Acids and bases differ in chemical structure and reactions, pH level and taste. When interacting with phenolphthalein and litmus, each changes to a different color. Substances with a pH greater than seven are basic and those with a pH less than seven are acidic. Acidic substances taste sour, while basic substances are bitter.
The chemical formulas for all acids begin with H, or hydrogen, with the one exception of acetic acid (vinegar), which has the formula CH3COOH. Base compounds' chemical formulas end with OH, or hydroxide. The formula for water, which is neutral, combines both acid, H, and base, OH.
When acid is placed in phenolphthalein, an indicator, it is colorless. In contrast, a base turns a pink color when placed in phenolphthalein. On a blue litmus strip, acid turns red; on a red strip of litmus paper, a base turns blue.
Science defines acid and base in a variety of ways, though there are three definitions that dominate: The Arrhenius Definition defines an acid as a compound that produces a hydrogen ion in aqueous solutions. Under the Brønsted-Lowry Definition, an acid is a proton donor whereas a base is a proton acceptor. The Lewis Theory defines acids as electron pair acceptors and bases as electron pair donors.