Acid, a chemical compound that is capable of releasing hydrogen ions (electrically charged hydrogen atoms) for a chemical reaction. All acids contain hydrogen, but not all hydrogen-containing compounds are acids. Most acids also contain oxygen.

Long, chainlike compounds called nucleic acids are found in all living cells and are essential to life. Amino acids, also found in living organisms, are the building blocks of proteins. Other acids, such as sulfuric acid and nitric acid, are of tremendous importance industrially. Sulfuric acid, familiar as the acid in automobile storage batteries, is the largest single product of the chemical industry.