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One common recommendation for watering house plants with tap water is to allow the water to sit overnight before watering, so that the fluorine and chlorine that are commonly added to tap water (which can be harmful to certain plants) will dissipate. I've seen this advice repeated many times, but never an original source or an explanation of the process behind it. So, does this actually work? And if so, how?

Example of claim, for fluoride, found here:

Tap water is acceptable for watering most plants. Some plants are susceptible to fluoride injury from treated water. Many susceptible plants have long slender leaves such as dracaena and spider plant. Injury is characterized by brown spots along the margin or leaf tip. Fluoridated water should be allowed to sit at room temperature over night before using. Potting soils containing perlite can also cause fluoride injury.

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Chlorine does evaporate, so if exposed to air (e.g. in a bucket) in warm water (especially under UV light or sunlight) it will probably mostly dissipate overnight.

Both fluoride and chloramine will not similarly dissipate, if you want them removed you need to filter them out (e.g. with activated carbon) or distill the water.

See:

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    I believe this answer is factually correct, but can you provide some more authoritative sources? about.com isn't really know for their scientific literature :)
    – Flimzy
    Jul 4, 2013 at 8:39
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    These links may be useful, they report also the appropriate chemical reactions. gewater.com/handbook/cooling_water_systems/ch_27_chlorine.jsp safewater.org/PDFS/resourcesknowthefacts/WhatisChlorination.pdf Finally, note that although Cl2 (in its liquid form) is used for water chlorination it is not the only option. Calcium hypochlorite, Ca(OCl)2 and sodium hypochlorite (NaOCl) are also used.
    – nico
    Jul 4, 2013 at 9:34
  • Probably mostly? I understand if you don't want to use absolutes, but could you clarify what you mean by "probably mostly"?
    – user5582
    Jul 4, 2013 at 15:14
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    @Sancho - there are many variables here (temperature, volume, surface area, atmospheric content, other water content, amount of chlorine dissolved in the water, UV radiation, flow, etc.) and the nature of the reactions involved is not linear (hence the amount of chlorine would never reach zero). The definitions involved are also a bit fluid - since water also evaporates, and since dissolved salt is technically half chlorine as well.
    – Ofir
    Jul 7, 2013 at 7:35

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