FPGEE for National Association of Boards of Pharmacy (NABP) Practice Exam 2026 - Free NABP Practice Questions and Study Guide

An acid-base reaction is primarily characterized by the transfer of hydrogen ions (protons) from one species to another. In this context, acids are substances that donate hydrogen ions, while bases are those that accept hydrogen ions. When an acid reacts with a base, the hydrogen ions released by the acid interact with the base, resulting in the formation of water and a salt in many cases.

This transfer of hydrogen ions is the fundamental principle of the Brønsted-Lowry definition of acids and bases, which expands upon earlier definitions by emphasizing proton transfer. For example, when hydrochloric acid (HCl) dissolves in water, it donates a proton to a water molecule, creating hydronium ions (H3O+) and chloride ions (Cl-). This process exemplifies the characteristic mechanism of an acid-base reaction.

The other options present related chemical processes, but they do not accurately define the central aspect of acid-base reactions. Electron transfer is more characteristic of redox reactions. While the transfer of hydroxide ions and the formation of water can occur in acid-base reactions (especially in neutralization reactions), they are not the defining characteristics of such reactions. Thus, the focus on hydrogen ions is what distinctly characterizes the acid-base reaction