Qualitative Analysis is the process of identifying unknown substances in a solution. In this lesson, we focus on using aqueous sodium hydroxide to identify common cations.
Syllabus Objectives: 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.)
For easy memory, you may wish to first divide the cations into 2 groups – precipitate formed and no precipitate formed.
No Precipitate formed
If no precipitate is formed and a colourless pungent gas is given off upon heating, the cation present is ammonium ion (NH₄⁺).
Ammonia gas turns moist red litmus paper blue.
Precipitate formed
Further divide the cations under ‘precipitate formed’ into white precipitate and coloured precipitate.
White Precipitate
Calcium, aluminium, zinc and lead(II) ions give white precipitate with a few drops of sodium hydroxide.
If the precipitate dissolves in excess to form a colourless solution, then zinc ion, aluminium ion or lead(II) ions could be present. Further tests can be done to identify which of these three ions are present.
If the white precipitate is insoluble in excess, it is calcium ion.
Coloured Precipitate
Light blue precipitate is formed if copper(II) ion (Cu²⁺) is present.
Green precipitate is formed if iron(II) ion (Fe²⁺) is present.
Reddish brown precipitate is formed if iron(III) ion (Fe³⁺) is present.
All coloured precipitates are insoluble in excess sodium hydroxide.
What is the Precipitate?
The precipitate formed in each of the reactions is the hydroxide of the metal ion.
Click here for more examples and chemical equations on identification of cations.
We will focus on the use of aqueous sodium hydroxide in this video.
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