ICSE Class 10 Chemistry Chapter 04 Analytical Chemistry

Analytical Chemistry - Use of Ammonium Hydroxide and Sodium Hydroxide

Important Points to Remember

1. Alkalies, i.e., Sodium hydroxide and Ammonium hydroxide react with the soluble salts of metal to precipitate different coloured hydroxides.

2. Reaction of Sodium hydroxide and Ammonium hydroxide with soluble salt of Zinc:

(a) With NaOH: On adding Sodium hydroxide drop by drop to a solution of Zinc salt a white precipitate is formed which is soluble in excess of Sodium hydroxide.

\[Zn(NO_3)_2 + 2NaOH \rightarrow Zn(OH)_2 \downarrow + 2NaNO_3\]

Zinc nitrate - Sodium hydroxide - Zinc hydroxide (White ppt.) - Sodium nitrate

\[Zn(OH)_2 + 2NaOH \rightarrow Na_2ZnO_2 + 2H_2O\]

Zinc hydroxide - Sodium hydroxide - Sodium zincate (Soluble salt) - Water

(b) With NH₄OH: On adding Ammonium hydroxide drop by drop to a solution of Zinc salt a white precipitate is formed which is soluble in excess of Ammonium hydroxide.

\[Zn(NO_3)_2 + 2NH_4OH \rightarrow Zn(OH)_2 \downarrow + 2NH_4NO_3\]

Zinc nitrate - Ammonium hydroxide - Zinc hydroxide (White ppt.) - Ammonium nitrate

\[Zn(OH)_2 + 4NH_4OH \rightarrow [Zn(NH_3)_4](OH)_2 + 4H_2O\]

Zinc hydroxide - Ammonium hydroxide - Tetraamine zinc hydroxide (Soluble salt) - Water

Teacher's Note

When you add cleaning solutions containing hydroxides to mineral deposits in water pipes, they can form precipitates that either dissolve to create smooth pipes or remain insoluble to form protective layers.

3. Reaction of Sodium hydroxide and Ammonium hydroxide with the soluble salt of Lead:

(a) With NaOH: On adding Sodium hydroxide drop by drop to a solution of Lead salt a white precipitate is formed which is soluble in excess of Sodium hydroxide.

\[Pb(NO_3)_2 + 2NaOH \rightarrow Pb(OH)_2 \downarrow + 2NaNO_3\]

Lead nitrate - Sodium hydroxide - Lead hydroxide (White ppt.) - Sodium nitrate

\[Pb(OH)_2 + 2NaOH \rightarrow Na_2PbO_2 + 2H_2O\]

Lead hydroxide - Sodium hydroxide - Sodium plumbate (Soluble salt) - Water

(b) With NH₄OH: On adding Ammonium hydroxide drop by drop to a solution of Lead salt a white precipitate is formed which is insoluble in excess of Ammonium hydroxide.

\[Pb(NO_3)_2 + 2NH_4OH \rightarrow Pb(OH)_2 \downarrow + 2NH_4NO_3\]

Lead nitrate - Ammonium hydroxide - Lead hydroxide (White ppt.) - Ammonium nitrate

Teacher's Note

Lead compounds in old plumbing systems can react with hydroxide-containing water treatments, creating insoluble deposits that either protect pipes or contaminate water depending on conditions.

4. Reaction of Sodium hydroxide and Ammonium hydroxide with the soluble salt of Copper:

(a) With NaOH: On adding Sodium hydroxide drop by drop to a solution of Copper salt, a bluish white precipitate is formed which is insoluble in excess of Sodium hydroxide.

\[CuSO_4 + 2NaOH \rightarrow Cu(OH)_2 \downarrow + Na_2SO_4\]

Copper sulphate - Sodium hydroxide - Copper hydroxide (Bluish white ppt.) - Sodium sulphate

(b) With NH₄OH: On adding Ammonium hydroxide drop by drop to a solution of Copper salt a pale blue or bluish white precipitate is formed which dissolves in excess of Ammonium hydroxide to give deep blue or inky blue or prussian blue solution.

\[CuSO_4 + 2NH_4OH \rightarrow Cu(OH)_2 \downarrow + (NH_4)_2SO_4\]

Copper sulphate - Ammonium hydroxide - Copper hydroxide (Bluish white ppt.) - Ammonium sulphate

\[Cu(OH)_2 + 4NH_4OH \rightarrow [Cu(NH_3)_4](OH)_2 + 4H_2O\]

Copper hydroxide - Ammonium hydroxide - Tetraamine copper hydroxide (Prussian blue solution) - Water

Teacher's Note

Copper sulfate fungicides used in agriculture form different colored compounds with ammonia and hydroxide solutions, which is how chemists identify and process copper-based treatments.

5. Reaction of Sodium hydroxide and Ammonium hydroxide with soluble salt of Calcium:

(a) With NaOH: On adding Sodium hydroxide drop by drop to a solution of Calcium salt, a white precipitate is formed which is insoluble in excess of Sodium hydroxide.

\[Ca(NO_3)_2 + 2NaOH \rightarrow Ca(OH)_2 \downarrow + 2NaNO_3\]

Calcium nitrate - Sodium hydroxide - Calcium hydroxide (White ppt.) - Sodium nitrate

(b) With NH₄OH: No visible reaction.

Teacher's Note

Calcium hydroxide is produced when lime reacts with water in construction materials, and understanding its behavior with different solutions helps in maintaining concrete structures.

6. Reaction of Sodium hydroxide and Ammonium hydroxide with soluble salt of Ferrous:

(a) With NaOH: On adding Sodium hydroxide drop by drop to a solution of Ferrous salt, a dirty green precipitate is formed which turns reddish brown after sometime and is insoluble in excess of Sodium hydroxide.

\[FeCl_2 + 2NaOH \rightarrow Fe(OH)_2 \downarrow + 2NaCl\]

Ferrous chloride - Sodium hydroxide - Ferrous hydroxide (Dirty green ppt.) - Sodium chloride

(b) With NH₄OH: On adding Ammonium hydroxide drop by drop to a solution of Ferrous salt a dirty green precipitate is formed which turns reddish brown after sometime and is insoluble in excess of Ammonium hydroxide.

\[FeCl_2 + 2NH_4OH \rightarrow Fe(OH)_2 \downarrow + 2NH_4Cl\]

Ferrous chloride - Ammonium hydroxide - Ferrous hydroxide (Dirty green ppt.) - Ammonium chloride

Teacher's Note

Iron in water supplies oxidizes and changes color over time, which is why rusty water appearing in old pipes shows the oxidation of ferrous compounds to reddish iron(III) compounds.

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