Changing color and back again

In grades 5-8 science today, we delved into heat, temperature, and chemical reactions accelerated by heat or by catalysts such as enzymes.  We ran one beautiful demonstration of a chemical reaction driven one way by rising temperature and back by decreasing temperature. It is the change in the molecules or ions surrounding cobalt II ions in solution with lots of hydrochloric acid to provide chloride ions.

Here are photos of the change from deep pink (ice-water temperature) to deep blue (near the boiling point to water), through purple = pink + blue in between – everyone had a great time!