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Chlorine evaporation is an overnight process from standing tap water in a gallon pan. Does fluoride evaporate in the same way/rate?

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migrated from skeptics.stackexchange.com Jan 28 '17 at 20:22

This question came from our site for scientific skepticism.

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    $\begingroup$ There's crucial difference between fluorine and fluoride. $\endgroup$ – Mithoron Jan 28 '17 at 20:27
  • $\begingroup$ Were these downvotes before or after migration? Seems like a good question to me. $\endgroup$ – fredsbend Jan 28 '17 at 23:18
  • $\begingroup$ And this whole -ine vs. -ide thing is pedantic. Clearly, the asker means the vernacular usages. "Chlorine" and "fluoride" are added to drinking water. How do they compare when using evaporating techniques? I sure hope the downvotes are not because of this. $\endgroup$ – fredsbend Jan 28 '17 at 23:20
  • $\begingroup$ @fredsbend Chlorination of water is often by adding actual chlorine gas ($\ce{Cl2}$), whereas fluorination of water can be actual sodium fluoride (NaF) which is a solid. NaF has no chance of evaporating unless you heat it to over 1000 degrees C. $\ce{Cl2}$ leaves the water easily like carbon dioxide coming out of soda. $\endgroup$ – DavePhD Jan 30 '17 at 19:45
  • $\begingroup$ @DavePhD Considering how the question was asked, I think that's exactly the information needed in an answer. The OP is clearly unaware of this. My complaint is if the downvotes are a result of the OP's apparent ignorance regarding what "added chlorine" and "added fluoride" mean chemically. $\endgroup$ – fredsbend Jan 30 '17 at 20:00
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Short answer: no, fluoride does not evaporate in the same way/rate as chlorine.

Longer answer: chlorine will be present as covalent, molecular $\ce{Cl2}$. This is, when not in solution, a gas. Chlorine molecules leaving the surface of the water will (on average) not come back (at least unless there is a very high concentration of $\ce{Cl2}$ in the room's air, in which case you have other problems).

Fluoride, on the other hand, is present as $\ce{F-}$ ions (or other, complex ions) along with corresponding positively charged ions (usually $\ce{Na+}$, I believe). Even if the $\ce{NaF}$ were to be removed from solution, $\ce{NaF}$ is a non-volatile salt, and so would not evaporate away.

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  • $\begingroup$ An imperfect answer because I slightly gloss over how ions and non-ionic molecules behave differently going into / coming out of solution. Suggestions and edits are very welcome! $\endgroup$ – owjburnham Jan 29 '17 at 0:04
  • $\begingroup$ Considering how the question was asked, I think you answered it perfectly. If the OP wants to know more about the differences between dissolved gases and dissolved salts, then he can look it up or ask a different question. $\endgroup$ – fredsbend Jan 30 '17 at 19:57

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