Analytical Chemistry — Part 2: Ammonium hydroxide and amphoteric substances: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

A weaker alkali, new clues

Part 2 — Ammonium hydroxide (aqueous ammonia) is a weak alkali. It precipitates the same hydroxides, but in excess it behaves differently from sodium hydroxide, giving us a second test.

In this lesson you will

Describe what a little and then excess ammonium hydroxide does to solutions of iron, copper, zinc and lead salts
Choose a reagent to distinguish between two salt solutions and state the observations
Define an amphoteric substance with examples
Write equations for sodium hydroxide with aluminium, zinc and lead and with their oxides and hydroxides

Table comparing sodium hydroxide and ammonium hydroxide and which precipitates each dissolves in excess
Reagent	Strength	Special action in excess
Sodium hydroxide, NaOH	Strong alkali	Dissolves Zn(OH)₂ and Pb(OH)₂
Ammonium hydroxide, NH₄OH	Weak alkali	Dissolves Zn(OH)₂ and Cu(OH)₂

Concept

Ammonium hydroxide, a few drops

A few drops of ammonium hydroxide give the same coloured precipitates as sodium hydroxide; the ammonium salt stays in solution.

Table of equations for a few drops of ammonium hydroxide with five salt solutions
Solution	Equation	Precipitate
Iron(II) sulphate	FeSO₄ + 2NH₄OH → Fe(OH)₂ + (NH₄)₂SO₄	Dirty green
Iron(III) chloride	FeCl₃ + 3NH₄OH → Fe(OH)₃ + 3NH₄Cl	Reddish brown
Lead nitrate	Pb(NO₃)₂ + 2NH₄OH → Pb(OH)₂ + 2NH₄NO₃	White, chalky
Zinc nitrate	Zn(NO₃)₂ + 2NH₄OH → Zn(OH)₂ + 2NH₄NO₃	White, gelatinous
Copper(II) sulphate	CuSO₄ + 2NH₄OH → Cu(OH)₂ + (NH₄)₂SO₄	Pale blue

Diagram

Excess ammonium hydroxide: copper and zinc

In excess, ammonium hydroxide dissolves two precipitates by forming soluble complex salts:

Copper(II) hydroxide → a deep (inky) blue solution of tetraamminecopper(II) sulphate, [Cu(NH₃)₄]SO₄.
Zinc hydroxide → a colourless solution of tetraamminezinc(II) sulphate, [Zn(NH₃)₄]SO₄.

Iron(II), iron(III) and lead hydroxides do not dissolve.

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