What is the definition of an acid in terms of its behavior in water?

An acid is defined by its ability to release hydrogen ions (H+) when dissolved in water. This release of hydrogen ions increases the concentration of H+ in the solution, resulting in a decrease in pH, typically below 7. This characteristic is fundamental to the definition of acids in the context of aqueous solutions, as the pH scale measures the hydrogen ion concentration in a solution, with values below 7 indicating acidity.

In contrast, the other definitions do not accurately describe the behavior of acids. A compound that releases hydroxide ions would be classified as a base, while a compound with a pH exactly at 7 is neutral, such as pure water. Additionally, a compound that has no effect on pH does not fit the definition of an acid, since acids, by their nature, change the pH of a solution by increasing the concentration of hydrogen ions. Thus, the correct interpretation of an acid's behavior in water is the release of hydrogen ions leading to a pH of less than 7.