The strength of acids and bases is measured on a scale called the pH scale, which is shown in the Figure below . By definition, pH represents the acidity , or hydrogen ion (H + ) concentration, of a solution. Pure water, which is neutral, has a pH of 7. With a higher the concentration of hydrogen ions, a solution is more acidic and has a lower pH. Acids have a pH less than 7, and the strongest acids have a pH close to zero. Bases have a pH greater than 7, and the strongest bases have a pH close to 14. It’s important to realize that the pH scale is based on powers of ten. For example, a solution with a pH of 8 is 10 times more basic than a solution with a pH of 7, and a solution with a pH of 9 is 100 times more basic than a solution with a pH of 7.
Sally is taking a college chemistry class. She is trying to determine whether her test substances are an acid or a base. After using test strips to determine the concentration of hydrogen ions in each substance, she learns that Substance A has a pH of 1, Substance B has a pH of 3, Substance C has a pH of 7, Substance D has a pH of 10, and Substance E has a pH of 13.
Is Substance D a stronger or weaker base than Substance E?