1.3.5 Disproportionation and Water Treatment

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Disproportionation and Water Treatment
Halogens undergo disproportionation with alkalis. The halogens will react with cold dilute
alkali solutions. In these reactions, the halogen is simultaneously oxidised and reduced, called
Chlorine and sodium hydroxide make bleach. If you mix chlorine gas with sodium hydroxide at
room temperature you get sodium chlorate (I) solution, which is common household bleach. The
bleach has lots of uses- it's used in water treatment, to bleach paper and textiles and it's good
for cleaning toilets.
Chlorine is used to kill bacteria in water. When you mix chlorine and water, it undergoes
disproportionation. You end up with a mixture of hydrochloric acid and chloric acid. Aqueous
chloric acid ionises to make ions called hypochlorite ions. These chlorate ions kill bacteria. So,
adding chlorine or a compound containing chlorine, to water can make it safe to drink or swim
Chlorine in water- there are benefits, risks and ethical implications. Clean drinking water
is amazingly important- around the world, almost two million people die every year from
waterborne diseases like cholera, typhoid and dysentery because they have to drink dirty water.
In the UK we're lucky because our drinking water is treated to make it safe. Chlorine is an
important part of water treatment:
> It kills disease causing microorganisms
> Some chlorine remains in water and prevents reinfection further down the supply
> It prevents the growth of algae, eliminating bad tastes and smells, and removes discolouration
caused by organic compounds
However, there are risks from using chlorine to treat water:
> Chlorine gas is very harmful if breathed in- it irritates the respiratory system. Liquid chlorine
on the skin or eyes causes severe chemical burns. Accidents involving chlorine could be really
serious or even fatal
> Water contains a variety of organic compounds e.g. from the decomposition of plants.
Chlorine reacts with these compounds to form chlorinated hydrocarbons e.g. chloromethane-
and many of these chlorinated hydrocarbons are carcinogenic. However, this increased cancer
risk is small compared to the risks from untreated water- a cholera epidemic, say, could kill
thousands of people.
There are ethical considerations too. We don't get a choice about having our water chlorinated-
some people object to this as forced mediation.
In some areas of the UK fluoride ions are also added to drinking water. Health officials
recommend this because it helps prevent tooth decay- there's loads of good evidence for this.
There's a small amount of evidence linking fluoridated water to a slightly increased risk of some
bone cancers. Most toothpaste is fluoridated, so some people think extra fluoride ions in water
is unnecessary.


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