Chemistry. a compound usually having a sour taste and capable of neutralizing alkalis and reddening blue litmus paper, containing hydrogen that can be replaced by a metal or an electropositive group to form a salt, or containing an atom that can accept a pair of electrons from a base. Acids are proton donors that yield hydronium ions in water solution, or electron-pair acceptors that combine with electron-pair donors or bases.
a substance with a sour taste.
something, as a remark or piece of writing, that is sharp, sour, or ill-natured:
His criticism was pure acid.
Slang. (def 2).

belonging or pertaining to acids or the anhydrides of acids.
having only a part of the hydrogen of an acid replaced by a metal or its equivalent:
an acid phosphate.
having a pH value of less than 7.
Compare (def 4).

sharp or biting to the taste; tasting like vinegar; sour:
acid fruits.
sharp, biting, or ill-natured in mood, manner, etc.:
an acid remark; an acid wit.
Geology. containing much silica.
Metallurgy. noting, pertaining to, or made by a process in which the lining of the furnace, or the slag that is present, functions as an acid in high-temperature reactions in taking electrons from oxide ions: usually a siliceous material, as sand or ganister.
Compare (def 3).
put on the acid, Australian Slang. to importune someone, as for money, sexual favors, or confidential information.
Historical Examples

It differs from common antimonic acid in being tetra, and forming two different classes of salts with the acids.
Cooley’s Practical Receipts, Volume II Arnold Cooley

Do not pour them into the sink, as acids ruin sink drainpipes.
Common Science Carleton W. Washburne

Resists the action of iodine, chlorine, alkalies, and acids.
Cooley’s Cyclopdia of Practical Receipts and Collateral Information in the Arts, Manufactures, Professions, and Trades…, Sixth Edition, Volume I Arnold Cooley

It turns green when it has been acted upon by acids, or exposed to the air.
A Handbook of Health Woods Hutchinson

In addition to the two elements present in binary acids, the great majority of acids also contain oxygen.
An Elementary Study of Chemistry William McPherson

The lesson is plain: it is that you and I are ferments of joy or acids of gloom.
Evening Round Up William Crosbie Hunter

With the acids it forms salts, which are mostly crystallisable.
Cooley’s Cyclopdia of Practical Receipts and Collateral Information in the Arts, Manufactures, Professions, and Trades…, Sixth Edition, Volume I Arnold Cooley

They bear a very interesting relation to the acids of nitrogen.
An Elementary Study of Chemistry William McPherson

They all contain nitrogen, and most of them are characterized by their power to combine with acids to form salts.
An Elementary Study of Chemistry William McPherson

Salts of all the acids are known, however, and some of them have commercial value.
An Elementary Study of Chemistry William McPherson

any substance that dissociates in water to yield a sour corrosive solution containing hydrogen ions, having a pH of less than 7, and turning litmus red See also Lewis acid
a sour-tasting substance
a slang name for LSD

of, derived from, or containing acid: an acid radical
being or having the properties of an acid: sodium bicarbonate is an acid salt

sharp or sour in taste
cutting, sharp, or hurtful in speech, manner, etc; vitriolic; caustic
(of rain, snow, etc) containing pollutant acids in solution
(of igneous rocks) having a silica content of more than 60% of the total and containing at least one tenth quartz
(metallurgy) of or made by a process in which the furnace or converter is lined with an acid material: acid steel

1620s, “of the taste of vinegar,” from French acide (16c.) or directly from Latin acidus “sour, sharp,” adjective of state from acere “to be sour,” from PIE root *ak- “sharp, pointed” (see acrid). Figurative use from 1775; applied to intense colors from 1916. Acid test is American English, 1892, from the frontier days, when gold was distinguished from similar metals by application of nitric acid. Acid rain is first recorded 1859 in reference to England.

1690s, from acid (adj.). Slang meaning “LSD-25” first recorded 1966 (see LSD).

When I was on acid I would see things that looked like beams of light, and I would hear things that sounded an awful lot like car horns. [Mitch Hedberg, 1968-2005, U.S. stand-up comic]

Acid rock (type played by or listen to by people using LSD) is also from 1966; acid house dance music style is 1988, probably from acid in the hallucinogenic sense + house “dance club DJ music style.”

acid ac·id (ās’ĭd)

Any of a large class of sour-tasting substances whose aqueous solutions are capable of turning blue litmus indicators red, of reacting with and dissolving certain metals to form salts, and of reacting with bases or alkalis to form salts.

A substance that ionizes in solution to give the positive ion of the solvent.

A substance capable of yielding hydrogen ions.

A proton donor.

An electron acceptor.

A molecule or ion that can combine with another by forming a covalent bond with two electrons of the other.

A substance having a sour taste.

See LSD.


Of or relating to an acid.

Having a high concentration of acid.

Having a sour taste.

Any of a class of compounds that form hydrogen ions when dissolved in water, and whose aqueous solutions react with bases and certain metals to form salts. Acids turn blue litmus paper red and have a pH of less than 7. Their aqueous solutions have a sour taste. Compare base.

acidic adjective

A sour-tasting material (usually in a solution) that dissolves metals and other materials. Technically, a material that produces positive ions in solution. An acid is the opposite of a base and has a pH of 0 to 7. A given amount of an acid added to the same amount of a base neutralizes the base, producing water and a salt. Common vinegar, for example, is a weak solution of acetic acid.

Note: Figuratively, acid applies to anything sour or biting; for example, an “acid wit” is sharp and unpleasant.


: an acid party


The hallucinogen LSD, which is chemically an acid; a (Narcotics)

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    to make somewhat . to sour; embitter. Historical Examples acidulate the filtered fluid slightly with hydrochloric acid, evaporate it to the bulk of 1⁄2 an ounce, and let it cool. Cooley’s Practical Receipts, Volume II Arnold Cooley A cup of roselle juice diluted is better to acidulate with than the lemon juice. The Khaki Kook […]

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    to make somewhat . to sour; embitter. Historical Examples The degree of acidulation does not appear to be very important, but about one-tenth per cent of strong acid does well. On Laboratory Arts Richard Threlfall verb (transitive) to make slightly acid or sour

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    Chemistry, . . an acidulating substance.

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