Q.Acidified potassium dichromate oxidizes $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$. Write the balanced ionic equation for this reaction and state the change in oxidation state of chromium.

The key here is the chromate–dichromate equilibrium. In aqueous solution, the yellow chromate ion ($\ce{CrO4^{2-}}$) and the orange dichromate ion ($\ce{Cr2O7^{2-}}$) exist in a pH-dependent equilibrium:

When you add an acid (increase $\ce{H+}$ concentration), the equilibrium shifts to the right, producing more orange dichromate. When you add a base (decrease $\ce{H+}$), it shifts left, giving yellow chromate. So the liquid left in the test tube must have been acidic to cause the immediate colour change.

Let’s check each option:

1. Sodium hydrogen carbonate solution — This is mildly basic (it hydrolyses to give $\ce{OH-}$). It would shift the equilibrium left, keeping the solution yellow, not orange. So not this.

2. Methyl orange solution — This is an indicator, not an acid or base. It would not change the pH significantly. The colour change from yellow to orange is not due to methyl orange itself (which is orange in acid, yellow in base — but here the chromate colour dominates). So not this.

3. Sodium hydroxide solution — This is a strong base. It would consume $\ce{H+}$, shifting the equilibrium left, making the solution stay yellow. So not this. …