You are given three white powders-calcium carbonate, lead carbonate and zinc carbonate. Describe the tests you would carry out in solution, to identify the metal ion in each of the above compounds. Indicate clearly how you would prepare the solutions for the test.

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Solution

The metal ions in calcium carbonate, lead carbonate and zinc carbonate are determined with the help of reaction with ammonium hydroxide. The cations are identified from their given salts in the following manner:

Reaction with ammonium hydroxide

Calcium carbonate Lead carbonate Zinc carbonate

Dissolve small amount of calcium carbonate in dilute nitric acid. To this solution, add ammonium hydroxide. No reaction takes place. Dissolve small amount of lead carbonate in dilute nitric acid. To this solution, add ammonium hydroxide. A white precipitate of lead hydroxide is formed, which is insoluble in excess of ammonium hydroxide.
${PbCO}_{3} + 2 {NH}_{4} OH \to Pb {(OH)}_{2} + {({NH}_{4})}_{2} {CO}_{3}$
Dissolve small amount of zinc carbonate in dilute nitric acid. To this solution, add ammonium hydroxide. A white gelatinous precipitate of zinc hydroxide is formed, which is soluble in excess of ammonium hydroxide.
${ZnCO}_{3} + 2 {NH}_{4} OH \to Zn {(OH)}_{2} + {({NH}_{4})}_{2} {CO}_{3} Zn {(OH)}_{2} + {({NH}_{4})}_{2} {CO}_{3} + 2 {NH}_{4} OH \to [Zn {({NH}_{3})}_{4}] {CO}_{3} + 4 H_{2} O$

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Reaction with ammonium hydroxide
Calcium carbonate	Lead carbonate	Zinc carbonate
Dissolve small amount of calcium carbonate in dilute nitric acid. To this solution, add ammonium hydroxide. No reaction takes place.	Dissolve small amount of lead carbonate in dilute nitric acid. To this solution, add ammonium hydroxide. A white precipitate of lead hydroxide is formed, which is insoluble in excess of ammonium hydroxide. ${PbCO}_{3} + 2 {NH}_{4} OH \to Pb {(OH)}_{2} + {({NH}_{4})}_{2} {CO}_{3}$	Dissolve small amount of zinc carbonate in dilute nitric acid. To this solution, add ammonium hydroxide. A white gelatinous precipitate of zinc hydroxide is formed, which is soluble in excess of ammonium hydroxide. ${ZnCO}_{3} + 2 {NH}_{4} OH \to Zn {(OH)}_{2} + {({NH}_{4})}_{2} {CO}_{3} Zn {(OH)}_{2} + {({NH}_{4})}_{2} {CO}_{3} + 2 {NH}_{4} OH \to [Zn {({NH}_{3})}_{4}] {CO}_{3} + 4 H_{2} O$