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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GCSE Chemistry > Ion Analysis