Reactions of aqueous ions with hydroxide and ammonia

Deprotonation (spec 17.23)

Hydroxide ions remove H+ from water ligands. With a small amount of OH- (or NH3, acting as a base), a neutral hydroxide precipitates:

[Cu(H2O)6]2+ + 2OH- -> [Cu(OH)2(H2O)4] + 2H2O

[Fe(H2O)6]3+ + 3OH- -> [Fe(OH)3(H2O)3] + 3H2O

Aqueous 3+ ions are acidic for the same reason: [Fe(H2O)6]3+ + H2O ⇌ [Fe(H2O)5(OH)]2+ + H3O+.

Observations (spec 17.22)

A few drops of ammonia gives the same precipitates as a few drops of NaOH.

Amphoteric hydroxides and ligand exchange (spec 17.23)

• Amphoteric: Cr(OH)3 and Zn(OH)2 dissolve in acid and in excess alkali:

[Cr(OH)3(H2O)3] + 3OH- -> [Cr(OH)6]3- + 3H2O

Zn(OH)2 + 2OH- -> [Zn(OH)4]2-

• Ligand exchange with excess ammonia:

[Cu(OH)2(H2O)4] + 4NH3 -> [Cu(NH3)4(H2O)2]2+ + 2OH- + 2H2O

[Co(OH)2(H2O)4] + 6NH3 -> [Co(NH3)6]2+ + 2OH- + 4H2O

[Ni(OH)2(H2O)4] + 6NH3 -> [Ni(NH3)6]2+ + 2OH- + 4H2O

[Cr(OH)3(H2O)3] + 6NH3 -> [Cr(NH3)6]3+ + 3OH- + 3H2O

Worked example

A solution gives a blue precipitate with a few drops of aqueous ammonia, which dissolves in excess to give a deep blue solution. Identify the ion and write the equation for the second step.

1. Blue precipitate: Cu(OH)2(H2O)4.
2. Deep blue solution: [Cu(NH3)4(H2O)2]2+, by ligand exchange.

Answer: Cu2+; [Cu(OH)2(H2O)4] + 4NH3 -> [Cu(NH3)4(H2O)2]2+ + 2OH- + 2H2O

Check yourself

1. Which ion gives an orange-brown precipitate with aqueous sodium hydroxide?
   • Fe2+
   • Cu2+
   • Fe3+
   • Cr3+
   Show answer

   Fe3+. Fe(OH)3(H2O)3 is orange-brown.

2. A green precipitate forms with NaOH(aq) and turns brown at the surface on standing. Which ion is present?
   • Fe2+
   • Ni2+
   • Cr3+
   • Cu2+
   Show answer

   Fe2+. Fe(OH)2 is oxidised by air to Fe(OH)3.

3. Which ion gives a white precipitate that dissolves in excess NaOH(aq) and in excess NH3(aq)?
   • Mn2+
   • Fe2+
   • Zn2+
   • Cu2+
   Show answer

   Zn2+. Zn(OH)2 is amphoteric and also forms [Zn(NH3)4]2+.

4. Which hydroxide is amphoteric?
   • Iron(III) hydroxide
   • Chromium(III) hydroxide
   • Copper(II) hydroxide
   • Iron(II) hydroxide
   Show answer

   Chromium(III) hydroxide. Cr(OH)3 dissolves in excess OH- to form [Cr(OH)6]3-.

5. What is seen when excess aqueous ammonia is added to nickel(II) hydroxide?
   • The precipitate turns brown
   • No change
   • A white precipitate forms
   • The green precipitate dissolves to give a blue-violet solution
   Show answer

   The green precipitate dissolves to give a blue-violet solution. Ligand exchange forms [Ni(NH3)6]2+.

6. Which equation shows chromium(III) hydroxide dissolving in excess sodium hydroxide?
   • [Cr(H2O)6]3+ + 3OH- -> [Cr(OH)3(H2O)3] + 3H2O
   • [Cr(OH)3(H2O)3] + 3OH- -> [Cr(OH)6]3- + 3H2O
   • [Cr(OH)3(H2O)3] + 6NH3 -> [Cr(NH3)6]3+ + 3OH- + 3H2O
   • Cr3+ + 3OH- -> Cr(OH)3
   Show answer

   [Cr(OH)3(H2O)3] + 3OH- -> [Cr(OH)6]3- + 3H2O. Further deprotonation gives the soluble hexahydroxo complex.

7. Why is a solution of iron(III) chloride acidic?
   • Chloride ions are acidic
   • Fe3+ reacts with dissolved oxygen
   • Iron(III) chloride contains hydrochloric acid
   • Water ligands on the small, highly charged Fe3+ ion release H+ ions
   Show answer

   Water ligands on the small, highly charged Fe3+ ion release H+ ions. [Fe(H2O)6]3+ + H2O ⇌ [Fe(H2O)5(OH)]2+ + H3O+.

8. What is seen when excess aqueous ammonia is added to a cobalt(II) solution?
   • A green precipitate forms that does not dissolve
   • A white precipitate forms that dissolves to a colourless solution
   • A blue precipitate forms, then dissolves to give a straw-coloured solution
   • An orange-brown precipitate forms
   Show answer

   A blue precipitate forms, then dissolves to give a straw-coloured solution. Co(OH)2 forms, then [Co(NH3)6]2+ by ligand exchange.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson Edexcel IAL Chemistry (2018) scheme of work

Last updated 7 November 2026. Spotted a mistake? Tell us.