Final answer:
The color change from pink to blue, and back to pink, when HCl is added to a cobalt chloride solution or when more water is introduced, demonstrates a shift in chemical equilibrium between Co(H₂O)₆²⁺ and CoCl₄²⁻, obeying Le Chatelier's principle.
Step-by-step explanation:
The observed color change is indicative of a shift in the chemical equilibrium between two forms of a cobalt complex in solution: the pink Co(H₂O)₆²⁺ and the blue CoCl₄²⁻ ions. When HCl is added to a solution of cobalt chloride, the increased chloride ions drive the equilibrium towards the formation of CoCl₄²⁻, thereby resulting in a blue color. Conversely, adding water decreases the concentration of chloride ions relative to the cobalt ion, pushing the equilibrium back towards the Co(H₂O)₆²⁺ complex and, thus, reverting the solution back to its pink color.
These color changes reflect Le Chatelier's principle, which predicts how a system at equilibrium reacts to disturbances. A change in concentration of one component of the system, such as adding more HCl or water, will result in the system adjusting itself to partially oppose the change and restore a new equilibrium. The concentration of reactants and products within a reversible reaction is thus altered accordingly, which is visually represented through the observed color transition.