Question about the temp of fire?

[Kravjitsu]

My girlfriend has been asking me for awhile now about what the coldest temperature fire can be sustained at? Im not a chemist of anything but i figured this would be the right forum to ask it on. She thinks it paper burning at 451 degrees but i have no idea. Any help would be appreciated

[iNow]

There are many chemists that frequent this site whom I'm sure will have a more clear answer to offer you. I'm not one of them. However, I seem to recall something about different substances burning at different temperatures. That would mean that some things burn hotter and others burn colder (relative to one another).

 

So, a question back to you (and your girlfriend) is "a fire which is burning what?" Good luck.

[insane_alien]

the temperature of a fire depends a lot on what is burning. also the conditions it is burning in.

 

what you'd want is a combustion reaction with a Heat of vapourization to heat of combustion ratio of approximately one(it will need to be in favour of the heat of combustion to be sustainable) and something with a high vapour pressure.

 

it should also be burning in a pure oxygen enviroment, possibly with low pressure (depends on the stoichiometry of the reaction) and the flame should be kept as small as possible.

 

with tweaking i don't see why you couldn't get a self sustaining flame below 0*C though the conditions would be incredibly far from normal atmospheric conditions.

 

the correct answer to this is: it depends.

 

also, paper will START burning at 451*F this is its ignition temperature. once you have it burning it will burn hotter as the reaction is dumping energy into it. this is the same with all combustibles. they have an ignition temperature and a general maximum temperature(that depends on conditions).

[chemkid]

What would you like to burn a substance in is also a factor. For example, many things will burn in elemental flourine however very few will burn in nitrogen. Also, 'fire' isn't easily defined, perhpas oxidation (if you want to burn things in oxygen) Another factor is how much of a material you are burning. As stated before, what temperature are you looking at? Auto ignition point? In what percent oxygen? If you want to use autoignition point what form of the substance are you using? Powdered magnesium can ignite spntaneously at room temperture but a lump of magnesium would never light at room temperature.

A rather complicated question

[Kravjitsu]

Thanks a lot guys, i know that different substances burn at different temps but i guess she just wanted to know what is the coldest fire? I know it sounds stupid and it might very well be but i think shes talking about the fire you can see thats red yellow and orange, whats the coldest that could be? This is for any substance and probably what ever conditions that would make it coldest.

 

Thanks again the help is appreciated

[Mr.Chockuls]

burning kno3 is no equal to burning a simple piece of paper.

[CaptainPanic]

All information has basically been given already. But I'll still add my reply.

 

I use the following definition of a fire: A self-sustaining reaction of a fuel and oxygen (from air). As stated in many schoolbooks, fire needs 3 components: oxygen, fuel and the right temperature.

 

Could we perhaps state that the coldest fire is the fire which has the lowest autoignition point?

 

Wikipedia states for "autoignition temperature": "The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark."

 

Wikipedia also lists a number of substances and their autoignition points.

 

It seems that Triethylborane autoignites at -20 °C (-4 °F). Assuming you are able to remove all heat of reaction by any means, the reaction (fire) would keep going at -20 °C. (If you don't remove the heat with some cooling-device, it will rapidly get hotter and hotter...).

 

Note that if you're able to keep the temperature low, this kind of fire probably doesn't have any light-effects.

 

p.s. (1) A catalyst may lower the ignition temperature a LOT, and I didn't take this into account. I consider a catalyst "cheating" in this question.

p.s. (2) This Triethylborane will probably autoignite at an even lower temperature in pure oxygen or at higher pressure... so changing the definition of "fire" also changes the answer.

p.s. (3) There may be exotic chemicals with an even lower autoignition point. I didn't look further than wikipedia.

[Mr Skeptic]

If you allow for just any kind of flame, you can make one that you can hold your hand in without getting burned. But if you want to talk about burning a carbon based object in regular atmosphere, it will have to be quite hot.