# What is Fire? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [August 29, 2007, 1:01pm UTC](https://boards.straightdope.com/t/what-is-fire/417353 "2007-08-29T13:01:59Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![brewha](https://avatars.discourse-cdn.com/v4/letter/b/91b2a8/32.png) [@brewha](https://boards.straightdope.com/u/brewha)\ **Post date:** [August 29, 2007, 1:01pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/1 "2007-08-29T13:01:59Z")
In [This](http://www.straightdope.com/columns/021122.html) thread, Cecil adresses the question, but doesn’t seem to answer it. More specifically, of what is the flame itself made? > [@Cecil](#): > > The dancing flames are glowing gas If the flame is a gas, what gas? What causes it to glow? You can put nitrogen (or any other gas) and a jar and it remains nitrogen. You can’t put a flame in a jar and expect it to remain a flame. When I tried to google the question, I got Cecil’s column. Anyone got the dope on this one? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 29, 2007, 1:54pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/2 "2007-08-29T13:54:55Z")
It’s various combustion products (which will vary depend on what is being burned)–in the case of hydrocarbons, these will include CO2, CO, water vapor and who-knows-what-else. It glows because it’s very hot. Hot atoms and molecules slam into each other with enough energy to emit a photon, the color of which will depend on temperature and/or the chemical composition of the hot material. ---
**Author:** ![kopek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kopek/32/337_2.png) [@kopek](https://boards.straightdope.com/u/kopek)\ **Post date:** [August 29, 2007, 4:21pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/3 "2007-08-29T16:21:56Z")
You may want to pick up a copy of The Chemical History of a Candle by Michael Faraday. Its been many years since I read that book but I think it may have some of the answers you are looking for. ---
**Author:** ![brewha](https://avatars.discourse-cdn.com/v4/letter/b/91b2a8/32.png) [@brewha](https://boards.straightdope.com/u/brewha)\ **Post date:** [August 29, 2007, 4:27pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/4 "2007-08-29T16:27:11Z")
So, the flame ends because the gas gets far enough away from the heat to stop glowing? If you were to put a flame in a jar, once it cooled, it would just be whatever gas that was glowing. Makes sense. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 29, 2007, 4:30pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/5 "2007-08-29T16:30:42Z")
[QUOTE=brewha] So, the flame ends because the gas gets far enough away from the heat to stop glowing? [/QUOTE] In a manner of speaking, yes. The dynamics are a bit complicated, but basically, the gas cools off to a point where it no longer glows in the visible, although it’ll still radiate in the infrared. Viewing in the IR, the flame would appear much larger. ---
**Author:** ![Troy\_McClure\_SF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troy_mcclure_sf/32/4150_2.png) [@Troy\_McClure\_SF](https://boards.straightdope.com/u/Troy_McClure_SF)\ **Post date:** [August 29, 2007, 6:06pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/6 "2007-08-29T18:06:18Z")
IANAScientist or anything resembling one, so please wait for confirmation from minds brighter than mine to take this for anything more than a WAG. Fire, as we observe it, is two main things: 1- a mixture of the substance that is being burned and whatever that reaction gives off (Wood + Oxygen -\> ashes) 2- energy, which we see as light and feel as heat. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [August 29, 2007, 9:24pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/7 "2007-08-29T21:24:11Z")
[QUOTE=Troy McClure SF] IANAScientist or anything resembling one, so please wait for confirmation from minds brighter than mine to take this for anything more than a WAG. Fire, as we observe it, is two main things: 1- a mixture of the substance that is being burned and whatever that reaction gives off (Wood + Oxygen -\> ashes) 2- energy, which we see as light and feel as heat. [/QUOTE] Or, you could simply read the relatively excellent, yet simple response already given by **Q.E.D.** ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [August 30, 2007, 5:45am UTC](https://boards.straightdope.com/t/what-is-fire/417353/8 "2007-08-30T05:45:07Z")
[QUOTE=brewha] If the flame is a gas, what gas? What causes it to glow? [/QUOTE] Heat. Anything heated enough will glow, with a colour matched to the temperature - this is where the phrases _red-hot_ and _white-hot_ come from; it doesn’t matter what the material is, if it’s heated sufficiently it will glow. Flame is merely ANY gas put off by the burning material heated to the extent that it glows; being gaseous it is free to dance around whereas a red-hot poker is not. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 30, 2007, 8:39am UTC](https://boards.straightdope.com/t/what-is-fire/417353/9 "2007-08-30T08:39:07Z")
[QUOTE=Askance] Heat. Anything heated enough will glow. [/QUOTE] What about water? - I acknowledge the basic truth of your answer, but I’m just curious as to how water might be rendered incandescent. Is it possible to heat water (well, _steam_) to an incandescent state, without turning it into something other than water? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 30, 2007, 2:01pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/10 "2007-08-30T14:01:32Z")
Actually, I thought that question might be worthy of its own thread: > **[Can water be heated until is is incandescent?](https://boards.straightdope.com/sdmb/showthread.php?t=434763)** > > Can water be heated until it emits visible light? I realise it won’t be liquid anymore - at least not at normal pressures - but can it be done without breaking the molecular bonds? ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [August 30, 2007, 3:27pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/11 "2007-08-30T15:27:30Z")
I thought I had talked about what causes different flame colours, and what the hottest part of a flame is, once. Oh yes, I did. [http://www.straightdope.com/mailbag/mhotflame.html](http://www.straightdope.com/mailbag/mhotflame.html) ---
**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:** [August 30, 2007, 3:50pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/12 "2007-08-30T15:50:53Z")
> [@](#): > > What about water? - I acknowledge the basic truth of your answer, but I’m just curious as to how water might be rendered incandescent. Is it possible to heat water (well, steam) to an incandescent state, without turning it into something other than water? Sure. It’s doing it right now, in fact. Anything above absolute zero (which is to say, anything at all) will glow due to its temperature. The hotter it is, the shorter the peak wavelength, and the more energy will be released at each wavelength, but it’ll always glow some. If you have something around room temperature, it’ll glow mostly in the infrared. Get something (like an electric stove burner, for instance) up to a few thousand degrees, and it’ll start glowing in the visible and be red-hot. Get something (like, say, the Sun) up to about 6000 degrees, and it’ll give off enough light in all the visible colors that it’ll appear yellowish white. People often think of infrared as “heat”, since that’s the color given off by most objects we’re familiar with (like, say, people), but it’s the exact same process which causes people to glow in the infrared as causes the Sun to glow in the visible. Now, if you’re asking “hot enough to produce a significant amount of visible light”, that’s possible, too. The water won’t be liquid any more, of course, but water molecules are observed in the upper layers of some of the cooler stars (those which are merely red-hot). As a matter of fact, water is one of the substances which can be heated the most, without breaking down: Mostly all of the molecules you’ll find in a star are simple things like water, carbon monoxide or dioxide, hydroxyl, and cyanide. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [August 30, 2007, 6:20pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/13 "2007-08-30T18:20:54Z")
Flame is plasma: heat released by combustion turns gas into ions. Send an alternating current through a flame, and it will flicker back and forth. ---
**Author:** ![SiXSwordS](https://avatars.discourse-cdn.com/v4/letter/s/49beb7/32.png) [@SiXSwordS](https://boards.straightdope.com/u/SiXSwordS)\ **Post date:** [August 30, 2007, 7:46pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/14 "2007-08-30T19:46:41Z")
…and yet the wise Master says: > [@](#): > > Ordinary fire isn’t plasma. ---
**Author:** ![bonzer](https://avatars.discourse-cdn.com/v4/letter/b/45deac/32.png) [@bonzer](https://boards.straightdope.com/u/bonzer)\ **Post date:** [August 30, 2007, 7:50pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/15 "2007-08-30T19:50:47Z")
[QUOTE=kopek] You may want to pick up a copy of The Chemical History of a Candle by Michael Faraday. Its been many years since I read that book but I think it may have some of the answers you are looking for. [/QUOTE] _The Chemical History of A Candle_ can be read [online](http://www.fordham.edu/halsall/mod/1860Faraday-candle.html). ---
**Author:** ![picunurse](https://avatars.discourse-cdn.com/v4/letter/p/7bcc69/32.png) [@picunurse](https://boards.straightdope.com/u/picunurse)\ **Post date:** [August 30, 2007, 9:03pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/16 "2007-08-30T21:03:20Z")
Fire is rapid oxidation of a given fuel. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 30, 2007, 9:43pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/17 "2007-08-30T21:43:07Z")
[QUOTE=Enola Straight] Flame is plasma: heat released by combustion turns gas into ions. Send an alternating current through a flame, and it will flicker back and forth. [/QUOTE] Thinking out loud here… If flames are conductive, then passing a current through them should generate a magnetic field, with which another magnetic field could interact so as to confine or move the other. I may be on the verge of inventing a candle-powered plasma beam weapon. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 30, 2007, 10:44pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/18 "2007-08-30T22:44:48Z")
[QUOTE=Mangetout] If flames are conductive, then passing a current through them should generate a magnetic field, with which another magnetic field could interact so as to confine or move the other. [/QUOTE] But that’s not the case, because as has been stated, ordinary fire is not a plasma. It’s not hot enough for the combustion gases to ionize. See [here](http://www-spof.gsfc.nasa.gov/Education/FAQs7.html#q97). ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 30, 2007, 11:20pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/19 "2007-08-30T23:20:48Z")
So, is **Enola Straight** fibbing about the effect of AC on a flame, or is there another effect at work? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [August 30, 2007, 11:38pm UTC](https://boards.straightdope.com/t/what-is-fire/417353/20 "2007-08-30T23:38:00Z")
[Flame rectification](http://www.fieldpiece.com/tech-articles-flame-rectifier.htm), a technique used in heating furnaces, among other applications, to detect if the burner is lit: > [@](#): > > Here’s the way it works. The controller applies alternating voltage between the flame sensing rod and the base of the flame (ground). The ions in the flame provide a high resistance current path between the two. Because the surface of the base flame is larger than the sensing flame rod, more electrons flow in one direction than the other. This results in a very small DC offset current. If there is a flame present, the DC offset is detected by the controller, which tells the gas valve to remain open. If there is no current flow, the controller will close the gas valve and the system will purge itself of any remnant gas before trying to re-ignite or lock-out. The DC offset is small, only in micro amps. It should be noted that the ions the article mentions are actually produced by the applied electric field, rather than the process of combustion itself. So, yeah, a flame is weakly conductive, but not because it’s a plasma. It can _become_ a plasma under the right conditions, though. Put a lit birthday candle in the microwave, and you’ll see. Use an oven you don’t care about–it will scorch the inside of the oven. [Next page](https://boards.straightdope.com/t/what-is-fire/417353.md?page=2)