Advanced Placement (AP) Biology Practice Exam

What does the pH scale measure?

1. The concentration of water in a solution

2. The number of molecules in a chemical reaction

3. How acidic or basic a solution is

4. The temperature of a solution

The correct answer is: How acidic or basic a solution is

The pH scale is specifically designed to measure how acidic or basic a solution is. It quantifies the concentration of hydrogen ions (H+) in a solution: the lower the pH value, the higher the concentration of hydrogen ions, indicating higher acidity; conversely, a higher pH value indicates lower hydrogen ion concentration, signifying a more basic solution. The pH scale typically ranges from 0 to 14, with 7 being neutral (pure water), values below 7 indicating acidity, and values above 7 indicating basicity. This is crucial in biology as many biological processes are sensitive to changes in pH, impacting enzyme activity, cell function, and metabolic pathways.