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

startanew
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3 Answers3

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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.

matt_black
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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.

Daniel Mahler
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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.

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

Eric Brown
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  • 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). – tomd Jun 11 '13 at 22:14
  • 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) – Eric Brown Jun 11 '13 at 23:25
  • 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? – tomd Jun 12 '13 at 08:13
  • @TomD I really don't care where the matter comes from. If the matter appears in the product, then it was a reactant (by my own definition). The combustion question itself is a bit "unanswerable" as most species can be oxidized by something. I have used water as the necessary product because this is what "average" chemists think of when they think of combustion processes (colloquialism), and I would rather not get dragged into some legalistic discussion on precisely what constitutes combustion. – Eric Brown Jun 12 '13 at 11:14
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    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... – tomd Jun 12 '13 at 11:59
  • ... Again from a biochemical standpoint, in general any X that contains a C-C or a C-H bond is potentially oxidizable (and may be used to 'make' ATP). – tomd Jun 12 '13 at 12:00
  • @TomD My equation is deliberately set up so that there is no hydrogen source, therefore no hydrogen to make water, and therefore no combustion (colloquial definition) is possible. Perhaps you would like to take a stab at answering the original question. Is there a molecule that does not combust? Can you give an example, or is it your opinion that all molecules can undergo combustion? – Eric Brown Jun 12 '13 at 12:14
  • let us continue this discussion in chat – tomd Jun 12 '13 at 16:57
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    @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. – bobthechemist Jul 09 '13 at 15:26
  • @bobthechemist I don't think my definition is out of line in a colloquial sense of "combustion." You are correct that combustion (proper) is anything that can be oxidized. But since anything can be oxidized by something, the legalistic answer to the question is "there are no examples." But I don't think that e.g., graphite is combustible (in a colloquial sense)--one reason being that there is no driving force by way of hydrogens to form water. BTW I am aware of $\ce{C(s)+ O2 --> CO2}$ but this doesn't seem very much like combustion ala a combustion engine. – Eric Brown Jul 09 '13 at 16:01
  • Nobody defines combustion that way and for good reason. Charcoal, for example, doesn't contain hydrogen but is quite effectively combusted in air. Otherwise barbeques would not be possible. – matt_black Mar 06 '20 at 23:09
  • @matt_black what is your definition of combustion? Before slamming mine, show me yours. (Mine is an attempt to cleverly answer a question in a way without exceptions.) – Eric Brown Mar 06 '20 at 23:14
  • Combustion is usefully defined as a sustainable exothermic reaction. This has the benefit of allowing the component substances to be anything you want to include but also of allowing the environment for the combustion reaction to be relevant. Many things we think of as combustible in air (such as paper) cannot sustain combustion if the oxygen level is too low (say <15%). Almost everything is combustible in chlorine trifluoride including concrete, rocket test engineers and asbestos. – matt_black Mar 06 '20 at 23:19
  • Is the exothermic association of O2 with a transition metal a combustion? – Eric Brown Mar 06 '20 at 23:21
  • @EricBrown Given what I said in the previous comment, it should be obvious why your definition is a poor one. Not every combustion reaction uses oxygen; and there are plenty of oxygen-consuming reactions that do not need the presence of any hydrogen. – matt_black Mar 06 '20 at 23:22
  • It is a constructed one to use to answer a question couched in absolutes. And it’s actually the definition that is taught from elementary school through university. – Eric Brown Mar 06 '20 at 23:22
  • @EricBrown Your constructed definition is useless because there are plenty of well-known examples of things we call combustion that clearly don't fit. Finely divided iron (a transition metal) will quite happily burn in air. Why would anyone not call that combustion? – matt_black Mar 06 '20 at 23:25
  • I would not call that combustion in the everyday parlance, I would call that reactivity. – Eric Brown Mar 06 '20 at 23:26
  • If you were taught this definition, you had poor teachers. And I have already given enough counterexamples for you to judge this for yourself. – matt_black Mar 06 '20 at 23:27
  • You have not written a generalized reaction that describes combustion uniquely. Please write it out so we can scrutinize it. – Eric Brown Mar 06 '20 at 23:29
  • And you have not written a definition that covers all the various things normally grouped together as "combustion". Exothermic, self-sustaining reactions that produce light might be both general enough and restrictive enough to cover the common use of term in chemistry and life. – matt_black Mar 06 '20 at 23:33
  • You’re just throwing out ad hominems and not providing a definition. My answer starts out “If you define combustion...”. Your most recent definition of combustion seems to include every single exothermic reaction possible. – Eric Brown Mar 06 '20 at 23:36
  • PS: http://web.fscj.edu/Milczanowski/psc/lect/Ch11/slide3.htm (First hit on my Google searching for definition of "combustion") Perhaps you want to write this professor and tell him/her that s/he is poor and his teachers were poor. (And tell Brown et al. who wrote perhaps the most used textbook in the chemical world that they are poor teachers for using the same definition that I invoked.) – Eric Brown Mar 07 '20 at 01:07