What characterizes an acid in terms of pH?

An acid is characterized as a proton donor, which refers to its ability to release hydrogen ions (H⁺) into a solution. This property is fundamental to the Brønsted-Lowry definition of acids and bases, where an acid is defined as a substance that can donate a proton to another substance.

In terms of pH, acids have a pH value of less than 7. The pH scale measures the concentration of hydrogen ions in a solution; lower pH values indicate a higher concentration of hydrogen ions, which is a key feature of acidic solutions. Therefore, when an acid is dissolved in water, it significantly increases the number of free hydrogen ions, consequently lowering the pH below the neutral point of 7.

This understanding is crucial in water quality assessment, as it allows for the determination of the acidity of water samples and helps in assessing environmental impacts such as acid rain or discharge from industrial processes.