When we add fluoride ions to water, to make it good for teeth, it's called fluoridation. When we add chloride ions to water, to kill microorganisms, it's called chlorination. In the latter case, the generally used name for the process doesn't match with the technical/jargon description.

Why is this? Is it that chlorination is an older invention, so the lay word for it reflects an older technical jargon? Or is there some subtlety about the processes themselves?

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    Aren't they just based on different base forms, viz., fluoride vs. chlorine? Commented May 19, 2014 at 8:03
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    I remember enough chemistry from over 50 years ago to know that chloride is not the same thing as chlorine!
    – WS2
    Commented May 19, 2014 at 9:16
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    If chloride ions in water killed microorganisms, this would be bad news for plankton :-) Commented May 19, 2014 at 15:03

3 Answers 3


First, I wish to say this question should have been asked in the chemistry forum.

It is possible to have free un-ionised chlorine, the form of which is used for chlorination.

However, fluorine is too reactive to be found in its un-ionised state, hence fluoride. It is impossible to find fluorine at room temperatures. Therefore, it is impossible to fluorinate water. Moreover, it is the dynamics of fluoride ions, rather than fluorine, which mediate in calcium re-crystallization,

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    Agree >>>>>100% Commented May 19, 2014 at 8:19
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    Correction: It is not actually impossible but extremely inconvenient, expensive to sequester and contain fluorine gas. But it is still true that fluorination of water is an impossible and pointless feat. Commented May 19, 2014 at 8:25
  • Perhaps I'm just thinking about it too hard. In both cases, once the chemical is dissolved in the water, it's in an ionised form. Are you saying the difference is just because the 'original' chlorine is in its elemental form (i.e it's because of a chemical difference in the process, not a difference of etymology)?
    – Dan Hulme
    Commented May 19, 2014 at 10:55
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    @DanHulme: No, it's more clear-cut than that. In the case of fluoridation, it's the fluoride ions which are the active agent. In the case of chlorination, it's the chlorine gas, and the subsequently generated hypochlorite ions, which are the active agent. The by-product chloride ions do absolutely nothing, which is why it's not called chloridation. Commented May 19, 2014 at 12:09
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    @Anonymous - If it exists in unionized form, you can bet the Koch brothers are against it.
    – Erik Kowal
    Commented May 20, 2014 at 9:16

The simplest (and oldest) way to chlorinate water is to dissolve gaseous chlorine in it, though various chlorine-containing compounds such as calcium hypochlorite (also known as chloride of lime), sodium hypochlorite, or a chloramine can be used instead.

When you fluoridate water, you don't add gaseous fluorine to it (though if you did, then linguistically speaking, you would be fluorinating it -- however, in practice this is not feasible [see the comments above]); rather, you add a fluorine-containing compound to it.

The oldest method is by adding sodium fluoride (hence the term fluoridation), but today fluorosilicic acid or sodium fluorosilicate are also used.






drop the last "e" - add "ation" - done!

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    This is not a terribly helpful answer, since chloride and fluorine are also things. Commented May 19, 2014 at 16:48
  • The OP was asking about the lack of parallelism: the elements are chlorine and fluorine, both ending with -ine, and yet the processes are fluoridation (with a d) and chlorination (with an n).
    – bdesham
    Commented May 19, 2014 at 19:35
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    @bdesham but fluoridation has rather little to do with fluorine, but rather with fluoride. The reason for the lack of parallelism is because the base words they are derived from are not parallel, not because we are doing something different with those base words. Commented May 20, 2014 at 1:52

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