What characterizes a substance with a pH less than 7?

A substance with a pH less than 7 is characterized as an acid. The pH scale ranges from 0 to 14, with values below 7 indicating increasing acidity. When the pH of a solution is below 7, it signifies an excess of hydrogen ions ((H^+)) in the solution, contributing to the acidic nature of the substance.

In contrast, a neutral solution has a pH of approximately 7, which indicates that the concentration of hydrogen ions is equal to the concentration of hydroxide ions ((OH^-)). Bases, or alkaline substances, have a pH greater than 7, demonstrating a deficiency of hydrogen ions relative to hydroxide ions. A salt, which results from the neutralization reaction between an acid and a base, does not inherently possess a pH lower than 7; its pH can vary depending on the strengths of the parent acid and base. Thus, the correct characterization of a substance with a pH less than 7 is that it is an acid.