imagine that a chunk of sodium in a pure flourine environment would fit everyone’s idea of what a fire is. > > Perhaps someone who recalls their college chemistry classes better than I do can confirm this. \*\* From the [Los Alamos National Laboratory’s Chemistry Division’s Periodic Table of the Elements](http://pearl1.lanl.gov/periodic/elements/9.html): > [@](#): > > Fluorine is the most electronegative and reactive of all elements. It is a pale yellow, corrosive gas, which reacts with most organic and inorganic substances. Finely divided metals, glass, ceramics, carbon, and even water burn in fluorine with a bright flame. ---
**Author:** ![Devil\_s\_Advocate](https://avatars.discourse-cdn.com/v4/letter/d/3d9bf3/32.png) [@Devil\_s\_Advocate](https://boards.straightdope.com/u/Devil_s_Advocate)\ **Post date:** [November 23, 2002, 8:38pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/31 "2002-11-23T20:38:16Z")
> [@](#): > > \*Originally posted by akrako1 \* > \*\*What is fire? > > Fire is alive. It grows, consumes, excreated and reproduces. Fits most of the criteria I can think of. > > Prove its not! \*\* It all depends on how we define “life”, of course. See Cecil’s column on the subject, [Why don’t we consider fire alive? What is life?](http://www.straightdope.com/classics/a971212.html). But that’s really a separate subject from the matter at hand (because it’s about classifying fire, not explaining it). ---
**Author:** ![PARbrowser](https://avatars.discourse-cdn.com/v4/letter/p/bcef8e/32.png) [@PARbrowser](https://boards.straightdope.com/u/PARbrowser)\ **Post date:** [November 25, 2002, 12:11am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/32 "2002-11-25T00:11:37Z")
Fire? Flame? Combustion? While I admire your attempt to clearly define and explain fire, there are a few areas that could use a little clarification. Your definition (a great improvement over that of EB) still lacks precision. Consider the following amended definitions. “Fire is the rapid, exothermic, self-sustaining, combustion of a gaseous fuel with oxygen.” “Flame is the usually visible body of incandescent gas formed by partially combusted reactants.” “Combustion is the change of chemical energy state due to rapid oxidation, from higher to lower, with differential energy being released as heat.” Now to pick flyshit out of pepper. “combustible material” is always gaseous fuel (even solids must be vaporized), oxygen is the oxidizer. “evolution of heat” don’t know what that is either-senseless. “usually accompanied by flame” – always accompanied by flame, sometimes visible. “contains and sustains the reaction” actual combustion occurs well before flame is visible, combustion sustains the flame, not vice versa. “fire…purely energy” correct, glowing gas is the result of energy release. “fire…ionized” yes indeed, since combustion is a chemical reaction at the elemental level, free radicals are produced which sustain the very complex chain reaction of the combustion process. “fire & gravity” of course, gas has mass. Gas lighter than air rises, and vice versa. Your essay being mostly correct, is skewed by one gross defect. NASCAR does not burn methanol as a fuel. They burn high octane, leaded, race gas supplied by 76, a unit of Conoco-Phillips Petroleum. Methanol is used by CART (old Indy cars) to help cool the turbocharger. Methanol is not clean burning, although the flame does lack color. The by-product of partially combusted methanol is formaldehyde-one of the most carcinogenic substances known. Ethanol, contrary to the proclamations of the agri-business lobby, also produces an harmful by-product called acetylaldehyde. With the vast intellectual resources available at two of the highest ranked universities in the nation (Northwestern and University of Chicago), issues of technical fact should be easily resolvable. Overall, an excellent response, keep up the good work. David Redszus, PhD. Precision AutoResearch ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [December 4, 2002, 8:31am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/33 "2002-12-04T08:31:17Z")
**confluenceofsouls** said: > [@](#): > > When we were taught about fires in boot camp, they told us it required four things, not just the three Cecil mentioned: fuel, oxygen, heat, and an uninhibited chemical reaction. Fuel, oxygen, and heat _is_ an uninhibited chemical reaction. Fuel and oxygen are the reactants of the chemical reaction. Heat is the trigger energy that activates it. After that, the reaction is exothermic (and thus the rapid release of heat that produces the flame). ---
**Author:** ![austinjp](https://avatars.discourse-cdn.com/v4/letter/a/a88e57/32.png) [@austinjp](https://boards.straightdope.com/u/austinjp)\ **Post date:** [December 10, 2002, 3:51pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/34 "2002-12-10T15:51:05Z")
My physics and chemistry are extremely basic, please forgive… A candle flame does indeed contain ionised material, and is hence affected by magnetism. However, in the usual conditions one would find on Earth, holding a magnet near a flame from eg a candle wouldn’t give a very noticable result. I believe this is because the convection currents in the air around the candle flame would be far more affective at moving/shaping the flame. In close-to zero-gravity conditions (aboard the Shuttle, or one of those crazy parabolic 747 flights perhaps) the convection currents would be minimised, because, as someone writes, “there is no up”. The candle flame would then be susceptible to weaker magnetic fields, and would indeed move. I’m sure the same could be achieved in an Earth-bound lab with a sufficiently powerful magnet. A couple of other pieces of candle flame trivia: A “Final Word” article in “New Scientist” magazine revealed that you can get a candle flame to point in a direction other than up quite easily. Securely place a candle toward the outer edge of a record player platter. Light the candle, place a sufficiently tall jam jar over the candle, and set the record player going. While there is available oxygen (or the oxidative gas of your choice) in the jam jar, the candle flame will point toward the centre of the platter. This is because the “centrifugal force” (I’m sure dealt with elsewhere on this site) causes the gas in the jar to become less dense towards the centre of the platter, and the convection currents around the candle “rise” in that direction. Sweet. Also, try this. Light a candle with a match. Blow the candle out, and wave the lit match through the smoke from the candle, a couple of centimetres from the wick. The candle will burst into light. Groovy. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 10, 2002, 10:25pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/35 "2002-12-10T22:25:18Z")
> [@](#): > > This is because the “centrifugal force” (I’m sure dealt with elsewhere on this site) Yes, it is, and you shouldn’t have put it in quotes. Centrifugal force is just as real as gravity. ---
**Author:** ![Devil\_s\_Advocate](https://avatars.discourse-cdn.com/v4/letter/d/3d9bf3/32.png) [@Devil\_s\_Advocate](https://boards.straightdope.com/u/Devil_s_Advocate)\ **Post date:** [December 10, 2002, 11:04pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/36 "2002-12-10T23:04:47Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\*Yes, it is, and you shouldn’t have put it in quotes. Centrifugal force is just as real as gravity. \*\* But it isn’t actually a force, but the effect of a force in the opposite direction (a centripital force) combined with inertia. Consequently, a lot of physicists don’t like to refer to “centrifugal force”, because the name sounds like it refers to an actual force acting on an object, when in reality there only _appears_ to be a force in the object’s accelerating frame of reference. It’s a “fictional force”. I think I got that all right. Someone correct me if I got something wrong. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [December 11, 2002, 6:38am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/37 "2002-12-11T06:38:22Z")
Yes and no. If you use an inertial reference system, yes. However, in a rotating reference system, the centrifugal force is a real force just like any other. Engineering schools (and perhaps physicists) just like to stress using inertial reference frames. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 12, 2002, 6:38pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/38 "2002-12-12T18:38:44Z")
By the same argument, gravity isn’t really a force, it’s just an effect of an accelerated reference frame (the surface of the Earth is accelerating upwards at 9.8 m/s[sup]2[/sup]). But since we’re usually working in a surface-of-the-Earth reference frame, it’s convenient to treat gravity as a force. Similarly, when one is in a rotating reference frame (say, inside the Tilt-a-whirl at the fair), it’s convenient to treat centrifugal force as a force. Such a force is sometimes referred to as a “fictitious force”, but it’s still exactly as real as gravity, and if it’s useful, why quibble about the label? ---
**Author:** ![Devil\_s\_Advocate](https://avatars.discourse-cdn.com/v4/letter/d/3d9bf3/32.png) [@Devil\_s\_Advocate](https://boards.straightdope.com/u/Devil_s_Advocate)\ **Post date:** [December 13, 2002, 4:23am UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/39 "2002-12-13T04:23:52Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\*By the same argument, gravity isn’t really a force, it’s just an effect of an accelerated reference frame (the surface of the Earth is accelerating upwards at 9.8 m/s[sup]2[/sup]). But since we’re usually working in a surface-of-the-Earth reference frame, it’s convenient to treat gravity as a force. > > Similarly, when one is in a rotating reference frame (say, inside the Tilt-a-whirl at the fair), it’s convenient to treat centrifugal force as a force. Such a force is sometimes referred to as a “fictitious force”, but it’s still exactly as real as gravity, and if it’s useful, why quibble about the label? \*\* How can the surface of the Earth be accelerating upwards in all directions at once? The Earth is round, after all; there’s more than one “up”. Unless you believe that gravity is actually caused by the constant expansion of matter. But then you would expect that gravity would only depends on size, not on mass, and there are other problems with that theory too. I can debunk it if you wish. Gravitation is caused by the attraction of massive bodies to one another. Centrifugal force is “caused” by using a non-inertial reference frame. It seems clear to me that there’s a fundamental difference between the two, and that the former is indeed more “real” than the latter. You can quibble about the semantics, of course; it all depends on what we mean by “force”, “real”, “ficticious”, etc. But the difference seems clear to me. ---
**Author:** ![BMalion](https://avatars.discourse-cdn.com/v4/letter/b/e274bd/32.png) [@BMalion](https://boards.straightdope.com/u/BMalion)\ **Post date:** [December 13, 2002, 1:45pm UTC](https://boards.straightdope.com/t/what-exactly-is-fire/138131/40 "2002-12-13T13:45:24Z")
Check this out for nifty photos of some flame in zero-gravity… [http://www3.cosmiverse.com/news/space/0802/space08220205.html](http://www3.cosmiverse.com/news/space/0802/space08220205.html) I hope that this shows us the dangers of campfires on our Lagrange-point space stations. It’s a complex universe out there, so be careful. Brian [Previous page](https://boards.straightdope.com/t/what-exactly-is-fire/138131.md?page=1) [Next page](https://boards.straightdope.com/t/what-exactly-is-fire/138131.md?page=3)