6.11.4 Identifying Cations

Test with Sodium Hydroxide and Ammonia Solution

Cations can be identified by their reaction with aqueous sodium hydroxide and aqueous ammonia.
Sodium hydroxide and aqueous ammonia produce hydroxide ion which will react with most anion to form precipitate.
NaOH + H₂O → Na⁺ + 2OH^– + H⁺

NH₃ + H₂O → NH₄⁺ + OH^–
Different cations like aluminium Al³⁺, calcium Ca²⁺, copper(II) Cu²⁺, iron(II) Fe²⁺, iron(III) Fe³⁺, lead(II) Pb²⁺, zinc Zn²⁺ produce different coloured precipitates, which may or may not dissolve in excess alkali.
Zn(OH)₂, Al(OH)₃ and Pb(OH)₃ dissolve in excess NaOH solution, this is because Zn(OH)₂, Al(OH)₃ and Pb(OH)₃ are amphoteric, they can react with NaOH to form salt and water.
Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O
Al₂(OH)₃ + 3NaOH → Na₃Al₂O₃ + 3H₂O
Pb(OH)₂ + 2NaOH → Na₂PbO₂ + 2H₂O
Table below shows the summary of the precipitate form by different cation.

	NaOH(ak)	NH₃(ak)
Na⁺	–	–
Ca²⁺	White precipitate.	–
Mg²⁺	White precipitate.	White precipitate.
Al³⁺	White precipitate. Dissolve in excess NaOH solution.	White precipitate.
Zn²⁺	White precipitate. Dissolve in excess NaOH solution.	White precipitate. Dissolve in excess NH₃ solution.
Pb²⁺	White precipitate. Dissolve in excess NaOH solution.	White precipitate.
Fe²⁺	Dirty green precipitate.	Dirty green precipitate.
Fe³⁺	Red brown precipitate.	Red brown precipitate.
Cu²⁺	Blue precipitate.	Blue precipitate. Dissolve in excess NH₃ solution and form a blue solution.
NH₄⁺	–	–

Out of the 10 cations, only lead(II) ions will form a precipitate with chloride ions.
This is because lead(II) chloride is insoluble in water.
The chemical reaction is a double decomposition reaction.
Pb²⁺ + 2Cl^– → PbCl₂
Lead(II) chloride will dissolve in hot water.

	HCl or NaCl
Na⁺	–
Ca²⁺	–
Mg²⁺	–
Al³⁺	.-
Zn²⁺	–
Pb²⁺	White precipitate. Dissolve in hot water
Fe²⁺	–
Fe³⁺	–
Cu²⁺	–
NH₄⁺	–

Out of the 10 cations, only calcium ions and lead(II) ions will form a precipitate with sulphate ions.
This is because both calcium sulphate and lead(II) sulphate are insoluble in water.
The chemical reaction is a double decomposition reaction.
Pb²⁺ + SO₄^2-→ PbSO₄
Ca²⁺ + SO₄^2-→ CaSO₄

All ions, except sodium ions and ammonium ions will form precipitate with carbonate.
This is because sodium carbonate and ammonium carbonate are soluble in water.

Iodide ions will form precipitate with lead(II) ions and copper(II) ions.
However, in SPM you only need to know the reaction between lead(II) ions and iodide ions.
The yellow precipitate formed will dissolve in hot water.
Pb²⁺ + 2I^– → PbI₂

	KI
Na⁺	–
Ca²⁺	–
Mg²⁺	–
Al³⁺	–
Zn²⁺	–
Pb²⁺	Yellow precipitate. Dissolve in hot water
Fe²⁺	–
Fe³⁺	A red brown solution formed.
Cu²⁺	White precipitate form in brown solution
NH₄⁺	–