Question

(i) How would you distinguish between Zn²⁺ and Pb²⁺ 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) After the addition of ammonium hydroxide to the given solution, if a gelatinous white precipitate is formed that dissolves in excess sodium hydroxide, then Zn^​2+ is present. But, if the white precipitate remains insoluble in excess ammonium hydroxide, then ​Pb^​2+ is present.

$$\begin{gathered} {\mathrm{Zn}}^{2+}+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Zn}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_4^{2+} \\ \mathrm{White}\mathrm{gelatinous}\mathrm{precipitate} \\ \mathrm{Zn}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_4^{2+}+2{\mathrm{NH}}_4\mathrm{OH}\to {\left[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}+4{\mathrm{H}}_2\mathrm{O} \\ {\mathrm{Pb}}^{2+}+2{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Pb}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NH}}_4^{2+} \\ \mathrm{White}\mathrm{precipitate} \end{gathered}$$

ii) The effect of heat on the given carbonates is listed in the table below:

Carbonate	Colour of residue on cooling
Zinc carbonate Lead carbonate Copper carbonate	Yellow in colour which tuns white on cooling Lime yellow in colour Black in colour