 We will look at the equilibrium between the chromate ion and the dichromate ion. The chromate ion in solution is yellow. The dichromate ion in solution is orange. We have two test tubes containing a solution of potassium chromate. We will add six molar nitric acid to one of the tubes. We notice that the solution turns orange. This indicates that the reaction has shifted in the reverse direction from the chromate to the dichromate. The nitric acid provides a source of hydrogen ion. The reaction shifts in the reverse direction to try and use up the hydrogen ions that have been added. We have two test tubes containing a solution of potassium dichromate. We will add some six molar sodium hydroxide to one of the tubes. We notice that the solution turns yellow, which indicates that the equilibrium has shifted from the dichromate to the chromate. The sodium hydroxide removes hydrogen ion from the solution, causing the reaction to shift in the forward direction. We have three test tubes, one containing a solution of potassium chromate and two test tubes containing solutions of potassium dichromate. We are going to add a solution of 0.1 molar barium chloride to the first tube. We are going to add a solution of barium chloride to the second tube. To the third tube, we are going to add some nitric acid to the tube first, and then we are going to add barium chloride to the solution. In the first tube, there is a high concentration of chromate ion, so we get a large amount of precipitate. In the second tube, we only get a small amount of precipitate. This indicates that the equilibrium lies very far towards the dichromate side and only a small amount towards the chromate side. When we add acid to the third tube, it shifts the equilibrium in the reverse direction and decreases the chromate ion concentration to the point where no barium chromate forms. If we take some of the solid barium chromate and add 6 molar nitric acid, we see that the solid goes back into solution and the solution turns orange. Adding the nitric acid provides a source of hydrogen ions. The hydrogen ions combine with the chromate ions and shift the equilibrium in the reverse direction to form dichromate. The solution turns orange. As we add more and more hydrogen ions, the equilibrium is forced further in the reverse direction and the solid completely dissolves.