Identification of Cations (Cambridge O Level Chemistry)

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Identification of Cations

  • 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

8.2.2 Identification of Ions & Gases, IGCSE & GCSE Chemistry revision notes

Examiner Tip

The ammonia or sodium hydroxide solution must be added very slowly. If it is added too quickly and the precipitate is soluble in excess, then you run the risk of missing the formation of the initial precipitate, which dissolves as quickly as it forms if excess solution is added.

Be sure to distinguish between the term “colourless” and “clear”. A solution that loses its colour has become colourless. A clear solution is one that you can see through such as water. Solutions can be clear and have colour eg. dilute copper sulphate.

  • 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

Identifying Metal Cations, IGCSE & GCSE Chemistry revision notes

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:

12-3-2-flame-tests-results

cie-flame-tests

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

Examiner Tip

The sample needs to be heated strongly, so the Bunsen burner flame should be on a blue flame.

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Alexandra

Author: Alexandra

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.