Potassium chromate(VI) and potassium dichromate(VI) solutions form an equilibrium mixture. The equilibrium system also includes hydrogen ions and water:

2CrO₄^2–(aq) + 2H⁺(aq) _∏ Cr₂O₇^2–(aq) + H₂O(l)

                                      chromate(VI) ions                dichromate(VI) ions

We can follow the effect of changing concentration on the position of this equilibrium because solutions containing chromate(VI) ions are yellow and solutions containing dichromate(VI) ions are orange:  

2CrO₄^2–(aq) + 2H⁺(aq) _∏ Cr₂O₇^2–(aq) + H₂O(l)

                                             yellow                                orange

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