What Is the Definition of "acid"?

Acid is defined as a chemical species that donates hydrogen ions or protons and accepts electrons. Scientists classify acids as weak or strong. Mineral acids include such compounds as sulfuric acid and hydrochloric acid. Organic acids include citric acid and acetic acid.

Acids are generally sour in taste. The sour flavor of citrus fruit is due to citric acid, and that of vinegar is due to acetic acid. In literature, this sour characteristic leads to the use of the term acid to indicate a biting or bitter person.

Acids have a pH below 7, which indicates a higher concentration of hydrogen ions than found in pure water. This low pH causes litmus paper to change in color from blue to red in the presence of an acid. Acids cause many metals, including iron, to corrode or rust. Nitrous oxide, a byproduct of burning fossil fuels, dissolves in the moisture in the air to cause acid rain.

Acids mix with bases to form neutral compounds. If they mix with carbonates, they release carbon dioxide bubbles. Mineral acids often mix with bases to form a salt and water. For example, hydrochloric acid mixes with sodium hydroxide to form sodium chloride and water. Since sodium chloride is soluble in water, the products require evaporation to recover sodium chloride crystals.