Definition of acid in English:

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Pronunciation: /ˈasɪd/


1A substance with particular chemical properties including turning litmus red, neutralizing alkalis, and dissolving some metals; typically, a corrosive or sour-tasting liquid of this kind. Often contrasted with alkali and base1. trees were exposed to mixtures of heavy metals, acids, and overdoses of nutrients [mass noun]: traces of acid
More example sentences
  • It can withstand high temperatures and is resistant to many corrosive substances such as acids and alkalis.
  • Zinc is a fairly active metal that dissolves in both acids and strong alkalis.
  • Copper is a moderately reactive metal that dissolves in most acids and alkalis.
1.1 [mass noun] Bitter or cutting remarks or tone of voice: she was unable to quell the acid in her voice
More example sentences
  • The bitterness and acid in his voice reassured me Peter was still in there, but he was very, very upset.
  • I guess I hadn't spoken with such acid in my tone for a while.
  • Her tone lacked its previous acid; she almost sounded sincere.

Acids are compounds which release hydrogen ions (H+) when dissolved in water. Any solution with a pH of less than 7 is acidic, strong acids such as sulphuric or hydrochloric acid having a pH as low as 1 or 2.

2 Chemistry A molecule or other species which can donate a proton or accept an electron pair in reactions.
Example sentences
  • They would like to use their carborane acids to bind protons to atoms of the inert gas xenon.
  • Weak acids have dissociable protons like strong acids, but they simply do not dissociate completely.
  • The acid's active ingredient is positively charged hydrogen, so a transfer of electrons takes place between the zinc and the acid.
3 [mass noun] informal The drug LSD: she didn’t have a clue the sweet had acid in it [as modifier]: a bad acid trip
More example sentences
  • He has used mescaline, cocaine and acid, although he has not indulged in intravenous drugs.
  • He looked around more, seeing many different things that he would never expect: cocaine, meth and acid.
  • That heroin and acid were and are illegal didn't seem to stop him.


1Containing acid or having the properties of an acid; having a pH of less than 7. Often contrasted with alkaline or basic. acid soils
More example sentences
  • In the case of acid soils, burning decreases acidity, which starts to increase again during the planting period.
  • Boxwood prefer slightly acid to slightly alkaline soil and do very well when planted in early fall.
  • It used to be that, if you had a very acid soil, the hydrangeas would be bluer; more alkaline soil, your hydrangeas would be pinker.
2Sharp-tasting or sour: acid fruit
More example sentences
  • Sour and acid tastes are liked, and are manifest in the use of lime juice, tamarind, etc.
  • On the visit to the GP I saw a locum who said I must sit about for the next week, not use the computer, and keep off acid fruits.
  • You can't eat it fresh because it's so acid, but often these highly acid fruits have the best flavour, and it makes the most wonderful fruit drinks, sorbets, ice creams and other things.
acidic, sour, tart, bitter, unsweetened, sharp, biting, acrid, pungent, acerbic, vinegary, vinegarish, acetic, acetous
rare acidulous, acidulated
2.1(Of a person’s remarks or tone) bitter or cutting: she was stung into acid defiance
More example sentences
  • He has become more prominent since Henry took over in November 1999 and adds the sugar to the coach's acid remarks when things are not going well.
  • Ignoring her acid tone, he mumbled, ‘You're very snappy this afternoon.’
  • ‘Please leave, I do not wish to speak to you,’ she said in quietly acid tones that immediately raised his concern.
acerbic, sarcastic, sharp, sardonic, satirical, scathing, cutting, razor-edged;
incisive, penetrating, piercing, biting, stinging, searing;
keen, caustic, trenchant, mordant, bitter, acrimonious, astringent;
harsh, severe, abrasive, wounding, hurtful, unkind, cruel, vitriolic, virulent, venomous, poisonous, waspish, spiteful, vicious, malicious;
North American  acerb
informal bitchy, catty
British informal sarky
North American informal snarky
rare mordacious, acidulous
2.2(Of a colour) strikingly intense or bright: an acid green
More example sentences
  • The art students brought their acid colour combinations, their lilacs, tangerines and lime greens from abstract painting.
  • She reckons you can get away with virtually anything, particularly acid colours.
  • Then her eyes started to glow an acid green colour that swirled in her eyes, almost manically. ‘Sweet dreams.’
3 Geology (Of rock, especially lava) containing a relatively high proportion of silica: the magma may start off fairly basic and end up at the close of the eruption much more acid
More example sentences
  • Acraman occurs in the Gawler Range Volcanics, a Mesoproterozoic continental suite of mainly acid lavas and ash flows.
  • In the Southern Central Iberian Zone there are minor acid volcanic rocks intercalated with Caradoc-Ashgill limestones.
  • These deposits are intimately associated with the porphyric parts of intermediate to acid plutons in orogenic belts.
3.1 Metallurgy Relating to or denoting steel-making processes involving silica-rich refractories and slags: the acid Bessemer process
More example sentences
  • In the acid processes, deoxidation can take place in the furnaces, leaving a reasonable time for the inclusions to rise into the slag and so be removed before casting.
  • Therefore, a business opportunity could be created for an entrepreneur interested in running the acid separation and purification process.
  • As it happened, Workington's adherence to acid steelmaking sealed the fate of the ore dock in the next few years.


put the acid on

Australian /NZ informal Seek to extract a loan or favour from (someone).
Acid from acid test, referring to possible resistance (because gold resists nitric acid)
Example sentences
  • Plans to erect fences around those sectors are underway with Labor Council likely to put the acid on at least 12 Sydney-based councils in the near future.
  • A building inspector, put the acid on a builder for initially $40,000, but reduced it to $20,000 and then foolishly took two cheques each for $10,000.
  • Labour Council will put the acid on the Environment Minister to fund training for delegates and organisers about how they can utilise existing legislation to become frontline environmental watchdogs.



Example sentences
  • The grilled lemon chicken special was great, and showed that the chef has real skill - there's nothing better than the blend of acidy lemon with a bit of charred poultry.
  • The taste changes to become sweet and a little acidy.
  • There's homemade tzatziki with everything, and it's acidy and flavourful.




Early 17th century (in the sense 'sour-tasting'): from Latin acidus, from acere 'be sour'.

  • Acid originally meant ‘sour-tasting’ and came from Latin acidus. The term seems to have been introduced by the scientist Francis Bacon, who in 1626 described sorrel as ‘a cold and acid herb’. The chemical sense developed at the end of that century because most common acids taste sour. The acid test was originally a method of testing for gold using nitric acid. An object made of gold will show no sign of corrosion if immersed in nitric acid, unlike one made of another metal. By the late 19th century the expression had come to mean any situation that proves a person's or thing's quality. The Australian expression put the acid on, meaning ‘to extract a loan or favour from’, comes from acid test—the would-be borrower is seen as ‘testing’ their victim for resistance or weakness. Acrid (early 18th century) is from the related Latin acer ‘sharp, pungent’ with spelling influenced by acid.

Words that rhyme with acid

Abbasid, antacid, flaccid, Hasid, placid

For editors and proofreaders

Line breaks: acid

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