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.

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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