What is an acid?

• A. A substance that donates protons (H+) in a chemical reaction.
• B. A substance that accepts protons (H+) in a chemical reaction.
• C. A substance that neutralizes bases.
• D. A substance that forms hydroxide ions on its own.

Answer: (A)

Acids are substances that donate protons (H+) in a chemical reaction. In water, donating a proton increases the hydronium ion (H3O+) concentration, which is why the solution becomes acidic. For example, hydrochloric acid donates a proton to water, forming H3O+ and Cl−. This proton-donating behavior is the defining feature used in common chemistry descriptions of acids (Bronsted–Lowry). The idea of accepting protons describes a base, not an acid. The notion of forming hydroxide ions on its own points to bases in the Arrhenius sense. Neutralizing bases can occur during acid–base reactions, but that outcome isn’t what defines an acid. So the best description is a substance that donates protons.