Syllabus Objectives:

Describe the use of aqueous sodium hydroxide and aqueous ammonia to identify the following aqueous cations: aluminium, ammonium, calcium, copper(II), iron(II), iron(III), lead(II) and zinc (formulae of complex ions are not required).

Cation Tests and Observations

What exactly happened?

Reaction with aqueous sodium hydroxide

Outcome 1: No precipitate formed (NH4+)

Ammonium salts will react with aqueous sodium hydroxide to form a soluble salt, water and ammonia gas.

Example: Ammonium chloride + Sodium Hydroxide → Sodium Chloride (soluble) + Water + Ammonia gas

NH4Cl (aq) + NaOH (aq) → NaCl (aq) + H2O (l) + NH3 (g)

On heating, ammonia gas is given off. Gas turns moist red litmus paper blue.

Why should the litmus paper be moist?

Ammonia, being an alkaline gas, produces hydroxide ions (OH–) in the presence of water.

H2O (l) + NH3 (g) → NH4+ (aq) + OH–  (aq)

Outcome 2: Precipitate formed (Zn2+, Al2+,Pb2+,Fe2+,Fe3+,Cu2+,Ca2+)

When sodium hydroxide is added to other metal cations, an insoluble hydroxide is formed.

Example 1:
Lead(II) chloride + Sodium Hydroxide → Lead(II) Hydroxide (insoluble) + Sodium Chloride (soluble)

PbCl2 (aq) + 2 NaOH (aq) → Pb(OH)2 (s)+ 2 NaCl (aq)

White precipitate of lead(II) hydroxide is formed.

Example 2:
Iron(II) nitrate + Sodium Hydroxide → Iron(II) Hydroxide (insoluble) + Sodium Nitrate (soluble)

Fe(NO3)2 (aq) + 2 NaOH (aq) → Fe(OH)2 (s)+ 2NaNO3 (aq)

Green precipitate of iron (II) hydroxide is formed.

Reaction with aqueous ammonia

Outcome 1: No precipitate formed (Ca2+)

No precipitate is formed as there is little amount of hydroxide ions present to form precipitate with calcium hydroxide, which is partially soluble.

Note: Aqueous ammonia is a weak alkali, hence only low concentration of hydroxide ions is present.

Outcome 2: Precipitate formed (Zn2+, Al2+,Pb2+,Fe2+,Fe3+,Cu2+)

When aqueous ammonia is added to other metal cations, an insoluble hydroxide is formed.

Example: Copper(II) chloride + Aqueous Ammonia → Copper(II) Hydroxide (insoluble) + Ammonium Chloride (soluble)

CuCl2 (aq) + 2 NH4OH (aq) → Cu(OH)2 (s)+ 2 NH4Cl (aq)

Light blue precipitate of copper (II) hydroxide is formed.

Example: Zinc nitrate + Aqueous Ammonia → Zinc Hydroxide (insoluble) + Ammonium Nitrate (soluble)

Zn(NO3)2 (aq) + 2 NH4OH (aq) → Zn(OH)2 (s)+ 2 NH4NO3 (aq)

White precipitate of zinc (II) hydroxide is formed.

Question: 

Lead(II) ions and aluminum ions give the same results in both aqueous sodium hydroxide and aqueous ammonia. How do we distinguish lead(II) ions from aluminium ions?

Answer

Add any solution containing chloride (or sulfate) ions. Only lead(II) ions give white precipitate with chloride ions (or sulfate ions).

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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