Question

When concentrated hydrochloric acid is added to a red solution containing the complex ion, the solution turns blue as the tetrahedral complex ion forms. Explain this color change.

Answer 1

Hydroxide ions (from, say, sodium hydroxide solution) remove hydrogen ions from the water ligands attached to the cobalt ion.

Once a hydrogen ion has been removed from two of the water molecules, you are left with a complex with no charge - a neutral complex. This is insoluble in water and a precipitate is formed.