The distinctive colours of the two cobalt(II) species in solution produce an attractive visual demonstration of a reversible reaction and the effect of concentration and temperature on the position of equilibrium.
The demonstration can be used to introduce reversible reactions and chemical equilibrium or to illustrate Le Chatelier’s principle once these concepts have been established. If students are unfamiliar with the formulae of complex ions this may confuse the issue. For the purposes of this discussion the equilibrium could adequately be represented by:
Pink cobalt species + chloride ions ⇌ Blue cobalt species + water molecules
A white background will help to show the colour changes to best effect. For big groups the reactions should be scaled up, using larger containers such as measuring cylinders or beakers, to improve visibility.
The demonstration could also be adapted for use as a class experiment with suitable groups.
Time taken should be about 10 min.