The Equilibrium of Chromate and Dichromate

We start this reaction with beakers containing the necessary solutions.  The chromate solution is yellow, while the dichromate is orange.  The hydrochloric acid, sodium hydroxide, and barium nitrate solutions are all clear and colorless.  Plastic dropper pipets are used to put about 1 mL of the chromate and dichromate solutions into test tubes.

The other solutions are then dropped into the various test tubes.  For example, in part I adding hydrochloric acid drop by drop makes the yellow dichromate solution turn orange, but does not change the color of the orange dichromate.

In part II of the procedure barium nitrate, a source of barium 2 plus ions is added to the various test tubes.  The formation of solid barium chromate is a positive test for the presence of chromate ions.  Adding various solutions of sodium hydroxide or hydrochloric acid will change the amount of chromate present.  The changes that are observed to place are the ones  predicted by le Chatelier's principle.