You are correct that the pH is a measure of the concentration of hydrogen ions (H+) in a solution. In pure water, which is neutral, the concentration of hydrogen ions (H+) is equal to the concentration of hydroxide ions (OH-), resulting in a balanced solution. In this case, the pH is 7.
However, when both the H+ and OH- concentrations increase simultaneously, the situation changes. This scenario typically arises when an acidic or basic substance is added to the water or when a chemical reaction occurs that generates or consumes H+ or OH-.
If the concentration of H+ increases while the concentration of OH- remains constant or decreases, the solution becomes more acidic, and the pH decreases. Conversely, if the concentration of OH- increases while the concentration of H+ remains constant or decreases, the solution becomes more basic, and the pH increases.
The pH scale is logarithmic, meaning that each unit represents a tenfold difference in acidity or alkalinity. Therefore, even small changes in the concentrations of H+ and OH- can lead to significant shifts in pH.
It's important to note that if the concentrations of H+ and OH- are equal, as in the case of pure water, the pH remains neutral at 7. However, any deviation from this balance will result in a change in pH.