A student dissolved a white powder in water and added blue litmus to it. The litmus turned red. What type of substance is the powder most likely to be?

Correct answer: Acid

The governing concept is the litmus test for acids and bases. When blue litmus turns red, the aqueous solution contains acidic substances, so the white powder is most likely an acid or an acidic compound. A base would produce the opposite change, turning red litmus blue, while a neutral salt would normally cause no colour change. An indicator is a testing substance, not the likely identity of the powder in this observation.