What contributes to the acidity or alkalinity measured by pH?

The pH scale measures the acidity or alkalinity of a solution based on the concentration of hydrogen ions (H⁺) present. A solution is considered acidic if it has a high concentration of hydrogen ions and a low pH value, while a solution is alkaline if it has a low concentration of hydrogen ions and a high pH value. The relationship is logarithmic; for every unit change in pH, the hydrogen ion concentration changes by a factor of ten. Thus, it is the concentration of hydrogen ions that directly influences the pH readings, confirming that this choice is the key factor in determining acidity or alkalinity.