Qualitative Analysis is the process of identifying unknown substances in a solution.

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+)
– zinc (Zn2+)
The 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

In ‘no precipitate formed’ category, if colourless and pungent gas is given out upon heating, then we can confirm ammonium ion is present. Of course, to confirm the identity, we should test the gas using moist red litmus paper. Ammonia gas will turn moist red litmus paper blue.

Precipitate formed

Further divide the cations under ‘precipitate formed’ into white precipitate and coloured precipitate.

White Precipitate formed

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 precipitate is insoluble in excess, then calcium ion must be present.

Coloured Precipitate formed

Things get easier with coloured precipitate.
Light blue precipitate is formed if copper(II) ion is present.
Green precipitate is formed if iron(II) ion is present.
Reddish brown precipitate is formed if iron(III) ion is present.
All the precipitate are insoluble in excess sodium hydroxide.

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.

Don’t forget to subscribe to the channel for more Chemistry learning videos. Have fun learning Chemistry 😊

More on
Experimental Chemistry

Measurement and Experimental Design

• Common Gases and their properties

• Common Laboratory Apparatus and their functions

• 40 Chemical Substances All Students Must Know

Purification:

• Setting up and Interpreting Paper Chromatography

• How to separate sand and copper(II) sulfate salt from its mixture

• Filtration to separate insoluble solid from liquid

• How to separate immiscible liquids using Separating Funnel

• Miscibility of liquids

• How to separate miscible liquids using Fractional Distillation

• What is Simple Distillation

Qualitative Analysis

• How to distinguish lead(II) ions from aluminium ions?

• Why calcium ions do not form precipitate with aqueous ammonia

• Identification of Anions

• Identification of Cations using aqueous Ammonia (Flowchart available)

• Identification of Cations using aqueous Sodium Hydroxide (Flowchart available)

• Identifying Anions

• Identifying Cations