Grade 12s are investigating equilibrium reactions. The cobalt ion changes colour depending on the conditions of the reaction, so it is easy to see which way the equilibrium leans. The explanation, using chemistry theory, is harder to get right. The cobalt ion goes blue when heated or when hydrochloric acid is added. It goes pink when cooled, or when water is added.
In the chromate-dichromate equilibrium, the students had to try a test to see which ion is orange and which ion is yellow. Adding sulphuric acid turned the solution orange, suggesting that the dichromate ion is orange in colour. Adding sodium hydroxide turned the solution yellow, suggesting that the chromate ion is yellow.