Fire

Cecil's Columns/Staff Reports
1

I was surfing the web and came across your comment about fire, your answer was ok but maybe needs more. Fire is indeed nothing more than Oxidation, the binding of an Oxygen molecule with another molecule for instance Carbon or Hydrogen. This binding is a chemical process that releases heat the more rapid the process the more heat. Rusting metal is also an Oxidation process though it is much slower.

You will find that objects that burn (Oxidize) have a relationship with heat that determines how they burn, there is also factors such as pressure and percentage of available Oxygen.

lets talk about those temperatures, two points on the thermometer are important one is called the “fire point” and the other is called the “Flash point”, fire point is the temperature that an substance can be ignited by an external source of ignition such as a match, flash point is the temperature at which a substance will self ignite.

These temperatures are depended on pressure and percentage of Oxygen, usually these are rated at sea level with 20.9% O2, if you increase the percentage of O2 what happens is the flash point temperature lowers, if a high enough increase in 02 percentage is supplied to the substance that the flash point falls below the fire point temperature the substance will self ignite.

This varies of course with altitude due to the change in pressure though it is not a significant change, if you put this in a pressure vessel and raise or lower the pressure significantly then the required temperature will raise or lower accordingly.
One of my favorites is " Do you know why water won’t burn?" the answer is simple, water is already burnt; The Hydrogen molecule is already bound to Oxygen molecules, but if you separate them say through electrolysis then you can burn the Hydrogen. And on that note we all grew up learning water’s chemical make up as H 2 O but recent studies suggest it might actually be H 1.5 O and only H 2 O when water is changing state ie evaporating or condensing.
One other note, Oxygen is not the only gas that can cause fire, there are other substances that Oxidize if you will, such as Fluorine that will cause a fire even in the absence of Oxygen.

2

fire hot. what ?

3

Welcome to the Straight Dope Message Boards, Kurtis, we’re glad to have you with us. When you start a thread, it’s helpful to other readers if you provide a link to the column you’re talking about. Cecil has written lots about fire. Providing a link saves searching time and helps keep everyone on the same page. I assume you’re talking about What exactly is fire?.

No biggie, you’ll know for next time. If that’s not the one you meant, let me know by email or PM and I’ll be glad to put in the correct link.

You’d also be interested in Water contains hydrogen and oxygen, why doesn’t it burn?

4

Not really, no.

You description is simplistic and misleading. There are numerous reactions occurring in a typical wood fire, including many reduction reactions.

Saying that fire is nothing more than Oxidation is only accurate if you also accept that life is also nothing more than Oxidation. In both cases the statement is broadly true since the net reactions are always oxidative. However it is equally misleading in both cases.

5

Also, the definitions are wrong. The fire point, which is rarely used, is the temperature at which a material will ignite and continue to burn for at least five seconds. The flash point is a lower temperature, at which the material will ignite, but usually will not continue to burn (i.e., flash). The temperature at which a material will auto-ignite is, unsurprisingly, the autoignition temperature.