Drinking water (e.g. from your home's tap) is spiked with chlorine, for an amount of just some ppm.
Purpose of this operation is to kill bacteria, for our health.

Background: chlorine in water is potentially dangerous for plants, or better, their soil.

A common solution to get rid of chlorine in tap water is leave it in an open pot all-night-long. In this way chlorine evaporates.

Now, in my mind, chlorine in dissolved in water as chloride ion ($\ce{Cl-}$). Just like salt in water.
And just salt in water, if you leave a pot of that mix in a room, after some time water evaporates completely and salt remains on the bottom of the pot.

Question is, how can chlorine evaporate from water?

  • $\begingroup$ And why do you think it shouldn't? $\endgroup$
    – Mithoron
    Sep 21, 2018 at 19:56
  • 1
    $\begingroup$ Because it has a higher vapor pressure than the water it's dissolved in? $\endgroup$
    – BillDOe
    Sep 21, 2018 at 20:08
  • $\begingroup$ I updated OP in the hope I well clarify question $\endgroup$
    – mattia.b89
    Sep 22, 2018 at 13:44
  • 2
    $\begingroup$ The chlorine in chlorinated water is NOT as chloride, it as molecular chlorine - a different oxidation state to chloride. $\endgroup$
    – Waylander
    Sep 22, 2018 at 16:09
  • 3
    $\begingroup$ @Waylander You have highlighted the key point. But the actual chemicals in the water will be a mixture of things like hypochlorite as the chlorine reacts with hydroxide in a complicated equilibrium. But the key part is that it isn't chloride. $\endgroup$
    – matt_black
    Sep 23, 2018 at 10:53

1 Answer 1


Chlorine and Free Chlorine

Chlorine is the term commonly used to describe the addition of chlorine gas, sodium hypochlorite (i.e., bleach), or calcium hypochlorite (i.e., bleach powder) to water to form free chlorine. Free Chlorine is the sum of hypochlorous acid ($\ce{HOCl}$) and hypochlorite ($\ce{OCl-}$); a monoprotic acid with a $\ce{pK_a} = 7.54$.

Chlorine gas ($\ce{Cl2}$) does not exist in water (except highly acidic) as it hydrolyzes to $\ce{HOCl}$. Chloride ($\ce{Cl-}$) forms from the redox reaction of $\ce{HOCl}$/$\ce{OCl-}$, but it is not reactive.

Both $\ce{HOCl}$ and $\ce{OCl-}$ are volatile, with $\ce{HOCl}$ being $\approx\!5\!\times$ greater (Henry's solubility constant being $H = 0.076$ and $0.013$, respectively).$^{[\text{citation needed}]}$ When you open a pitcher of water containing free chlorine to the atmosphere, the free chlorine comes into equilibrium with the atmosphere and thus it volatilizes. As such, you do not lose "$100\%$ or all" of the free chlorine, but enough is removed that the concentrations drops below your taste threshold.

Side Note

If the water contains dissolved organic carbon and organic nitrogen (e.g., natural organic matter, pathogens), then the free chlorine could be reacting while it is sitting and thus you could be losing free chlorine this way as well. Further you can also add combined chlorine (e.g., monochloramine) to water in place of free chlorine. This also forms from free chlorine reaction with nitrogen compounds in water (e.g., urine) and is the common "pool smell" observed in swimming pools that are not properly chlorinated.


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