An acid is a substance that typically donates protons (hydrogen ions, H⁺) in a chemical reaction and has a pH level less than 7 in aqueous solutions. Acids have a sour taste, can conduct electricity in solution due to the presence of ions, and can react with bases to form salts and water in neutralization reactions.
Chemically, acids can be classified into strong acids, which completely dissociate in water (e.g., hydrochloric acid, sulfuric acid), and weak acids, which partially dissociate (e.g., acetic acid, citric acid). Acids can also react with metals, producing hydrogen gas and a corresponding salt.
In the context of Brønsted-Lowry acid-base theory, an acid is defined as any substance that can donate a proton to a base. In Lewis acid-base theory, acids are defined as electron pair acceptors.