To test for cations, you usually use aqueous sodium hydroxide or aqueous ammonia. To identify the cation, you look at three things:
- The colour of the precipitate formed
- The solubility in excess alkali (base)
- Reactivity to a flame test (only if required for further identification)
COLOUR OF PRECIPITATE
When NaOH is combined with cations, they form metal hydroxides. Soluble metal hydroxides are all those in Group I and ammonia. Meanwhile, any other metal hydroxide is insoluble. Insoluble means? THEY'LL FORM A PRECIPITATE. <-- Great.
Adding NaOH to any salt solutions containing transition metals usually form coloured solutions, while salt solutions with other metal ions(except Group I and ammonia) form white precipitate.
For ammonia, however, since it has a lower concentration of hydroxide ions, slightly soluble hydroxides such as calcium and barium hydroxides are not precipitated. Can't remember your solubility table? Here it is:
SOLUBILITY IN EXCESS ALKALI/BASE
So there is this trio of metal hydroxides that are fortunately or unfortunately complicated. Aluminium (II) Oxide, Zinc (II) Oxide and Lead (II) Oxide are amphoteric in nature, meaning they react with both acids and bases. When this happens, they form complex salts that are soluble to give colourless solutions.
So what happens exactly when you add excess alkali (base)?
Acid-base neutralisation takes place to form salt and water. That's why when you add excess base, the white precipitate you saw would dissolve and form a colourless solution.
For ammonia, only two metal ions form precipitates which then dissolve to form soluble complexes when excess ammonia is added: Zinc and Copper. Copper would form a deep blue solution whilst zinc would form a colourless solution.
REACTIVITY TO FLAME TEST
After all those troublesome things you did, you still have more?? Yeah.
Fortunately or unfortunately, this isn't covered in the syllabus, so this'll be for another post another day...To sum up, here is everything in a pretty table:
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