Chlorine
<p>Chlorine is a component of various compounds, including <a href="http://en.wikipedia.org/wiki/Table_salt" title="Table salt" class="mw-redirect" >table salt</a>. It is the <a href="http://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust" title="Abundance of elements in Earth's crust" >second most abundant halogen and 21st most abundant chemical element</a> in Earth's crust. The great oxidizing potential of chlorine led it to its <a href="http://en.wikipedia.org/wiki/Bleach_(chemical)" title="Bleach (chemical)" class="mw-redirect" >bleaching</a> and disinfectant uses, as well as uses of an essential reagent in the chemical industry. </p>
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As a common disinfectant, chlorine compounds are used in <a href="http://en.wikipedia.org/wiki/Swimming_pool" title="Swimming pool" >swimming pools</a> to keep them clean and <a href="http://en.wikipedia.org/wiki/Swimming_pool_sanitation" title="Swimming pool sanitation" >sanitary</a>. In the <a href="http://en.wikipedia.org/wiki/Upper_atmosphere" title="Upper atmosphere" class="mw-redirect" >upper atmosphere</a>, chlorine-containing molecules such as <a href="http://en.wikipedia.org/wiki/Chlorofluorocarbons" title="Chlorofluorocarbons" class="mw-redirect" >chlorofluorocarbons</a> have been implicated in <a href="http://en.wikipedia.org/wiki/Ozone_depletion" title="Ozone depletion" >ozone depletion</a>. </p>
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Elemental chlorine is extremely dangerous and poisonous for all lifeforms; however, chlorine is necessary to most forms of life, including <a href="http://en.wikipedia.org/wiki/Human" title="Human" >humans</a>, in form of <a href="http://en.wikipedia.org/wiki/Chloride" title="Chloride" >chloride</a> ions.</p>
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By 1918, the <a href="http://en.wikipedia.org/wiki/US_Department_of_Treasury" title="US Department of Treasury" class="mw-redirect" >US Department of Treasury</a> called for all drinking water to be disinfected with chlorine. Chlorine is presently an important chemical for<a href="http://en.wikipedia.org/wiki/Water_purification" title="Water purification" >water purification</a> (such as water treatment plants), in <a href="http://en.wikipedia.org/wiki/Disinfectant" title="Disinfectant" >disinfectants</a>, and in <a href="http://en.wikipedia.org/wiki/Bleach" title="Bleach" >bleach</a>. Chlorine in water is more than three times as effective as a disinfectant against <i><a href="http://en.wikipedia.org/wiki/Escherichia_coli" title="Escherichia coli" >Escherichia coli</a></i> than an equivalent concentration of <a href="http://en.wikipedia.org/wiki/Bromine" title="Bromine" >bromine</a>, and is more than six times more effective than an equivalent concentration of <a href="http://en.wikipedia.org/wiki/Iodine" title="Iodine" >iodine</a>.<sup id="cite_ref-54" class="reference" ><a href="http://en.wikipedia.org/wiki/Chlorine#cite_note-54" >[55]</a></sup></p>
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Chlorine is usually used (in the form of <a href="http://en.wikipedia.org/wiki/Hypochlorous_acid" title="Hypochlorous acid" >hypochlorous acid</a>) to kill <a href="http://en.wikipedia.org/wiki/Bacteria" title="Bacteria" >bacteria</a> and other microbes in <a href="http://en.wikipedia.org/wiki/Drinking_water" title="Drinking water" >drinking water</a> supplies and public swimming pools. In most private swimming pools, chlorine itself is not used, but rather <a href="http://en.wikipedia.org/wiki/Sodium_hypochlorite" title="Sodium hypochlorite" >sodium hypochlorite</a>, formed from chlorine and <a href="http://en.wikipedia.org/wiki/Sodium_hydroxide" title="Sodium hydroxide" >sodium hydroxide</a>, or solid tablets of chlorinated isocyanurates. The drawback of using chlorine in swimming pools is that the chlorine reacts with a human's hair and skin because hair and skin are made from protein.<a href="http://en.wikipedia.org/wiki/Hypochlorous_acid" title="Hypochlorous acid" >(Reaction with protein amino groups)</a> Even small water supplies are now routinely chlorinated.<sup id="cite_ref-CRC_5-1" class="reference" ><a href="http://en.wikipedia.org/wiki/Chlorine#cite_note-CRC-5" >[6]</a></sup></p>
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It is often impractical to store and use poisonous chlorine gas for water treatment, so alternative methods of adding
chlorine are used. These include <a href="http://en.wikipedia.org/wiki/Hypochlorite" title="Hypochlorite" >hypochlorite</a> solutions, which gradually release chlorine into the water, and compounds like <a href="http://en.wikipedia.org/wiki/Sodium_dichloro-s-triazinetrione" title="Sodium dichloro-s-triazinetrione" class="mw-redirect" >sodium dichloro-s-triazinetrione</a> (dihydrate or anhydrous), sometimes referred to as "dichlor", and <a href="http://en.wikipedia.org/wiki/Trichloro-s-triazinetrione" title="Trichloro-s-triazinetrione" class="mw-redirect" >trichloro-s-triazinetrione</a>, sometimes referred to as "trichlor". These compounds are stable while solid and may be used in powdered, granular, or tablet form. When added in small amounts to pool water or industrial water systems, the chlorine atoms hydrolyze from the rest of the molecule forming hypochlorous acid (HOCl), which acts as a general biocide, killing germs, micro-organisms, algae, and so on.<sup id="cite_ref-FOOTNOTEGreenwood1997860_55-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Chlorine#cite_note-FOOTNOTEGreenwood1997860-55" >[56]</a></sup><sup id="cite_ref-FOOTNOTEWiberg2001411_56-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Chlorine#cite_note-FOOTNOTEWiberg2001411-56" >[57]</a></sup>
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