What is pH a measure of?

The correct interpretation is that pH measures the acidity or alkalinity of a solution. It is a numerical scale that quantifies the concentration of hydrogen ions (H⁺) in a solution. A lower pH value indicates a higher concentration of hydrogen ions, which correlates with increased acidity, while a higher pH value suggests a lower concentration of hydrogen ions and increased alkalinity.

Understanding pH is crucial in various scientific and practical applications, including chemistry, biology, medicine, and environmental science, as it can influence chemical reactions, biological processes, and the behavior of substances in solutions. This concept is central to fields such as water quality assessment, where the pH level can affect aquatic life, or in medicine, where pH variations can indicate specific health conditions.

The other choices do not accurately reflect what pH represents. Temperature relates to how hot or cold a solution is, concentration of dissolved substances is broader than just hydrogen ions, and viscosity pertains to the thickness or flow characteristics of a fluid, none of which are directly tied to the concept of pH.