Correct Option: (D) HCl solution

Step 1 (Governing Chemical Equilibrium): [0.5 Mark]

The interconversion between chromate ions ($CrO_4^{2-}$, yellow) and dichromate ions ($Cr_2O_7^{2-}$, orange) is pH-dependent. In an acidic medium, the equilibrium shifts towards the formation of the dichromate ion according to the following balanced chemical equation:

Step 2 (Step-by-Step Logical Deduction): [0.25 Mark]

• Potassium chromate ($K_2CrO_4$) provides $CrO_4^{2-}$ ions, which are yellow in color.
• The addition of an acid ($H^+$ ions) increases the concentration of hydrogen ions in the solution.
• According to Le Chatelier's Principle, increasing the concentration of a reactant ($H^+$) shifts the equilibrium to the right, favoring the production of $Cr_2O_7^{2-}$ ions.
• $Cr_2O_7^{2-}$ ions are orange in color.
• Among the given options, only HCl is a strong acid that provides the necessary $H^+$ ions to drive this reaction.

Step 3 (Final Concluded Answer): [0.25 Mark]

Since the color change from yellow to orange indicates the formation of dichromate ions in an acidic environment, the substance present in the test tube must be (D) HCl solution.

💡 EL10 Concept Insight: Think of the chromate-dichromate balance like a seesaw that tips toward "orange" whenever it gets acidic; since HCl is the only acid in the list, it’s the only one that can tip the balance and change the color.