Tests for Cations (Edexcel IGCSE Chemistry: Double Science)

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Tests for cations

  • Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide 
  • If only a small amount of NaOH is used then normally the metal hydroxide precipitates
  • The table below contains the results for each of the cations included in the syllabus
  • If a precipitate is formed from NaOH then the hydroxide is insoluble in water

Metal ion Effect of adding NaOH

Cu2+

Light blue precipitate formed

Fe2+

Green precipitate formed

Fe3+

 Brown precipitate formed 

NH4+

Ammonia gas is produced which turns damp red litmus paper blue

Examiner Tip

Sometimes you may not see much of a precipitate because the cation you are testing is present is very small amounts. However, every a slight cloudiness or colour change can indicate a positive test result.

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.