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.
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.
