Advanced Placement (AP) Biology Practice Exam

Which statement is true regarding pH changes?

1. A change of one unit represents a 10-fold difference in H+ concentration

2. A change of one unit means only a slight change in acidity

3. A pH of 7 indicates a base

4. Only strong acids affect pH values significantly

The correct answer is: A change of one unit represents a 10-fold difference in H+ concentration

A change of one unit in the pH scale represents a tenfold difference in hydrogen ion (H+) concentration, which is a fundamental concept in understanding acids, bases, and their effects on pH. The pH scale is logarithmic, meaning every one-unit change in pH corresponds to a tenfold increase or decrease in H+ concentration. For instance, a solution with a pH of 4 has ten times more H+ ions than a solution with a pH of 5. This principle is crucial in various biological processes, as even small changes in pH can significantly impact enzymatic activity, metabolic reactions, and cellular functions. In contrast, a change of one unit does not indicate only a slight change in acidity, as it actually signifies a substantial change. A pH of 7 is considered neutral, indicating a balance between H+ and hydroxide (OH−) ions, rather than being indicative of a basic solution. Lastly, while strong acids do have a pronounced effect on pH, weak acids can also significantly influence pH depending on their concentration and dissociation in solution. Thus, the understanding of pH changes and their implications is a critical part of studying biological systems.