Explain how the colour of K₂Cr₂O₇ solution depends on pH of the solution.

Dichromate ($\mathrm{Cr_2O_7^{2-}}$, orange) and chromate ($\mathrm{CrO_4^{2-}}$, yellow) ions are in equilibrium: $\mathrm{Cr_2O_7^{2-} + 2OH^- \rightleftharpoons 2CrO_4^{2-} + H_2O}$. On adding alkali the pH increases and the solution changes from orange to yellow; on adding acid the pH decreases and it changes from yellow back to orange. $\mathrm{K_2Cr_2O_7 + 2KOH \rightarrow 2K_2CrO_4 + H_2O}$ (orange to yellow) $\mathrm{2K_2CrO_4 + H_2SO_4 \rightarrow K_2Cr_2O_7 + K_2SO_4 + H_2O}$ (yellow to orange)