Investigation 13: Equilibrium and Le Chatelier's Principle

The classic version uses the equilibrium between Co(H2O)6²+, which is pink, and CoCl4²-, which is blue. Co(H2O)6²+ (aq) + 4 Cl^- (aq) <→ CoCl4²- (aq) + 6 H2O (l). Both species sit in the same flask at once, and the color you see is just whichever one dominates. Add chloride ions, say from concentrated HCl, and the forward reaction gets pulled forward to use some of that extra chloride up, so the solution turns more blue. Add water or cool the mixture, and the equilibrium shifts back toward the pink hydrated form. The color is not the reaction itself, it is a readout of which side currently has more product.

Two flasks made from the same stock solution can end up looking different only if something about their conditions differs now, like one sitting in an ice bath or one having chloride added. Since Le Chatelier's principle says a system responds to a stress by shifting to partially relieve it, different stresses produce different shifts, and that shows up as different colors. This is why color is treated as evidence in this investigation: it lets you track a shift you cannot see directly by watching something you can.

The reversibility point matters for interpreting results: equilibrium is dynamic, so forward and reverse reactions keep running even when nothing looks like it's happening. If a flask turns blue when you add chloride and then fades back to pink as you dilute it with water, that is the system shifting back to a new equilibrium position, not proof the original stress did nothing. This connects to the error analysis and experimental design practices used across the other labs: you always ask what evidence actually supports your claim, not just what the final appearance suggests.