Question

In cobalt nitrate test Zn and Mg gives:
A. Blue and green colour
B. Green and pink colour
C. Green and blue colour
D. None of the above

Answer 1

Hint: Try to recall that cobalt nitrate test is applied to those salts which leave white residue in charcoal cavity test and the test is based on the fact cobalt oxide combines with metallic oxides like ZnO, MgO etc. and form colored compounds. Now by using this you can easily find the correct option from the above given options.

Complete step by step solution:
It is known to you that cobalt nitrate test is an extension of charcoal test and is used for qualitative inorganic analysis of cations.
Cobalt nitrate test is applied to those salts that leave white residue in the charcoal cavity test.
This test is based on the fact that cobalt nitrate decomposes on heating and forms cobalt oxide, $CoO$.
In cobalt nitrate test, a drop of cobalt nitrate solution is added to the residue left in charcoal activity and when it is heated in oxidizing flame, cobalt nitrate decomposes and forms cobalt oxide, $CoO$. This cobalt oxide combines with metallic-oxides present as white residue in charcoal activity and forms colored compounds having characteristic colors.
So, cobalt nitrate test for Zn and Mg:
$
  MgO + CoO \to MgO.CoO(pink) \\
  ZnO + CoO \to ZnO.CoO(green) \\
$
Hence, it is observed that Zn gives green and Mg gives pink color in cobalt nitrate test.
Therefore, you can say that option B is the correct option to the given question.

Note: It should be remembered that in the cobalt nitrate test, cobalt oxide cannot be used directly because cobalt oxide is not soluble in water.
Also, you should remember that charcoal test is performed by mixing given salt with sodium carbonate and then heating the mixture on the block of charcoal.