How does the pH scale measure acidity or alkalinity?

The pH scale is a logarithmic scale that measures the concentration of hydrogen ions (H⁺) in a solution, providing insight into the acidity or alkalinity of that solution. A pH value of 7 is considered neutral, indicating a balanced concentration of H⁺ ions.

When the pH value falls below 7, it signifies an increase in hydrogen ion concentration, thus indicating acidity. Conversely, a pH value greater than 7 reflects a decrease in hydrogen ion concentration, which means the solution is more alkaline (basic). This relationship is fundamental in characterizing solutions as acidic or basic.

Therefore, the correct answer accurately summarizes this relationship, stating that a pH less than 7 indicates acidity, while a pH greater than 7 indicates alkalinity.