A substance's aqueous solution has excess hydrogen ions. What will happen to blue litmus?

Correct answer: It will turn red

The governing concept is the indicator test for an acidic solution. Excess hydrogen ions in water indicate that the solution is acidic, meaning its pH is below 7. Blue litmus changes to red in an acidic medium, so option A follows directly. Blue litmus does not normally become pink or white in this test, while remaining blue would indicate a neutral or basic condition rather than excess hydrogen ions.