Chemguide: Support for CIE A level Chemistry

Group 17 (previously Group 7)

Learning outcome 11.4.2

This statement is about 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.

Cl₂ + H₂O    HCl + HOCl

You will also sometimes find HOCl written as HClO.

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

The chloric(i) acid is a weak acid and ionises to an extent to give hydrogen ions and chlorate(I) ions, ClO^-. These are also powerful oxidising agents.

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