What is an acid?

An acid is defined as a substance that, when dissolved in water, dissociates to produce hydrogen ions (H+). This definition stems from the Brønsted-Lowry acid-base theory, which states that an acid is a proton donor. The presence of hydrogen ions in solution is what gives acids their characteristic properties, such as sour taste and the ability to conduct electricity.

For example, hydrochloric acid (HCl) dissociates in water to form H+ and Cl- ions. The increase in hydrogen ion concentration in a solution results in a lower pH value, which is a hallmark of acidic solutions.

In contrast, other options do not capture the essential characteristic of acids. Compounds that form hydroxide ions in solution are classified as bases, not acids. Additionally, a substance that reacts only with metals is too narrow and does not encompass the broader range of reactions that acids can have, particularly with bases in neutralization reactions. Lastly, a neutral substance in solution, which maintains a balanced pH and does not increase the concentration of either H+ or OH- ions, cannot be considered an acid since it does not meet the basic criteria of producing hydrogen ions.

Therefore, the defining feature of an acid is its