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According to this (and a bunch of other sources), NO (nitric oxide) is not combustible. But that same source says NO rapidly reacts with O2 to produce NO2, which I thought was the definition of combustion: being rapidly oxidized by oxygen. So why isn't it considered "combustible"?

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    $\begingroup$ You should search properly before asking. // See en.wikipedia.org/wiki/Combustion $\endgroup$
    – Poutnik
    Commented May 25 at 20:02
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    $\begingroup$ NO doesn't react nearly that rapidly. While it can be oxidised to NO2, it's prone to decompose to elements. $\endgroup$
    – Mithoron
    Commented May 25 at 20:44
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    $\begingroup$ Yes, so why aren't NOx species considered combustible according to Wikipedia? It doesn't explain. $\endgroup$
    – unstable
    Commented May 25 at 20:56
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    $\begingroup$ Use a good dictionary like Merriam Webster, can you add the definition of the word combustible in the question: merriam-webster.com/dictionary/combustible $\endgroup$
    – AChem
    Commented May 25 at 20:59
  • $\begingroup$ "a usually rapid chemical process (such as oxidation) that produces heat and usually light" According to this, NO qualifies, but I'm not sure Merriam-Webster is the best source for definitions of chemistry terms. $\endgroup$
    – unstable
    Commented May 25 at 21:10

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Combustion is general scientific term. The OP originally asked, what is meant by combustible. A typical dictionary definition is:

Capable of being burnt or consumed by fire, fit for burning, burnable.

So let us look at the detailed definition of combustion from an authentic source (behind paywall), the multivolume Oxford English Dictionary. It is actually better than many chemistry dictionaries.

Combustion:

The ordinary term in scientific use. As all ordinary combustion consists in the energetic combination of a body with oxygen, with evolution of heat and light, this was alone contemplated in earlier definitions of the term; but since it has been known that similar phenomena attend the combination of other elements, e.g. that of hydrogen and metals with chlorine, bromine, and iodine, combustion has been defined more generally as ‘The development of light and heat accompanying chemical combination’. internal combustion engine, etc.:

Formerly, oxygen was regarded as essentially the supporter of combustion, the bodies which burned in it being called combustibles. Afterwards the former term was extended to all substances capable of forming vapours in which others can burn, as chlorine, bromine, iodine, sulfur; but since the same substance may sometimes act in both capacities, and since the vapours in question are actually consumed in the process, as truly as the so-called ‘combustible’, the distinction has gradually become obsolete. (Watts, etc.)

If you want to be esoteric, $\ce{NO}$ may be termed as comburent. A comburent causes combustion of certain materials. Nitric oxide is not combustible in the sense that it will not combine with oxygen with the emission of visible light or large amount of heat. $\ce{NO}$ is instantly oxidized to a brown colored gas $\ce{NO2}$. One can further split hairs with relatively cold flames but this should be enough at this stage.

Does the fact that NO can act as an oxidizing agent have anything to do with it not being considered "combustible", or is the fact that NO doesn't release light or heat when reacted with oxygen like the quintessential combustion reaction the only reason?

Mostly the latter. It does not react with oxygen with the emission of light or significant amount of heat.

Note that molecules do not care what labels humans have given them.

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  • $\begingroup$ Thanks! Does the fact that NO can act as an oxidizing agent have anything to do with it not being considered "combustible", or is the fact that NO doesn't release light or heat when reacted with oxygen like the quintessential combustion reaction the only reason? $\endgroup$
    – unstable
    Commented May 25 at 23:05
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    $\begingroup$ The nitrogen oxides are all endothermic compounds with relatively small energy differences among them. Similar behavior is seen among higher oxidation states of many elements such as chlorine, manganese, iron , chromium . $\endgroup$
    – jimchmst
    Commented May 26 at 1:00
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    $\begingroup$ Combustible gases form a flame with oxygen, when ignited. NO does not, nor it can get ignited. $\endgroup$
    – Poutnik
    Commented May 26 at 10:42

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