Final answer:
A reducing agent such as ethanol in dilute sulfuric acid will reduce the hexavalent chromium (Cr6+) in potassium dichromate to trivalent chromium (Cr3+), changing its color from orange to green.
Step-by-step explanation:
The substance that will change the color of aqueous potassium chromate from orange to green is a reducing agent that can reduce the chromium (Cr) from its hexavalent state (Cr6+) to its trivalent state (Cr3+). This change is associated with the reduction of dichromate ions (Cr2O72-) to chromium ions (Cr3+). It's the chromium ions (Cr3+) that impart a green color to the solution. An example of this chemical reaction occurs when potassium dichromate (K2Cr2O7) reacts with a reducing substance such as ethanol in dilute sulfuric acid. The balanced equation for the reduction in acidic medium is:
Cr2O72- (aq) + 14H+ (aq) + 6e- ⇒ 2Cr3+ (aq) + 7H2O(l)
This redox reaction illustrates the conversion from the orange color due to the dichromate ions to the green color due to the formation of trivalent chromium ions.