Will fluoride and chlorine dissipate from tap water left to sit overnight? 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.

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:

* Can You Remove Fluoride By Boiling Water?
* What You Must Absolutely Know About Water