# why are some substances flammable and others not?

Discussion in 'Chemistry' started by visceral_instinct, Feb 6, 2009.

1. ### visceral_instinctMonkey see, monkey denigrateValued Senior Member

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What determines whether a substance can be ignited or not?

3. ### leopoldValued Senior Member

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substances with low vapor pressures are easily ignited.
there is a difference between ignition and detonation.
ignition implies a rearrangement with an oxidizer, usually oxygen.
detonation is a spontaneous rearrangement into a more stable structure, nitrogen based explosives fall into this category.

5. ### TrippyALEA IACTA ESTStaff Member

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All it really means is that the reaction is exothermic, and the energy released by the reaction occuring is more than the activation energy of that reaction.

In terms of classification, the UN GHS classifies flammability on the basis of flash point.

7. ### RomanBannedBanned

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Certain structures are more stable than others in typical earth conditions. Less stable structures are more easily turned into more stable structures, typically with the release of heat. Methane, for example, is CH4, but is more stable as carbon dioxide and water. There also happens to be a lot of oxygen present, which makes it quite easy for us to turn it into CO2 & H2O. If the atmosphere was composed of only nitrogen, and oxygen was very rare, we would not consider CH4 flammable, since it would be difficult to get it to undergo any reactions. If the atmosphere was mostly fluorine gas, CH4 would be even more reactive, and would spontaneously turn into CF4 (only a little energy is needed- 25°C or light, to generate radicals), since fluorine is more electronegative (and thus more reactive) than oxygen.

Hope that helps.

If I'm wrong about spontaneous formation of carbon tetrafluoride, could a real chemist correct me?

8. ### NasorValued Senior Member

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As others have sort of said, it's a combination of reactivity and vapor pressure. Something will be more flammable if their vapor burns more exothermically when combined with oxygen, and also if the vapor pressure is high (because there will be more vapor around).

9. ### ElectricFetusSanity going, going, goneValued Senior Member

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We live in an atmosphere of oxygen, things get oxygenated, if they are already oxygenated than its hard or impossible to oxygenate them more. Now this grossly simplifying it (at the very least it does not cover halogenation, nor are we talking about fire specifically) but if chemical A does not have oxygen in its equations that's usually a sign it can be oxygenated, if it does than that means its going to be harder to attach more oxygen.

10. ### visceral_instinctMonkey see, monkey denigrateValued Senior Member

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Thanks for replies!

11. ### cosmictravelerBe kind to yourself always.Valued Senior Member

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Usually if you pee on it, it usually won't fire up, so don't pee on stuff.

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13. ### S.A.M.uniquely dreadfulValued Senior Member

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The ignition point, which is the lowest temperature at which something will combust. Combustion itself is a process that requires an oxidant acting on a "fuel source" that produces radical intermediates in the presence of atmospheric oxygen, leading to an increase in temperature. This acts as a positive feedback, in which the increase in temperature causes a change in the products, further increasing heat, in a cyclical fashion, without the need to supply any more external heat.

14. ### superluminal.Registered Senior Member

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The whim of the god of fire, Ignitotron (yes, he's a robot god with a built-in flame-thrower-of-death).

15. ### dixonmasseyValued Senior Member

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Quantitatively, change of Gibbs potential, G=H-TS, is used to estimate possibility of a chemical reaction under given conditions.

To compress all thermodynamics in a single sentence, all processes in the world go in the direction of maximum energy dispersal.

Therefore, if combining a substance X with oxygen results in a substance XyOx +/- heat, it means that energy contained in the reacted amounts of X and O2 was more "concentrated" than energy of chemical bonds of XyOx +/- dispersed heat.

Concentrated means - "total amount energy of all chemical bonds" divided "total number of bonds/energy levels". As a chemical reaction progresses heat is being released (exothermic) and energy is being absorbed by zillions of atoms of environment (energy disperses). Thus, total energy of chemical bonds decreases, the number of bonds changes too.

It's little bit convoluted, but the main point is that for a reaction to proceed spontaneously energy of chemical bonds must disperse in the process.

Last edited: Feb 14, 2009
16. ### Dr MabusePercipient ThaumaturgistRegistered Senior Member

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Everything is flammable.

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Even water?

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Yep.

19. ### superluminal.Registered Senior Member

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You should see what happens when you throw water on a burning hunk of magnesium.

In fairness though, the water isn't burning. It's being dissociated by the extreme heat of the burning Mg and then explosively recombining into H2O. Umm... Right?

20. ### TrippyALEA IACTA ESTStaff Member

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No, the water reacts directly with the heated magnesium to form Magnesium Hydroxide and hydrogen gas, and the hydrogen gas then reacts with the air.

$Mg + 2H_2O \rightarrow Mg(OH)_2 + H_2$

21. ### Fraggle RockerStaff Member

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And then

$2H_2 + O_2 \rightarrow 2H_2O$

It looks like half of the water molecules serve as catalysts, since they exist at both the start and end of the cycle of reactions? And the hydrogen is merely an intermediate result that quickly vanishes. So, ultimately (and rather quickly I assume)...

$2Mg + 4H_2O + O_2 \rightarrow 2Mg(OH)_2 + 2H_2O$

Last edited: Feb 27, 2009
22. ### TrippyALEA IACTA ESTStaff Member

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Yup.

But I was only really addressing the assertion that the Hydrogen was produced by the heat of the fire - it's not, it's produced by a direct reaction with the Magnesium, but yes, the hydrogen then goes on to ignite and cause further problems.