All acids contain the element hydrogen. When an acid is dissolved in water, it releases hydrogen ions, which combine with water molecules to form hydronium ions, H3O+.
Acids are classified as weak or strong. A weak acid only partially ionizes in water. Not all of its hydrogen atoms break away to form hydronium, but a strong acid effectively fully ionizes. Hydrochloric acid, HCl, and nitric acid, HNO3, are examples of strong acids. Acetic acid, HC2H3O2, is a weak acid.
Acidity is measured by the pH scale; pH means potential hydrogen although sometimes it is written as power of hydrogen. An acid has a pH of 0 to 7. Anything wit a pH higher than 7 is considered to be a base.
French chemist Antoine Lavoisier first put forth the theory in the late 18th century that oxygen was the element common to all acids. However, a few acids were discovered that did not contain oxygen, such as hydrochloric acid, and in the mid-19th century, Justus von Liebig of Germany proposed that hydrogen was the element that gave acids their properties.
In 1890, Swedish chemist and physicist Svante Arrhenius formulated the first modern theory of acids, detailing how an acid dissociates in water to form hydronium and a negative ion.