Question

The solution of which of the following salts will become a deep blue when an excess of ammonium hydroxide solution is added to it?

• A. $\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2$
• B. ${\mathrm{FeSO}}_4$
• C. $\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2$
• D.
  • ${\mathrm{MgSO}}_4$
• E. ${\mathrm{ZnCl}}_2$
• F.
  • ${\mathrm{FeCl}}_3$

Answer 1

The correct option is A

$\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2$

Correct answer is option A) $\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2$

Explanation of correct answer

• When ammonium hydroxide is added to copper nitrate, a pale blue precipitate is formed .
• This is due to the formation of copper hydroxide
• $\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Cu}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_4{\mathrm{NO}}_3$
• When excess reagent is added, it dissolves and a deep blue coloured solution is formed

Explanation of incorrect answer

Option B) ${\mathrm{FeSO}}_4$

• Ferrous sulphate react with ammonium hydroxide, a dirty green precipitate is formed.
• This precipitate indicates the presence of ferric hydroxide.
• ${\mathrm{FeSO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\to \underset{\mathrm{a}\mathrm{dirty}\mathrm{green}\mathrm{precipitate}}{\mathrm{Fe}{\left(\mathrm{OH}\right)}_2}+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$

Option C) $\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2$

• When lead nitrate react with ammonium hydroxide, a white precipitate is formed.
• This white precipitate indicates the formation of lead hydroxide Pb(OH)₂
• $\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Pb}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_4{\mathrm{NO}}_3$

Option D)${\mathrm{MgSO}}_4$

• When magnesium sulphate react with ammonium hydroxide, a white precipitate is formed
• The precipitate is Magnesium hydroxide and ammonium sulphate is formed
• MgSO₄ +NH₄OH$\to$Mg(OH)₂+ 2(NH₄)₂SO₄

Option E)${\mathrm{ZnCl}}_2$

• When zin chloride react with ammonium hydroxide, A white precipitate of Zn(OH)₂ is formed.
• ZnCl₂+ 2NH₄OH $\to$ Zn(OH)₂ + NH₄Cl
• It dissolves in excess of the ammonia solution to form a soluble colourless ammine complex.
• Zn(OH)₂+ 4 NH₃ $\to$ Zn(NH₃)₄](OH)₂

Option F) ${\mathrm{FeCl}}_3$

• When Ferric chloride react with sodium hydroxide, a reddish brown precipitate of Fe(OH)₃ is formed.
• ${\mathrm{FeCl}}_3+3{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_3+3{\mathrm{NH}}_4\mathrm{Cl}$
• When excess reagent is added, no reaction takes place