ac·id / ˈasid/ • n. a chemical substance that neutralizes alkalis, dissolves some metals, and turns litmus red; typically, a corrosive or sour-tasting liquid of this kind. Often contrasted with alkali or base¹ . ∎ fig. bitter or cutting remarks or tone of voice. ∎  inf. the drug LSD. ∎ Chem. a molecule or other entity that can donate a proton or accept an electron pair in reactions.• adj. 1. containing acid or having the properties of an acid; in particular, having a pH of less than 7: poor, acid soils. Often contrasted with alkaline or basic. ∎  Geol. (of rock, esp. lava) containing a relatively high proportion of silica. ∎  Metallurgy relating to or denoting steelmaking processes involving silica-rich refractories and slags.2. sharp-tasting or sour: acid fruit. ∎  (of a person's remarks or tone) bitter or cutting. ∎  (of a color) intense or bright: an acid green.DERIVATIVES: ac·id·y adj.

acid A type of compound that contains hydrogen and dissociates in water to produce positive hydrogen ions. The reaction, for an acid HX, is commonly written: HX ↔ H⁺ + X^–

In fact, the hydrogen ion (the proton) is hydrated, and the complete reaction is: HX + H₂O ↔ H₃O⁺ + X^–

The ion H₃O⁺ is the hydroxonium ion. The strength of an acid depends on the extent to which it dissociates: strong acids (e.g. sulphuric acid and hydrochloric acid) are almost completely dissociated in water; weak acids (e.g. carbonic acid) are only partially dissociated. See also buffer; pH scale. Compare base.

acid Chemical compound containing hydrogen that can be replaced by a metal or other positive ion to form a salt.

Acids dissociate in water to yield aqueous hydrogen ions (H⁺), thus acting as proton donors. The solutions are corrosive and have a pH below 7. Strong acids, such as sulphuric acid, are fully dissociated into ions, making good electrolytes that conduct electricity. Weak acids, such as ethanoic acid, only partially dissociate. See also base

acid Chemically, compounds that dissociate (ionize) in water to give rise to hydrogen ions (H⁺); they taste sour. See also alkali; amino acids; buffers; esters; fatty acids; pH; salt.

acid According to the Brønsted–Lowry theory (proposed in 1923 by both J. N. Brønsted in Copenhagen and T. M. Lowry in Cambridge, who were working independently of one another), a substance that in solution liberates hydrogen ions or protons. The Lewis theory (proposed by G. N. Lewis, also in 1923) states that it is a substance that acts as an electron-pair acceptor. An acid reacts with a base to give a salt and water (neutralization), and has a pH of less than 7.