The pH Scale

The pH scale is a scale used by chemists in order to compare the relative strengths of acids or bases. pH stands for percentage of hydrogen ions, as they are the active mediator in acid and base reactions. pH uses a logarithmic scale to determine the concentration of hydrogen ions there are, given a substance.
It is known that pH can be defined as:

Thus a pH of 7 indicates there are 10-7 hydronium ions in concentration within the solution.

Common pHs:
The pH of a very strong acid may be 3 or lower.
The pH of a very strong base may be 10 or higher.
The pH of a neutral substance is 7.