CIE IGCSE Chemistry

Revision Notes

7.3.2 Redox Reactions

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Redox Reactions

Identifying redox reactions

  • Redox reactions can be identified by the changes in the oxidation states when a reactant goes to a product


Chlorine + potassium iodide → potassium chloride + iodine
Cl2 + 2KI → 2KCl + I2
  • Chlorine has become reduced as its oxidation state has decreased from 0 to -1 on changing from the chlorine molecule to chloride ions:
Cl2(g) → 2Cl-(aq)
  • Iodine has been oxidised as its oxidation state has increased from -1 to 0 on changing from iodide ions to the iodine molecule:
2I-(aq) → I2(s)


Identifying redox reactions by colour changes

  • The tests for redox reactions involve the observation of a colour change in the solution being analysed
  • Two common examples are acidified potassium manganate(VII), and potassium iodide
  • Potassium manganate (VII), KMnO4, is an oxidising agent which is often used to test for the presence of reducing agents
  • When acidified potassium manganate (VII) is added to a reducing agent its colour changes from pink-purple to colourless
  • Potassium iodide, KI, is a reducing agent which is often used to test for the presence of oxidising agents
  • When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide, the solution turns a brown colour due to the formation of iodine

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.

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