# How does chlorine in water evaporate?

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.

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?

• And why do you think it shouldn't? – Mithoron Sep 21 '18 at 19:56
• Because it has a higher vapor pressure than the water it's dissolved in? – BillDOe Sep 21 '18 at 20:08
• I updated OP in the hope I well clarify question – mattia.b89 Sep 22 '18 at 13:44
• The chlorine in chlorinated water is NOT as chloride, it as molecular chlorine - a different oxidation state to chloride. – Waylander Sep 22 '18 at 16:09
• @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. – matt_black Sep 23 '18 at 10:53

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.