Question

Zinc on being tested with cobalt nitrate solution gives green colour residue with chemical composition of:
A.${\text{ZnO}}{\text{.CoO}}$
B.${\text{ZnCoO}} + {\text{ nitrogen dioxide}}$
C.${\text{ZnO}} + {\text{ Nitrogen dioxide}}$
D.None of the above

Answer 1

To answer this question, you must recall the cobalt nitrate test. It is based on the fact that cobalt nitrate on heating decomposes to form cobalt oxide.

Complete step by step solution:
We know that the cobalt nitrate test is an extension of the charcoal cavity test used for the qualitative analysis of cations inorganic compounds. This test is used to test those salts which give a white residue in the charcoal test, namely zinc and magnesium. It is based on the fact that cobalt oxide forms coloured compounds in combination with certain metal oxides.
In the test, we add a drop of cobalt nitrate to the residue obtained from the charcoal cavity test $\left( {{\text{ZnO}}} \right)$ and it is heated in an oxidising flame. The cobalt nitrate decomposes and forms cobalt oxide $\left( {{\text{CoO}}} \right)$. The cobalt oxide thus formed combines with zinc oxide present in the residue giving a green coloured compound as:
${\text{ZnO}} + {\text{CoO}} \to {\text{ZnO}}{\text{.CoO}}$

Thus, the correct option is A.

Additional information:
The charcoal cavity test is based on the fact that the metallic carbonates on heating inside a charcoal cavity decompose into their oxides. The oxides are obtained as residue inside the cavity and in some cases even the metallic state is formed. The colour of the residue helps us to identify the cation present in the test sample. In case of metals like zinc and magnesium, this test cannot be used as they form white colored oxides as residues.

Note:
In the cobalt nitrate test, we do not use cobalt oxide directly as it is insoluble in water. Thus, we use cobalt nitrate and the main reactant cobalt oxide is prepared in-situ.