How is an acid defined in terms of hydrogen ions?

An acid is defined as a substance that donates hydrogen ions, H⁺, when dissolved in water. This definition stems from the Brønsted-Lowry theory of acids and bases, which describes acids as proton donors. When an acid dissociates in an aqueous solution, it releases hydrogen ions into the solution, contributing to an increase in the concentration of H⁺ ions. This presence of hydrogen ions is what characterizes the acidic properties of the solution, leading to a lower pH value (typically less than 7).

The incorrect options reflect misunderstandings of what constitutes an acid. For instance, a source of hydroxide ions would refer to a base, not an acid. A pH greater than 7 indicates a basic solution, which also does not align with the definition of an acid. Furthermore, defining an acid as a neutral compound in water contradicts the fundamental properties of acids, as they create an acidic environment through the release of H⁺ ions.