Question

(i) How would you distinguish between Zn²⁺ and Pb²⁺ ions, using ammonium hydroxide solution?
(ii) Copy and complete the following table which refers to the action of heat on some carbonates:

Carbonate	Colour of residue on cooling
Zinc carbonate
Lead carbonate
Copper carbonate

Answer 1

(i) Zinc and lead ions can be distinguished by using ammonium hydroxide solution in the following manner:

Reaction with ammonium hydroxide
Zinc ion	Lead ion
Zinc salts react with ammonium hydroxide solution to form white gelatinous precipitate of zinc hydroxide. On adding excess of ammonium hydroxide, soluble, colourless tetramine zinc complex salt solution along with water is formed. $$\begin{gathered} {\mathrm{ZnSO}}_4+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_2+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4 \\ \mathrm{Zn}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_4\mathrm{OH}+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\to \left[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4\right]{\mathrm{SO}}_4+4{\mathrm{H}}_2\mathrm{O} \end{gathered}$$	Lead salts react with ammonium hydroxide solution to form white precipitate of lead hydroxide, which is insoluble in excess of ammonium hydroxide. $\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Pb}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_4{\mathrm{NO}}_3$

(ii) The action of heat on carbonates causes the formation of metal oxides (residue) along with the liberation of carbon dioxide gas.

Carbonate	Colour of residue on cooling
Zinc carbonate	White residue
Lead carbonate	Yellow residue
Copper carbonate	Black residue