Cobalt complex ions: Le Châtelier’s Principle

On the front cover, the pink colour in the test tube comes from cobalt(II) ions in water, Co(H2O)62+. The blue colour is the result of cobalt chloride complex ions (CoCl42–) in less dense acetone. This classic Le Châtelier’s Principle lab explores the reversible chemical reaction:

Co(H2O)62+ + 4Cl + heat   --> CoCl42–  +  6H2O

     pink                                     blue

Students experiment with shifting the equilibrium and observing the colour change by

  • adding HCl(aq).
  • adding H2O.
  • adding AgNO3(aq).
  • adding acetone.

The blue layer on top of a pink layer in the cover photo is accomplished by SLOWLY adding 10 mL of acetone to
5 mL of aqueous 0.2 M CoCl2. The acetone undergoes hydrogen bonding with the H2O, causing the equilibrium to shift to the right producing more of the blue CoCl42- ion. The difference in density is what allows the colour change to occur at the top of the test tube.

This lab comes from Chemistry: A Modern Course, Teacher Resource Book, Merrill (1987) — still enlightening students 30 years later. It is a favourite of mine because
I really enjoy experiencing students' surprised reactions when the two colourful layers form. In fact, once a grade 11 chemistry student asked about the experiment after eyeing the set of these test tubes in the fume hood where I was evaporating off the acetone. After a discussion about the lab, the student decided to take grade 12 chemistry, just to experience it for themselves. The following year, they concluded that it was worth the wait.

[Flinn has a PDF handout and video with a similar lab except acetone is not used. In their experiment, the effect of temperature on the equilibrium shift is explored.

This Flinn handout also gives important disposal instructions.

Flinn Scientific Chem Fax PDF Handout:


Another good video can be found on
Mr Grodski Chemistry YouTube channel   ]