Chapter 13, Problem 93AE

Chromium(VI) forms two different oxyanions, the orange dichromate ion, Cr₂O₇²⁻ , and the yellow chromate ion, CrO₄²⁻ .

(See the following photos.) The equilibrium reaction between the two ions is

${\text{Cr}}_2{\text{O}}_7{}^{2-}\left(aq\right)+{\text{H}}_2\text{O}\left(l\right)\rightleftharpoons 2{\text{CrO}}_4{}^{2-}\left(aq\right)+2{\text{H}}^{+}\left(aq\right)$

Explain why orange dichromate solutions turn yellow when sodium hydroxide is added.