Steve Marsden’s

The starting color of the system is a pale yellow-orange. When acid is added to the system, the color becomes more orange. A sensible conclusion would be that adding acid causes additional Cr2O72- to form in the mixture. A possible explanation involves the increased chance for collisions among the products (which include H+---provided by the acid) that temporarily increases the rate of the reverse reaction until a new equilibrium is established. By the time the rate of the reverse reaction has begun to slow again and the forward reaction has caught up with it, there is a net increase in dichromate ion and thus the color of the mixture is more orange.

Adding base to the original mixture seems to have the opposite effect. The mixture becomes definitely yellow. The color change signals an increase in the chromate ion concentration and a decrease in the dichromate ion concentration. Added base supplies OH- ions which react with the H+ ions, decreasing their co