Chemguide: Support for CIE A level Chemistry

Learning outcomes 11.5(a) and 11.5(b)

These statements are about various uses and environmental problems concerning chlorine and its compounds.

Before you go on, you should find and read the statements in your copy of the syllabus.

The use of chlorine in water purification

Chlorine is added to water to kill bacteria. Some water treatment companies also add it early in the treatment process to prevent the growth of algae in the treatment works.

Chlorine reacts with water to some extent to give a mixture of hydrochloric acid and chloric(I) acid.

You will also sometimes find HClO written as HOCl.

The chloric(I) acid is a powerful oxidising agent, and kills bacteria by oxidation.

Because of the formation of acids in this reaction, in addition to any acids that might be present in the original water, the pH of the treated water is corrected after chlorination. This might typically be done by adding calcium hydroxide to the water.


Most domestic bleach (smelling of chlorine) is a mixture of sodium chloride and sodium chlorate(I) (sodium hypochlorite). See the first few lines of learning outcome 11.4(a).

Sodium chlorate(I) is sometimes written as NaClO, and sometimes as NaOCl. It is a powerful oxidising agent, and bleaches things by oxidising many coloured substances.


For the structure and uses of PVC, find the section on PVC on the page about the polymerisation of alkenes. You don't need the fine detail about the structure of PVC. Just recognise its repeating unit, and know a few uses.

You also need to know about problems with the disposal of chlorinated polymers like PVC. If these are burnt at too low a temperature, poisonous products such as phosgene (COCl2) and dioxins are formed.


Read the "CFCs and their replacements" part of the page about uses of halogenoalkanes. You should also follow the link in the second green box on that page to find the mechanism for ozone depletion.

The syllabus also mentions "halogenated hydrocarbons" used as solvents. They have much the same problems as CFCs, but can also contaminate water supplies. They include molecules like trichloroethene, CCl2=CHCl.


These are effective but persistent insecticides which are both chlorinated hydrocarbons. Unfortunately, they are broken down extremely slowly in the environment, and accumulate in food chains. The use of these insecticides is now limited.

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© Jim Clark 2011 (last modified May 2014)