What indicates the number of free H-ions in a solution?

The pH of a solution is a direct measure of the concentration of free hydrogen ions (H+) present in that solution. The pH scale ranges from 0 to 14, with lower values indicating a higher concentration of hydrogen ions (more acidic conditions) and higher values indicating a lower concentration (more basic or alkaline conditions).

When the pH decreases, it signifies that there is an increase in the number of free H+ ions, whereas an increase in pH reflects a decrease in free H+ ions. Therefore, pH is fundamentally linked to the activity of hydrogen ions and serves as a crucial indicator of the acidity or alkalinity of a solution.

Other options provided relate to concepts that do not specifically measure or indicate the concentration of free hydrogen ions. For instance, the hydrogen index may refer to the relative amount of hydrogen in various contexts but isn’t a standard measure of free H+ ions. The alkalinity index refers to the capacity of water to resist changes in pH, not a direct measure of hydrogen ion concentration. Acid-base indicators change color at certain pH levels but do not quantify the number of free hydrogen ions, rather they visually indicate whether the solution is acidic or basic. Hence, pH remains the most accurate and