The correct 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 whole number change on the scale represents a tenfold change in acidity. Therefore, going from pH 3 to pH 6 represents a decrease in acidity by a factor of 10 three times (10 x 10 x 10 = 1,000).