1. Tasting sour like acid; SYN. acidulent, acidulous.
2. (Chemistry) Relating to or containing an acid; having a pH of less than 7.

Compound that, in solution in an ionizing solvent (usually water), gives rise to hydrogen ions (H+ or protons). In modern chemistry, acids are defined as substances that are proton donors and accept electrons to form ionic bonds. Acids react with bases to form salts, and they act as solvents. Strong acids are corrosive; dilute acids have a sour or sharp taste, although in some organic acids this may be partially masked by other flavor characteristics.
Acids can be detected by using colored indicators such as litmus and methyl orange. The strength of an acid is measured by its hydrogen-ion concentration, indicated by the pH value. Acids are classified as monobasic, dibasic, tribasic, and so forth, according to the number of hydrogen atoms, replaceable by bases, in a molecule. The first known acid was vinegar (ethanoic or acetic acid). Inorganic acids include boric, carbonic, hydrochloric, hydrofluoric, nitric, phosphoric, and sulfuric. Organic acids include acetic, benzoic, citric, formic, lactic, oxalic, and salicylic, as well as complex substances such as nucleic acids and amino acids.
Any of various water-soluble compounds having a sour taste and capable of turning litmus red and reacting with a base to form a salt.