Why does a solution containing the following equilibrium system turns yellow when NaOH is added:?

Question

Cr2O7(valence of -2)(aq)+H2O(l)<->2CrO4(valence of -2)(aq)+2H
orange yellow

Answer 1

When we add NaOH the hydroxide ions react with the H+ ions in the water, reducing the concentration of H+ ions. The equilibrium therefore shifts in the direction that produces more H+ ions, i.e. the forward direction. So the colour shifts towards the colour of CrO4.