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 |