Identification of Cations (Cambridge O Level Chemistry)

• Metal cations in aqueous solution can be identified by the colour of the precipitate they form on addition of sodium hydroxide and ammonia
• If only a small amount of NaOH is used then normally the resulting metal hydroxide precipitates out of solution
• In excess NaOH some of the precipitates may re-dissolve
• A few drops of NaOH is added at first and any colour changes or precipitates formed are noted
• Then the NaOH is added in excess and the reaction is observed again
• The steps are then repeated for the test using ammonia solution

 Analysing results

• The table below contains the results for each of the cations included in the syllabus
• If a precipitate is formed from either NaOH or aqueous ammonia then it means that the hydroxide is insoluble in water
• Zinc chloride, for example, reacts as such:

ZnCl₂ (aq) + 2NaOH (aq) →  Zn(OH)₂ (s) + 2NaCl (aq)

• Ca²⁺ ions can be distinguished from Zn²⁺ and Al³⁺ as calcium hydroxide precipitate does not dissolve in excess NaOH but both zinc hydroxide and aluminium hydroxide do
• Zn²⁺ ions can be distinguished from Al³⁺ ions as Zn(OH)₂ dissolves in excess aqueous ammonia but Al(OH)₃ does not
• Most transition metals produce hydroxides with distinctive colours

• The flame test is used to identify the metal cations by the colour of the flame they produce
  • Ions from different metals produce different colours
• Dip the loop of an unreactive metal wire such as nichrome or platinum in concentrated acid and then hold it in the blue flame of a Bunsen burner until there is no colour change
  • This is an important step as the test will only work if there is just one type of ion present
    • Two or more ions means the colours will mix, making identification erroneous
    • This cleans the wire loop and avoids contamination
• A small sample of the compound is placed on an unreactive metal wire loop such as nichrome or platinum
• Dip the loop into the solid sample / solution and place it in the edge of the blue Bunsen flame
  • Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour

Diagram showing the technique for carrying out a flame test

• The colour of the flame is observed and used to identify the metal ion present:

Diagram showing the colours formed in the flame test for metal ions