Write the equation of the equilibrium!
$Cr_2O_7^{2-}$ (orange) + $H_2O$ $\rightleftarrows $ $2CrO_42^{-}$ (yellow) + $2H^+$
Why addition of ion $H^+$ shifts the equilibrium to the left?
According to the principle of the Châtelier, the addition of a product tends to remove a portion of this product to reform the reactants.
Why addition of ion $OH^-$ shifts the equilibrium to the right?
The $H^+$ is consumed by $OH^-$: $H^+$ $+$ $OH^-$ $\longrightarrow$ $H_2O $ According to the principle of the Chatelier, the removal of a product tends to make reappear a part of this product at the expense of the reactants.