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If I am not mistaken, distilling water does not remove all possible impurities. Chemicals with boiling points less than $100\ \mathrm{^\circ C}$ will be transferred as well. Does the usual tap water contain such (I think there is at least fluoride.)? I read that activated carbon can be used to purify water as well. How is it done in practice? Suppose I first use activated carbon and then distill the water (or in the reverse order). How much impurities there would be still left?

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  • $\begingroup$ Welcome to chemistry.SE! If you had any questions about the policies of our community, you can ‎visit the help center or take a ‎‎tour of the website.‎ $\endgroup$ – M.A.R. Feb 1 '15 at 20:25
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    $\begingroup$ Don't confuse the fluoride anion, $\ce{F-}$ with fluorine $\ce{F2}$, which is a gas under standard conditions. $\endgroup$ – Klaus-Dieter Warzecha Feb 1 '15 at 20:38
  • $\begingroup$ Correct. Fluoride ion is what that's in tap water and is helpful for maintaining teeth health. Plus, it's REALLY hard (in an uppercase sense) to make something 100 percent pure. $\endgroup$ – M.A.R. Feb 1 '15 at 20:47
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The main reason activated carbon is used in water treatment is that its large surface area is useful for adsorbing volatile organic contaminants and chlorine. It is not effective at removing inorganic ions (dissolved minerals, including fluoride), but these are removed quite well by distillation. I think distillation first would make more sense—distillation will remove less volatile contaminants so that less has to be adsorbed by the carbon.

Tap water has to meet certain standards (that depend on your area) for dissolved minerals and volatile organic compounds, so depending on the end use of the water, removing the small amount of remaining contaminants may not be necessary. Formerly, double-distilled water was the standard for making highly purified water, but certain applications are more or less tolerant of certain types of contaminants, and double-distillation requires a lot of energy, so it has largely been replaced for many purposes.

Consumer water filters usually contain filters to remove particulates, ion exchange resin to remove inorganic ions, and activated carbon to improve the taste of the water, and as far as purification for laboratory use, combinations of filtration, deionization, reverse osmosis, ultrafiltration, etc. are common these days. Different processes are better at removing some types of contaminants than others, so combinations are used. Distillation of tap water followed by treatment by activated carbon will produce quite pure water by consumer standards, but might not be pure enough for say semiconductor microfabrication or biomedical applications.

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Activated Carbon is used in water filter purifiers because activated carbon removes from the water most toxic organic compounds in water like pesticides and heavy metal organic compounds.

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