Question

Distinguish between the following pairs of compounds using the test given within brackets a lead salt and a zinc salt (using excess ammonium hydroxide).

Answer 1

Distinguish between a lead salt and a zinc salt:

A lead salt & a zinc salt can be distinguished by using excess Ammonium hydroxide.

Part 1: Lead salt

• Let's take the example of Lead nitrate.
• When excess Ammonium hydroxide is added to lead nitrate salt, a chalky white precipitate of lead hydroxide is formed.
• A chalky white precipitate of lead hydroxide is insoluble in excess of ammonium hydroxide.

$\underset{\mathrm{Ammonium}\mathrm{hydroxide}}{2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)}+\underset{\mathrm{Lead}\mathrm{nitrate}}{\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)}\to \underset{\mathrm{Ammonium}\mathrm{nitrate}}{2{\mathrm{NH}}_4{\mathrm{NO}}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Lead}\mathrm{hydroxide}}{\mathrm{Pb}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)\downarrow }$

Part 1: Zinc salt

• Let's take the example of Zinc sulphate.
• When excess Ammonium hydroxide is added to Zinc sulphate salt, a white gelatinous precipitate of Zinc hydroxide is formed.
• A white gelatinous precipitate of Zinc hydroxide is soluble in excess of ammonium hydroxide.

$\underset{\mathrm{Ammonium}\mathrm{hydroxide}}{2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)}+\underset{\mathrm{Zinc}\mathrm{sulphate}}{{\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)}\to \underset{\mathrm{Ammonium}\mathrm{sulphate}}{{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Zinc}\mathrm{hydroxide}}{\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)\downarrow }$