Making my way through old questions, I constantly wish that something regarding decomposition existed. It's not , it's not , and it definitely isn't . Sometimes it's hard to find ancillary tags when the main focus of the question is regarding the act of decomposition.

To be honest, I wish there was a single tag for each of the general reaction classes. For the sake of discussion, this is the one I'd want the most.

  • Counterargument: it will just serve as a dumping ground for old questions.

  • Another counterargument: Tag for dissolution process? gives . This is not really correct here, since the focus isn't usually around some precipitate, it's more general.

Potential candidates:

In these cases, I'm not making the argument that these are good questions, but that the present tags are poor because they are incomplete.

Here is it less of an issue, because the focus isn't on the decomposition itself:


I did initially have certain reservations about this tag. The best way I can phrase it is that decomposition isn't traditionally a concept or subfield in chemistry. In that sense, I mean that textbooks don't contain chapters specifically on decomposition reactions (do they?), and there aren't specialised books on decomposition (2-second Google search doesn't reveal any). A quick glance through our current tag list shows that most of our current tags don't have this problem. So its usefulness in helping people filter questions they like/don't like is probably not as good as other tags. See, for example, Q2 and Q9 on MAR's tag test.

Overall, though, I'd probably still be in favour of creating it. Reasons:

  1. Especially given that we already have , , , and , making a new tag does seem like a logical step to take.

    Most decomposition reactions can be described as internal redox. For example, the decomposition of hydrogen peroxide is a disproportionation, so technically would be applicable. However, simply lumping redox on them sort of misses the point IMO. e.g. the Teflon question is probably redox, but it makes little sense to describe it as such.

  2. As you said, it's not good when we have questions that don't have any appropriate tags.

  3. Its scope is well-defined: IUPAC Gold Book

  4. Regarding your counterarguments: since is a more specific tag than , so I don't see it becoming the same kind of indiscriminate dumping ground with 900+ questions. On top of that it doesn't suffer from the newbie "idk what tag to put, let's put reaction" issue. So in these respects it certainly outdoes its predecessor.

P/S I am not a fan of , though. As people learn more chemistry, they start to realise that there are more descriptive and specific terms to classify reactions: in this case, either acid-base, redox, or precipitation.

  • $\begingroup$ My choices are "No, Partially, Yes, No, No, No, Yes, Partially (it depends), Yes, Yes" to put it at 6. For myself, I would put it under ignored tags. $\endgroup$ Aug 14 '17 at 15:28
  • $\begingroup$ I do still think that questions whether the focus is on specific decomposition reactions rather than decomposition as a concept are often...lower quality. $\endgroup$ Aug 14 '17 at 15:30
  • $\begingroup$ Regarding that score, my take on it is NPYYY YYNNY for a score of 10. Then again it's just a crude test. // Re: question quality, I actually thought so too, which is why I initially asked for some examples. Those seem pretty ok to me, though. I'm not super interested in them, but they're within the scope of the site. My main gripe is that they're overly specific, but we don't have a close reason for that. $\endgroup$
    – orthocresol Mod
    Aug 14 '17 at 15:35

One possible problem with is that there's potentially a subjective element to its definition. In one situation a reaction might be a decomposition, while in another it's a desired transformation. I don't have the organic chops to come up with a really punchy example, but take ester hydrolysis as representative:

  • If one wants to work with a particular ester, say ethyl chloroacetate, but the conditions one needs for other reasons favor hydrolysis to ethanol and chloroacetic acid, then I'd say the ester is decomposing in that milieu.

  • If one specifically desires an ester to hydrolyse, though, e.g. when one is using it (if one ever would...) as a protecting group, then that hydrolysis is not decomposition but a specific reaction of interest.

So, in the end, I'm of divided opinion about it. On one hand, any given OP will have in mind whether they're inclined to call the reaction/process of interest a "decomposition", and could apply or not accordingly. On the other hand, it's not a term that constrains the topic to a particular type of chemical transformation, and thus it might be too hard to clearly define a useful scope for it.

A counter-question, possibly to help clarify our thinking: What would be a good tag excerpt for ? If we can come up with an excerpt that's satisfactory to a quorum of the community, it would be a big step toward demonstrating that the tag is worth having.

  • 1
    $\begingroup$ Esters are certainly used as protecting groups ;) I think decomposition is fairly well-defined, in terms of A -> B + C (+ ...) see IUPAC Gold Book link in my answer. Ester hydrolysis doesn't fall under that, as the reaction is ester + water -> carboxylic acid + alcohol. However, I can see what you're aiming for, and there is quite likely an example of such a process. Maybe the second step of the Balz–Schiemann reaction is a case in point. That's formally a decomposition but I wouldn't label it as such. I like your last paragraph, too. $\endgroup$
    – orthocresol Mod
    Aug 15 '17 at 5:20
  • $\begingroup$ @orthocresol Oh, interesting, I didn't realize there was a 'unimolecularity' aspect to the definition. Makes sense, though -- and it does provide for a reasonably non-ambiguous definition. $\endgroup$
    – hBy2Py
    Aug 15 '17 at 15:58

I object for the same reasons as orthocresol (that is, I share the "contra" part of his reasoning, but not the "pro"). Reaction classes aren't really a thing anyway. Most of these questions fall under all right. For the rest we probably should have a tag like "reaction-products" or something, and use it on new questions like "what happens if I mix such-and-such chemicals".


I used the MATT and this tag gets a score of 0. Questions 1,2,3,8,9,10 all are answered no; questions 4,7 are answered partially and questions 5,6 are answered yes. Based on this result, I'd say this isn't a good tag to have.

  • $\begingroup$ Like most things on the site, it's a subjective test and everyone can come up with different answers/reasoning. I think that considering some of the other tags on the site and their general misuse, this one is ok. $\endgroup$ Aug 26 '17 at 15:23
  • 2
    $\begingroup$ MATT is a decent start, but should never be used on its own to judge the viability of a tag. As you've probably already noticed, 3 people gave this tag scores of 0, 6, and 10. $\endgroup$
    – orthocresol Mod
    Aug 26 '17 at 16:30

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