pH is a measure of the acidity or alkalinity of a solution. It is a scale from 0-14, where 0 is most acidic, 7 is neutral, and 14 is most alkaline. The acidity or alkalinity of a solution is determined by the concentration of hydrogen ions (H+) in the solution. The higher the concentration of hydrogen ions, the lower the pH level. pH affects many chemical reactions, such as the solubility of ionic compounds, the activity of enzymes and proteins, and the rate at which chemical reactions occur.