What is a base?

• A. A substance that accepts protons (H+) in a chemical reaction.
• B. A substance that donates protons (H+).
• C. A substance that forms salts when dissolved.
• D. A substance that decreases pH to acidic.

Answer: (A)

The main idea here is how bases are defined in acid–base chemistry. A base is a substance that accepts a proton (H+) in a reaction. This is the Bronsted–Lowry view of bases. When a base meets an acid, the base grabs a hydrogen ion from the acid, forming a conjugate acid for the base and a conjugate base for the acid. For example, ammonia accepts an H+ from water to form ammonium, while water becomes hydroxide (OH−).

Donating protons describes acids, not bases, so that option isn’t the defining action of a base. While forming salts can occur after acid–base reactions, it’s not the defining trait of a base. Decreasing pH to become acidic describes acids themselves, not bases.