View synonyms for acid


[ as-id ]


  1. 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.
  2. a substance with a sour taste.
  3. something, as a remark or piece of writing, that is sharp, sour, or ill-natured:

    His criticism was pure acid.

  4. Slang. LSD 1.


  1. Chemistry.
    1. belonging or pertaining to acids or the anhydrides of acids.
    2. having only a part of the hydrogen of an acid replaced by a metal or its equivalent:

      an acid phosphate.

    3. having a pH value of less than 7. Compare alkaline ( def 4 ).
  2. sharp or biting to the taste; tasting like vinegar; sour:

    acid fruits.

  3. sharp, biting, or ill-natured in mood, manner, etc.:

    an acid remark; an acid wit.

    Synonyms: tart, vitriolic, biting, acerbic

  4. Geology. containing much silica.
  5. 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 basic ( def 3 ).


/ ˈæsɪd /


  1. 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
  2. a sour-tasting substance
  3. a slang name for LSD
“Collins English Dictionary — Complete & Unabridged” 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012


  1. chem
    1. of, derived from, or containing acid

      an acid radical

    2. being or having the properties of an acid

      sodium bicarbonate is an acid salt

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

    acid steel

“Collins English Dictionary — Complete & Unabridged” 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012


/ ăsĭd /

  1. 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.
  2. Compare base


  1. 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.

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

  • ˈacidly, adverb
  • ˈacidness, noun
  • ˈacidy, adjective
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Other Words From

  • ac·id·ly adverb
  • ac·id·ness noun
  • non·ac·id noun adjective
  • pre·ac·id adjective
  • pre·ac·id·ness noun
  • sem·i·ac·id adjective
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Word History and Origins

Origin of acid1

First recorded in 1620–30; from Latin acidus “sour,” akin to ācer “sharp,” acētum “vinegar”; acescent, acicula
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Word History and Origins

Origin of acid1

C17: (first used by Francis Bacon): from French acide or Latin acidus, from acēre to be sour or sharp
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Idioms and Phrases

  1. put on the acid, Australian Slang. to importune someone, as for money, sexual favors, or confidential information.
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Synonym Study

Acid, astringent are terms used figuratively of wit or humor. Acid suggests a sharp, biting, or ill-natured quality: an acid joke about an opponent. Astringent connotes severity but usually also a bracing quality, as of something applied with curative intent: astringent criticism.
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Example Sentences

The researchers’ process involves first bathing the bricks in a hydrochloric acid vapor, which seeps into the pores and reacts with the iron oxide that gives the bricks their red color.

Other methods exist to determine whether people are capable of infecting others, including monoclonal antibodies and CRISPR gene-editing techniques to detect viral proteins or bits of nucleic acid.

There, Gilbert uses amino acids to get proteins to communicate with each other.

Most potential drug targets are proteins, and a protein’s structure—meaning the way a 2D sequence of amino acids folds into a 3D protein—determines its function.

Sasselov has encouraged Globus and Blandford to consider whether cosmic rays might join forces with polarized light to shape the amino acids on asteroids.

I write the lyrics and work with Murv Douglas from Lords of Acid.

During the height of his disenchantment, he visited his hometown where an old friend gave him some liquid acid.

Eggs, he says, are a good source of cysteine, an amino acid that helps the liver break down alcohol faster.

This at-home blood test kit gives a full reading of antioxidant, fatty acid, or vitamin panels.

The recent spate of acid attacks on women is only the latest manifestation of this dangerous trend.

There are a number of bacilli, called acid-fast bacilli, which stain in the same way as the tubercle bacillus.

The amount of the other purin bodies together is about one-tenth that of uric acid.

Uric acid is decreased before an attack of gout and increased afterward, but its etiologic relation is still uncertain.

An increase is also noted in the uric-acid diathesis and in diseases accompanied by respiratory insufficiency.

Indol is absorbed and oxidized into indoxyl, which combines with potassium and sulphuric acid and is thus excreted.


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More About Acid

What does acid mean?

In science, an acid is a sour-tasting substance that releases hydrogen ions when added to water. Acids will turn litmus to a red color and have a pH lower than 7.

An acid is any substance that will release hydrogen ions when mixed with water. The amount of hydrogen ions that a substance releases is measured on the pH scale, which ranges from 0 to 14. The lower the pH number is, the more hydrogen ions that are being released. Water has a pH of 7, which is neutral on the pH scale. All acids have a pH lower than 7. A pH greater than 7 indicates an alkali, or base.

Acids also have a sour taste, such as the acid found in lemons. A final quality that all acids share is that when they come into contact with litmus paper, they will turn the paper red.

Acids are all around us and are used for a wide variety of purposes. Some common acids you can find in your house include lactic acid (in milk), ascorbic acid (in citrus fruits), and acetic acid (in vinegar).

Why is acid important?

The first records of the term acid come from around 1620. It comes from the Latin word acidus meaning “sour.” Acids have a sour taste, although sometimes it is a very bad idea to put an acid in your mouth.

Several types of acids, such as the ones present in foods we commonly eat, are not only safe to eat but your body actually needs them to stay healthy. Some acids, however, are dangerous and can damage your skin if it touches it or organs if ingested. Hydrochloric acid, which is used in manufacturing, and sulfuric acid, which is used in making fertilizers, are two acids that can burn your skin, damage your organs, and lead to serious health problems with prolonged exposure. Substances like these are considered to be hazardous materials, and there are usually strict laws about handling and transporting these dangerous acids.

Did you know … ?

Your stomach uses a substance called gastric acid to digest the food you eat. It’s also able to kill many bacteria and other organisms that might be in your food or that enter your stomach. Some diseases, spicy foods, or even stress can cause your stomach acid to enter your esophagus. This condition is known as acid reflux. This is why medicines that treat acid reflux by neutralizing stomach acid are referred to as antacids.

What are real-life examples of acid?

Students of chemistry learn about acids early in their studies.


What other words are related to acid?

Quiz yourself!

True or False?

Acids have a pH higher than 7.

Definitions and idiom definitions from Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.




aciculumacid anhydride