Question

Colour of cobalt chloride solution is:
(A) Pink
(B) Black
(C) Colourless
(D) Green

Answer 1

Hint: Due to their ability to absorb lower-energy yellow, orange, and red light, coordination compounds of transition metals with weak-field ligands are frequently blue-green, blue, or indigo in colour. The ligand field is what matters because each of these complex ions has the same metal and oxidation state.

Complete Step by Step Solution:
The cobalt compounds' red and blue hues, as well as how they alternate between the two, have garnered a lot of attention. It has been suggested that both in the crystal and in solution, the cobalt atom is in association with six other atoms or groups when the colour is red and with four when the colour is blue.

This conclusion is based on an investigation of the absorption spectra of a number of red and blue cobaltous pigments with known crystal structures as well as of a number of red and blue cobalt (II) compounds in solution. This theory has been supported by later determinations of the crystal structures of a number of other substances.

The ion \[\text{ }\!\![\!\!\text{ Co}{{\left( {{\text{H}}_{\text{2}}}\text{O} \right)}_{\text{6}}}\text{ }\!\!]\!\!\text{ }\] is responsible for the cobalt chloride's red colour in an aqueous solution, whereas the ion \[\text{ }\!\![\!\!\text{ Co}{{\left( Cl \right)}_{4}}\text{ }\!\!]\!\!\text{ }\] is responsible for the cobalt chloride's blue colour in concentrated hydrochloric acid.

Anhydrous \[\text{ }\!\![\!\!\text{ Co}{{\left( Cl \right)}_{4}}\text{ }\!\!]\!\!\text{ }\] is blue in colour and turns pink when it comes into touch with moisture.
Hence option (A) is correct.

Note: By measuring the densities, viscosities, refractive indices, surface tensions, and electrical conductivities of a series of solutions containing the same amount of cobalt chloride with increasing concentrations of hydrochloric acid and of an identical series containing no cobalt, it has been possible to track the transition from red to blue.