Perhaps no two classes of compounds are more important in chemistry than acids and bases. All acids have several properties in common: They have a sour taste, and they all react with most metals to form hydrogen gas (H 2) and with baking soda to form carbon dioxide (CO 2). All acids turn blue litmus paper red, and their solutions conduct electricity because acids form ions when dissolved in water. All bases also share several common properties: They have a bitter taste; their solutions feel slippery like soapy water; and they turn red litmus paper blue (the opposite of acids). Solutions of bases also conduct electricity because they, too, form ions in water. Acids are similar because they produce a hydronium ion, H 3O + ( aq), in water. Bases, on the other hand, all form a hydroxide ion, OH –( aq), in water. These ions are responsible for the properties of acids and bases.
The pH scale was developed to express, in a convenient way, the concentration of hydrogen ion in solutions and is widely used when discussing acids and bases.
In the late 1800s, Svante Arrhenius defined an acid as a substance that increases the hydronium ion (H 3O +) concentration in water, and a base as any substance that increases the hydroxide ion (OH –) concentration in water. Acids and bases react with one another in a process called neutralization to form a salt and water. Hydrochloric acid neutralizes potassium hydroxide forming potassium chloride (a salt) and water: