A student dissolves a metal oxide in water and the resulting solution turns red litmus blue. What does this prove about the metal oxide?

Correct answer: It is a basic oxide

The governing concept is the indicator test for bases. A substance that turns red litmus blue produces a basic aqueous medium, so the metal oxide behaves as a basic oxide under the stated conditions. Many metal oxides form hydroxides with water, for example sodium oxide forms sodium hydroxide. An acidic oxide would turn blue litmus red; colour alone cannot determine chemical nature, and the substance is not a gas.