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Are there substances that cannot undergo combustion? If so, what are they?

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    $\begingroup$ I think this is a bit to broad in its current form. Could you narrow down your scope a bit? Are you trying to choose a material for a particular application? $\endgroup$ – jonsca Jun 9 '13 at 11:23
  • $\begingroup$ Also, please consider using the "Contact Us" link below to merge your accounts together, as you keep creating new ones each time, that way you can consolidate all of your questions. Thanks! $\endgroup$ – jonsca Jun 9 '13 at 11:26
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    $\begingroup$ On burning "unburnable" substances: Sand won't save you this time $\endgroup$ – EnergyNumbers Jun 9 '13 at 13:16
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It depends a little on your definition of combustion.

Most combustion we see involves things reacting with the oxygen in air. Most combustion reactions are a self-sustaining exothermic reaction of some substance with oxygen gas. But atmospheric oxygen is relatively dilute (only about 21% of air is oxygen) and this limits what will burn sustainably in air (also, at slightly lower concentrations many commonly flammable things won't stay lit). The requirement for things to be combustible in air is, crudely, a reaction path with oxygen that releases energy.

Examples include many types of compound. Hydrogen reacts with oxygen to give water; hydrocarbons react to give carbon dioxide and water. Many metals react to give metal oxides (eg magnesium, spectacularly when lit; iron, only when hot and finely divided).

Many things won't burn in air for two reasons: there are activation barriers to a potential oxidation meaning a potential reaction can't be sustained (aluminium has a strong oxide layer on its surface protecting the reactive metal from further oxidation, for example); other compounds are already highly oxidised an no further oxidation can happen (eg silicon dioxide, a major ingredient of sand and glass).

But there are many more things that will combust in pure oxygen as Apollo 1 discovered, tragically. And as many demonstrations show spectacularly (see demo here). But highly oxidised things like silica can't be oxidised further and won't burn not matter how concentrated the oxygen is.

But oxygen isn't the only gas that can support exothermic reactions. Rather more things will burn in fluorine, for example. Even relatively unreactive substances can support combustion if there is an available reaction path that releases enough energy and a reaction can get started. One of the best known examples is that magnesium will burn in carbon dioxide (the reaction producing magnesium oxide releases energy) and even nitrogen (producing magnesium nitride). See the explanation here and the video here.

So, to return to the original question, are there any substances that won't combust? Whatever the definition of combustion is the answer is still yes. Noble gases, for example, though some can be coerced into reacting under extreme circumstances, never combust or support combustion.

If we restrict our definition to combust in air, many substances won't combust though many of them might do so under different circumstances.

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If one takes combustion to mean redox reaction then it is hard to see how lithium fluoride could take part in any further redox reactions since it is composed of strongest reducing agent and the strongest oxidising agent.

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If you define combustion as a chemical process that consumes $\ce{O2}$ and produces $\ce{H2O}$, then the answer is: any $\ce{X}$ that does not contain hydrogen is not combustible.

$\ce{X + O2 -> Y + H2O}$

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  • $\begingroup$ Even if we accept your definition of combustion, it does not follow that the hydrogens in water must come from the combustible material. They may be picked up from solution, for example (and often are). $\endgroup$ – tomd Jun 11 '13 at 22:14
  • $\begingroup$ If the solvent is providing H's to the reaction, then the solvent is actually part of X -- it is a reactant. So by my definition, the solvent better not have any H's (e.g. tetrachloromethane) $\endgroup$ – Eric Brown Jun 11 '13 at 23:25
  • $\begingroup$ Even if the solvent is water, as it is most (?all) biochemicl processes? I presume you agree that if X is an organic substance and the reaction you describe is done in water (at pH 7, say), water does not supply any electrons to oxygen, merely (it may be envisaged) a pair of protons, and that protonation (by definition) is not an oxidation (or a reduction). Water may be considered part of the reaction, but it does not in any sense combust, surely? $\endgroup$ – tomd Jun 12 '13 at 8:13
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    $\begingroup$ But you say: 'any X that does not contain hydrogen is not combustible'. What about, for example, oxalate (the dianion): can the carbon-carbon bond not be oxidized? Furthermore, in the (biochemical) combustion of glucose it is electrons 'held' in both carbon-carbon and carbon-hydrogen bonds that are passed (via the Respiratory Redox chain/Krebs Cycle/Glycolysis) to oxygen. I too don't want to get dragged down into a pedantic discussion, but from a biochemical standpoint, I think your final statement (quoted) is misleading... $\endgroup$ – tomd Jun 12 '13 at 11:59
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    $\begingroup$ @EricBrown can you support your definition of combustion as a reaction that consumes $\ce{O2}$ and produces $\ce{H2O}$? I think a more realistic definition is an exothermic reaction that involves a fuel and an oxidant. $\endgroup$ – bobthechemist Jul 9 '13 at 15:26

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