The substance with the lowest pH (0) is battery acid. It is a highly concentrated solution of sulfuric acid in water. Battery acid is used in lead-acid batteries, which are commonly found in cars and other vehicles. The pH scale measures the acidity or basicity of a substance on a scale from 0 to 14, with 0 being the most acidic and 14 being the most basic. Battery acid has a pH of 0, which means that it is extremely acidic.