The pH scale measures the acidity or basicity of a solution and ranges from 0 to 14, with 7 being neutral. Acids have a pH below 7, while bases have a pH above 7. The pH of an acid is lower than the pH of a base because acids release hydrogen ions (H+) into a solution, while bases release hydroxide ions (OH-). The higher the concentration of hydrogen ions, the lower the pH of the solution. Conversely, the higher the concentration of hydroxide ions, the higher the pH of the solution.