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GCSE Chemistry > Ion Analysis

Testing for Cations

Cations are positively charged (+) ions. There are 2 basic tests for them:

  • Flame test
  • Adding NaOH (aq) to solution of ionic compound
The flame test involves a clean nichrome wire first being dipped into a solution of Hydrochloric acid. This is then dipped into the solid powdered compound of the metal being tested. This is then held over a Bunsen Burner and the flame colour is observed.

The following results are gained for each ion:

  • Na+ - Yellow flame
  • K+ - Lilac (light purple) flame
  • Ca2+ - Red flame
  • Cu2+ - Blue / green flame
The second test of adding NaOH (sodium hydroxide) gives a coloured precipitate. The colour of this precipitate reveals which ion is present. The following results are gained for each ion:
  • Ca2+     White precipitate - Ca(OH)2 (s)
  • Cu2+     Blue precipitate - Cu(OH)2 (s)
  • Fe2+     Green precipitate - Fe(OH)2 (s)
  • Fe3+     Brown precipitate - Fe(OH)3 (s)
To detect the Hydrogen ion (H+), which is not a metal, but still has a positive charge, Universal indicator can be used, which will turn red. (because anything with the hydrogen ion in it is an acid)

Testing for Anions

Anions are negatively charged (-) ions and there are many different tests for them. The following table shows the tests and positive results:

Ion Test Positive Result
Hydroxide (OH-) Add an indicator (red litmus)
or add an acid
Litmus turns blue
or test tube becomes warm
Sulphate (SO4-2) Add Barium Chloride solution White solid
Carbonate (CO3-2) Add an acid Gas given off which should be CO2 (test with limewater)
Chloride (Cl-) Add acidified solution of AgNO3 (aq) White precipitate
Bromide (Br -) Add acidified solution of AgNO3 (aq) Cream precipitate
Iodide (I-) Add acidified solution of AgNO3 (aq) Yellow precipitate

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