Acids are proton-donating chemical species that play a fundamental role in biochemical reactions, pH regulation, and molecular transformations in laboratory protocols. Their behavior is central to a wide range of applications in analytical chemistry, molecular biology, and pharmaceutical research.
Definitions
Acids can be defined according to several complementary theoretical frameworks. According to the Arrhenius definition, acids release H⁺ ions in aqueous solutions, forming hydronium ions (H₃O⁺). The Brønsted–Lowry theory defines acids as proton donors capable of transferring H⁺ to a base, while the Lewis definition considers acids as electron pair acceptors. Acidity is quantitatively expressed by pH (pH = −log[H⁺]), where values below 7 indicate acidic conditions. Strong acids, such as HCl, H₂SO₄, and HNO₃, fully dissociate in solution (Ka > 1), whereas weak acids, such as acetic acid (CH₃COOH, pKa = 4.76), only partially ionize.
Properties
Acids exhibit characteristic chemical and physical properties. They react with metals to release hydrogen gas (H₂), turn blue litmus paper red, and often present a sour taste, as observed in organic acids such as citric or lactic acid. In acid–base reactions, acids neutralize bases to form salts and water. Upon proton donation, they generate conjugate bases (e.g., acetate CH₃COO⁻ from acetic acid). Certain compounds, such as phosphoric acid (H₃PO₄), display amphoteric behavior, meaning they can act as both proton donors and acceptors depending on the pH, typically across a range from 2 to 12.
