How to Test for Cations (Edexcel IGCSE Chemistry (Modular))

Revision Note

Test yourself

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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