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Author Topic: Is fire a gas or not?  (Read 12866 times)

Offline evan_au

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Re: Is fire a gas or not?
« Reply #25 on: 12/12/2014 23:50:06 »
Quote from: jeffreyH
The flame moves away from a gravitational source but does a spark?
A flame rises because the hot gas is less dense than the surrounding cool air, and the "lowest energy state" is with the denser air closer to the center of the Earth.

A spark is a glowing piece of iron or magnesium, which is more dense than cool air, so I expect that a spark will fall in a parabola towards the ground (if it doesn't burn out first).
Quote from: PmbPhy
small grey/blackish balls of some sort of burned out material
  • Candle wax is a hydrocarbon, whose products of combustion are CO2 and water vapor (plus a bit of carbon soot, carried away by convection)
  • Wood has a more complex composition; in addition to the volatile products produced by a candle, it leaves behind a small amount of ash, consisting of non-volatile compounds of Potassium, Sodium, Copper, etc. But it is undoubtedly a "fire".
  • A Sparkler produces a variety of gaseous compounds, but the spark remnants are non-volatile compounds of iron or magnesium. Does the large amount of "ash" disqualify a spark from being a fire? 
 

Offline yor_on

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Re: Is fire a gas or not?
« Reply #26 on: 13/12/2014 12:53:06 »
Fire is a result of transformations, interacting. It's also a temperature. Temperatures is also what seems to create 'emergences'. So, can you give me a 'emergence' without a temperature being involved?
=

The point is, I think I can argue at least one, myself.
(Sometimes you find it defined as different 'regimes', cosmologically.)
« Last Edit: 13/12/2014 12:58:45 by yor_on »
 

Online jeffreyH

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Re: Is fire a gas or not?
« Reply #27 on: 13/12/2014 14:35:41 »
Quote from: jeffreyH
The flame moves away from a gravitational source but does a spark?
A flame rises because the hot gas is less dense than the surrounding cool air, and the "lowest energy state" is with the denser air closer to the center of the Earth.

A spark is a glowing piece of iron or magnesium, which is more dense than cool air, so I expect that a spark will fall in a parabola towards the ground (if it doesn't burn out first).
Quote from: PmbPhy
small grey/blackish balls of some sort of burned out material
  • Candle wax is a hydrocarbon, whose products of combustion are CO2 and water vapor (plus a bit of carbon soot, carried away by convection)
  • Wood has a more complex composition; in addition to the volatile products produced by a candle, it leaves behind a small amount of ash, consisting of non-volatile compounds of Potassium, Sodium, Copper, etc. But it is undoubtedly a "fire".
  • A Sparkler produces a variety of gaseous compounds, but the spark remnants are non-volatile compounds of iron or magnesium. Does the large amount of "ash" disqualify a spark from being a fire? 

With a higher energy comes an increase in kinetic energy. If we start a flame in a freefall orbit we get a spherical flame even though there will be a gas in the spacecraft. To simply state this as buoyancy due to density is an oversimplification. Where is the buoyancy/density relationship in freefall? Just to add to this a little. What if we were to ignite a flame inside a sealed container. If we were to apply a force throughout this container such that any gas density variations were evened out would the flame then become spherical or would it still move upwards? This is not a trivial point to make. If it is simply a density relationship with buoyancy being a factor then we will have removed this relationship from the experiment. In which case gravity would be the only factor under consideration.
« Last Edit: 13/12/2014 15:13:43 by jeffreyH »
 

Offline Bored chemist

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Re: Is fire a gas or not?
« Reply #28 on: 13/12/2014 21:13:05 »
Fire isn't a substance, it's a process.
 

Offline David Cooper

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Re: Is fire a gas or not?
« Reply #29 on: 13/12/2014 22:45:05 »
When energy is released in our muscle cells, is that the same process as burning? Could it be described as a fire?
 

Offline chiralSPO

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Re: Is fire a gas or not?
« Reply #30 on: 13/12/2014 22:51:08 »
I don't think metabolism would count as fire. Fire is typically a self-sustaining, uncontrolled chain reaction (even if the fuel/air ratio is very carefully controlled, there is no control at a molecular level). Metabolism, in muscles or elsewhere, is very tightly regulated cellular process in which enzymes oxidize organic molecules and direct that energy into converting ADP to ATP, or enzymes can convert ATP to ADP and use that energy to do other things.
 

Offline evan_au

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Re: Is fire a gas or not?
« Reply #31 on: 14/12/2014 05:56:23 »
Quote from: jeffreyH
If we were to apply a force throughout this container such that any gas density variations were evened out would the flame then become spherical or would it still move upwards?
We could eliminate the effects of gravity on a flame inside a sealed container by dropping the container in a very long vacuum tube.
I expect that the flame would become spherical.
 

Offline Bored chemist

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Re: Is fire a gas or not?
« Reply #32 on: 14/12/2014 16:52:32 »
 

Offline yor_on

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Re: Is fire a gas or not?
« Reply #33 on: 19/12/2014 20:25:59 »
I've seen it defined as 'burning' Chiral? Maybe oxidation would be more to the point though?
=

I also have a weak memory of reading of 'two dimensional' fires acting quite strange, looking at the cool link above :). Think they just 'sandwiched' the fire, maybe that too was in a weightless environment?
« Last Edit: 19/12/2014 20:30:00 by yor_on »
 

Offline chiralSPO

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Re: Is fire a gas or not?
« Reply #34 on: 03/01/2015 17:57:28 »
I've seen it defined as 'burning' Chiral? Maybe oxidation would be more to the point though?


There are plenty of oxidation (redox) processes that don't involve combustion, burning or fire: batteries, fuel cells, metabolism, browning apples, rusting ships etc.

I am hard pressed to think of a reaction that could qualify as burning without involving redox, but that doesn't mean it's impossible. I can think of a few cases of "explosive polymerization" (involving ethylene oxide or acetylene or similar compounds) which could potentially release enough energy to reach incandescence without involving any redox processes, and also perhaps exothermic decomposition of ozone, but these are special cases...
 

Offline evan_au

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Re: Is fire a gas or not?
« Reply #35 on: 04/01/2015 05:37:40 »
Quote from: yor_on
oxidation processes that don't involve combustion, burning or fire: ... rusting ships, etc
Rusting is an oxidation process that occurs very slowly due to Arrhenius' equation: reactions get exponentially faster as temperature rises. If you put steel in a fire, it rusts very quickly.

If you get iron hot enough (eg finely divided steel wool in a Bunsen burner), it burns quite well. With a stronger oxidiser, steel wool burns at room temperature (eg steel wool in fluorine - start video at 2 minutes 45 seconds).

What defines a fire in common usage is that the temperature produced by the reaction is high enough to emit light and heat*. If the temperature is high enough, any hot substance will emit both heat and light through black-body radiation.

To get bulk steel hot enough to sustain a flame in atmospheric oxygen, you have to get it red-hot anyway, by which time it is already emitting light and heat from the external heat source. (Injection of pure oxygen bubbles into molten steel during refining provides much larger surface area than you have with bulk steel.)

So I would say that an oxidation reaction could be called fire if the reaction is self-sustaining, and is exothermic enough to emit heat and visible light much brighter than the surrounding environment.

*Room-temperature bioluminescence in fireflies fits the light-emitting part of the description, but does not conform to our expectation that a fire will also emit lots of heat.
« Last Edit: 04/01/2015 05:40:04 by evan_au »
 

Offline lightarrow

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Re: Is fire a gas or not?
« Reply #36 on: 05/01/2015 00:38:35 »
I've seen it defined as 'burning' Chiral? Maybe oxidation would be more to the point though?

I am hard pressed to think of a reaction that could qualify as burning without involving redox, but that doesn't mean it's impossible. I can think of a few cases of "explosive polymerization" (involving ethylene oxide or acetylene or similar compounds) which could potentially release enough energy to reach incandescence without involving any redox processes, and also perhaps exothermic decomposition of ozone, but these are special cases...
Write "aluminium + palladium" in a search engine and choose "video".
When you'll find a video of that intermetallic reaction, you'll be amazed 😊

--
lightarrow
 

Offline chiralSPO

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Re: Is fire a gas or not?
« Reply #37 on: 05/01/2015 15:27:51 »
Yes this reaction is quite impressive. I'm not entirely convinced that the surrounding air didn't contribute to the effect, but the reaction between Pd and Al is apparently exothermic enough on its own, that it probably would have "burnt." This isn't really a redox neutral process, though. Pd is electronegative enough that it could well have oxidized the aluminum (maybe not quite so far as to produce an ionic compound, but the bonding character between aluminum and noble metals is not entirely metallic either--I know gold and aluminum will react to form white or purple compounds that are very poor conductors, which is why Au-Al contacts are typically avoided in circuits.)

Thanks for link!
 

Offline lightarrow

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Re: Is fire a gas or not?
« Reply #38 on: 05/01/2015 16:41:48 »
Yes this reaction is quite impressive. I'm not entirely convinced that the surrounding air didn't contribute to the effect,
Here it says no, but who knows... :
http://en.wikipedia.org/wiki/Pyrotechnic_initiator
(see paragraph "Intermetallic", subparagraph "Palladium-aluminium")
Quote
but the reaction between Pd and Al is apparently exothermic enough on its own, that it probably would have "burnt." This isn't really a redox neutral process, though. Pd is electronegative enough that it could well have oxidized the aluminum (maybe not quite so far as to produce an ionic compound, but the bonding character between aluminum and noble metals is not entirely metallic either--I know gold and aluminum will react to form white or purple compounds that are very poor conductors, which is why Au-Al contacts are typically avoided in circuits.)
You are probably right, I didn't know of such compounds between Au and Al; so it's likely that between Pd and Al similar compounds forms.

--
lightarrow
 

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Re: Is fire a gas or not?
« Reply #38 on: 05/01/2015 16:41:48 »

 

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