Which statement best defines an acid?

• A. A substance that increases the hydrogen ion concentration of a solution.
• B. A substance that binds to a surface.
• C. A substance that neutralizes bases.
• D. A substance that lowers pH.

Answer: (A)

Acids are defined by what they do in water: they increase the hydrogen ion concentration in the solution. When an acid dissolves, it donates protons (H+), which in practice become hydronium ions (H3O+). This rise in H3O+ lowers the pH, which is the observable sign of acidity. The other ideas describe things that aren’t what makes something an acid: binding to a surface isn’t related to acidity, neutralizing bases is a common acid–base reaction but not the defining feature, and simply lowering pH is a consequence of increased H+ rather than the defining property itself.