Define pH and describe the scale used for pH.
pH: Numerical value that represents the acidity or alkalinity of a substance. The lower the number the more acid (H+). The higher the number, more alkaline (OH-). The scale for pH runs 0-14. 0=pure acid. 14=pure alkaline. Each pH unit is a tenfold difference, meaning each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6.