Which statement best describes acids?

• A. A substance that produces hydroxide ions in water.
• B. A substance that produces hydrogen ions in water.
• C. A substance that neutralizes bases.
• D. A substance that increases pH.

Answer: (B)

Acids are substances that increase the concentration of hydrogen ions in solution. In water, they donate protons, which effectively becomes hydronium ions (H3O+), lowering the pH of the solution. This proton-donating behavior is the defining feature scientists use to classify acids. Producing hydroxide ions describes bases, not acids. Acids can neutralize bases, but that describes a reaction, not the fundamental nature of an acid. And acids do not raise pH; they typically make the solution more acidic.