Scientific chemistry question

[TomAtlanta]

Chlorine breaks down over time due to sunlight and other factors and has to be replaced in the pool.

I don't understand this.

Chlorine is a basic element, CL, number 17 on the periodic table of elements. Molecules composed of basic elements break down into the individual elements, but the individual elements, such as CL, only break down during nuclear fission, if I remember correctly from science classes years ago.

What is really happening to the CL in the pool water?

 

[PiratesLoveBacon]

Sodium hypochlorite breaks down and has to be replaced. Chlorine itself is a gas.

 

[chem geek]

When we say "break down" we don't mean it changes to a different element. We mean that the chlorine chemical changes from one type to another and in this case it changes its oxidation state from +1 in hypochlorous acid (HOCl) or hypochlorite ion (OCl^-) to -1 in chloride ion (Cl^-). Some technical details of how chlorine gets used/consumed are in this post.

 

[march2012]

Chlorine breaks down over time due to sunlight and other factors and has to be replaced in the pool.

I don't understand this.

Chlorine is a basic element, CL, number 17 on the periodic table of elements. Molecules composed of basic elements break down into the individual elements, but the individual elements, such as CL, only break down during nuclear fission, if I remember correctly from science classes years ago.

What is really happening to the CL in the pool water?

Wikipedia is your friend

http://en.wikipedia.org/wiki/Sodium_hypochlorite

This is probably the normal reaction, probably somewhat catalyzed by UV
4 NaClO + 2 H2O ? 4 Na+ + OH- + 2 Cl2 + O2

 

[chem geek]

4 NaClO + 2 H2O ? 4 Na+ + OH- + 2 Cl2 + O2

As is sometimes the case, Wikipedia is wrong about this. Look at the above reaction. It isn't even balanced for charge or for hydrogen or oxygen. Even if you add a "4" before the hydroxyl, it still isn't what happens unless you continue to simply the formula by dissolving the chlorine gas into water. However, the above is just plain wrong in terms of explaining what goes on. I already gave a link to the post that shows the correct reactions and it has a link to another post that goes into the more detailed free radical reactions that show that a UV photon of light breaks apart the bond between the oxygen and the chlorine in hypochlorous acid and in hypochlorite ion. From there it's just a matter of eventually getting to a chloride ion and oxygen gas.