Question

The aqueous solution of transition metal salt changes color from pink to blue, when concentrated hydrochloric acid is added to it. The change in color is due to
 $ A) $ Evolution of hydrogen that changes the oxidation state of the metal ion.
 $ B) $ Changes in the coordination number of the metal ion from $ 6 $ to $ 4 $ and formation of new species in solution.
 $ C) $ Formation of a coordination complex of the metal ion with hydrochloric acid
 $ D) $ Protonation of the metal ion

Answer 1

Hint :When we add hydrochloric acid in hexaaquacobalt $ (II) $ ions. There is no change in the oxidation state of the compound and the compound changes its geometry from octahedral to tetrahedral. So, we know that the most of the color changes takes place in these two geometry.

Complete Step By Step Answer:
If we add concentrated hydrochloric acid to a solution containing hexaaquacobalt $ (II) $ ions. A new species $ {[MC{l_4}]^ - } $ is formed.
  $ {[M{({H_2}O)_6}]^{2 + }} $ is pink in color but it changes to deep blue when we add hydrochloric acid. The six water molecules are replaced by four chloride ions. The reaction taking place is reversible.
The coordination number of the central metal ion changes from $ 6 $ to $ 4 $
So, the correct answer is $ B) $ changes in the coordination number of the metal ion from $ 6 $ to $ 4 $ and formation of new species in solution.

Note :
Color in transition-series metal compounds is generally due to electronic transitions of two principal types. Charge transfer transitions in this an electron may jump from a predominantly ligand orbital to a predominantly metal orbital, giving rise to a ligand-to-metal charge-transfer transition. $ d - d $ transitions in which an electron jumps from one $ d - $ orbital to another. In complexes of the transition metals the d orbitals do not all have the same energy.