Qualitative Analysis is the process of identifying unknown substances in a solution.
At ‘O’ levels, students are required to describe the use of aqueous sodium hydroxide and aqueous ammonia to identify the following aqueous cations: aluminium (Al3+), ammonium (NH4+), calcium (Ca2+), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (Fe3+), lead(II) (Pb2+) and zinc (Zn2+) (formulae of complex ions are not required).
White Precipitate formed
Aluminium, zinc and lead(II) ions give white precipitate with a few drops of aqueous ammonia. If the precipitate dissolves in excess to form a colourless solution, then zinc ion is present. If the precipitate remains insoluble in excess aqueous ammonia, aluminium ion or lead(II) ions could be present. Further tests can be done to identify which of these two ions are present.
How to distinguish lead(II) ion from aluminium ion?
Method 1:
Add salt solutions containing iodide ions (e.g. potassium iodide, KI).
Yellow precipitate of lead(II) iodide, PbI2 is formed if lead(II) ions are present.
No precipitate is formed if aluminium ions are present.
Method 2:
Add any solution containing chloride or sulfate ions (e.g. sodium chloride, NaCl or potassium sulfate, K2SO4).
White precipitate is formed if lead(II) ions are present.
No precipitate is formed if aluminium ions are present.
Check out the video for further explanation on the solubility of salts formed.
Click here for more examples and chemical equations on identification of cations.
Click here to find out why calcium ion do not form precipitate with aqueous ammonia.
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