The statement that best describes the difference between a substance with a pH of 3.0 and a substance with a pH of 6.0 is:
The substance with the lower pH has 1,000 times as many hydrogen ions per volume of water.
This is because the pH scale is logarithmic, meaning that each decrease of 1 in pH corresponds to a tenfold increase in the concentration of hydrogen ions (H⁺). Therefore, a pH of 3.0 (which is 3 units lower than pH 6.0) indicates that the hydrogen ion concentration is \(10^3\) (or 1,000) times higher.