Question

$\text{Zn}$ is tested with cobalt nitrate test instead of charcoal cavity test because:
A. on heating metal carbonate we get a colourless residue which cannot be surely said to be $\text{ZnO}$.
B. in charcoal cavity test $\text{Zn}$ reacts with carbon present in it.
C. heat of formation of $\text{ZnO}$ is very high
D. none of the above

Answer 1

The two tests are related to each other, as cobalt nitrate test is done after charcoal cavity test only. Charcoal cavity test uses charcoal, a form of carbon for reduction of metallic oxides to give metals. Cobalt nitrate test uses the oxide of cobalt for the reaction to form coloured compounds with metallic oxides. Write the reactions and check the differences.

Complete step by step answer:
Let us discuss charcoal cavity and cobalt nitrate test, to distinguish the two after:
(1) Charcoal cavity test: The basic principle behind this test is metallic carbonates heated with charcoal cavity to give corresponding oxides. The reactions with zinc carbonate and zinc sulphate are $\text{ZnS}{{\text{O}}_{4}}+\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}\to \text{ZnC}{{\text{O}}_{3}}+\text{N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{4}}$ and $\text{ZnC}{{\text{O}}_{3}}\xrightarrow{\vartriangle }\text{ZnO}+\text{C}{{\text{O}}_{2}}\uparrow$. Zinc oxide is white coloured compound.
The oxides undergo partial reduction to metallic state and thus give metallic beads, which is not given by zinc.
(2) Cobalt nitrate test: This test is tested on those compounds which left white residue in the charcoal cavity test. Initially, cobalt nitrate decomposes to give cobalt oxide $\left( \text{CoO} \right)$, which in turn reacts with metallic oxide to form a coloured compound. The reactions involved in this test with zinc oxide formed are $2\text{Co}{{\left( \text{N}{{\text{O}}_{3}} \right)}_{2}}\xrightarrow{\vartriangle }2\text{CoO}+4\text{N}{{\text{O}}_{2}}+{{\text{O}}_{2}}$ and $\text{ZnO}+\text{CoO}\to \text{ZnO}\text{.CoO}$. The compound formed is green.
$\text{Zn}$ is tested with cobalt nitrate test instead of charcoal cavity test because on heating metal carbonate we get a colourless residue which cannot be surely said to be $\text{ZnO}$.

The correct option is option ‘a’.

Additional Information: The cobalt nitrate test is given by magnesium oxide also to form pink coloured compounds. The reaction is $\text{MgO}+\text{CoO}\to \text{MgO}\text{.CoO}$.

Note: While performing cobalt nitrate test, some points need to be considered:
(1) Do this test only, when oxide of metal leaves a white coloured residue in charcoal cavity test.
(2) Use dilute cobalt nitrate solution.
(3) Do not put excess cobalt nitrate, if it is done, then cobalt nitrate will decompose to give its oxide that is cobalt oxide $\left( \text{CoO} \right)$ which is a black coloured compound. So, put only 2 drops of cobalt nitrate.