Acids and Bases

An important consideration when dealing with acids and bases is their strength; that is, how chemically reactive they act as acids and bases. The strength of an acid or base is determined by the degree of ionization of the acid or base in solution—that is, the percentage of dissolved acid or base molecules that release hydrogen or hydroxide ions. If all of the dissolved acid or base separates into ions, it is called a strong acid or strong base. Otherwise, it is a weak acid or weak base. There are only a few strong acids: hydrochloric acid (HCl), hydrobromic acid (HBr), hydriodic acid (HI), perchloric acid (HClO₄), nitric acid (HNO₃), and sulfuric acid (H₂SO₄). Similarly, there are only a few strong bases: lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca[OH]₂), strontium hydroxide (Sr[OH]₂), and barium hydroxide (Ba[OH]₂).

These strong acids and bases are 100% ionized in aqueous solution. All other Arrhenius acids and bases are weak acids and bases. For example, acetic acid (HC₂H₃O₂) and oxalic acid (H₂C₂O₄) are weak acids, while iron hydroxide, Fe(OH)₃, and ammonium hydroxide, NH₄OH (which is actually just ammonia, NH₃, dissolved in water), are examples of weak bases. The percentage of the acid and base molecules that are ionized in solution varies and depends on the concentration of the acid. For example, a 2% solution of acetic acid in water, which is about the concentration found in vinegar, is only 0.7% ionized. This means that fully 99.3% of the acetic acid molecules are unionized and exist in solution as the complete acetic acid molecule.