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acids and bases

acids and bases, two related classes of chemicals; the members of each class have a number of common properties when dissolved in a solvent, usually water.


Acids in water solutions exhibit the following common properties: they taste sour; turn litmus paper red; and react with certain metals, such as zinc, to yield hydrogen gas. Bases in water solutions exhibit these common properties: they taste bitter; turn litmus paper blue; and feel slippery. When a water solution of acid is mixed with a water solution of base, water and a salt are formed; this process, called neutralization, is complete only if the resulting solution has neither acidic nor basic properties.


Acids and bases can be classified as organic or inorganic. Some of the more common organic acids are: citric acid, carbonic acid, hydrogen cyanide, salicylic acid, lactic acid, and tartaric acid. Some examples of organic bases are: pyridine and ethylamine. Some of the common inorganic acids are: hydrogen sulfide, phosphoric acid, hydrogen chloride, and sulfuric acid. Some common inorganic bases are: sodium hydroxide, sodium carbonate, sodium bicarbonate, calcium hydroxide, and calcium carbonate.

Acids, such as hydrochloric acid, and bases, such as potassium hydroxide, that have a great tendency to dissociate in water are completely ionized in solution; they are called strong acids or strong bases. Acids, such as acetic acid, and bases, such as ammonia, that are reluctant to dissociate in water are only partially ionized in solution; they are called weak acids or weak bases. Strong acids in solution produce a high concentration of hydrogen ions, and strong bases in solution produce a high concentration of hydroxide ions and a correspondingly low concentration of hydrogen ions. The hydrogen ion concentration is often expressed in terms of its negative logarithm, or pH. Strong acids and strong bases make very good electrolytes (see electrolysis), i.e., their solutions readily conduct electricity. Weak acids and weak bases make poor electrolytes.

See buffer; catalyst; indicators, acid-base; titration.

Acid-Base Theories

There are three theories that identify a singular characteristic which defines an acid and a base: the Arrhenius theory, for which the Swedish chemist Svante Arrhenius was awarded the 1903 Nobel Prize in chemistry; the Brönsted-Lowry, or proton donor, theory, advanced in 1923; and the Lewis, or electron-pair, theory, which was also presented in 1923. Each of the three theories has its own advantages and disadvantages; each is useful under certain conditions.

The Arrhenius Theory

When an acid or base dissolves in water, a certain percentage of the acid or base particles will break up, or dissociate (see dissociation), into oppositely charged ions. The Arrhenius theory defines an acid as a compound that can dissociate in water to yield hydrogen ions, H+, and a base as a compound that can dissociate in water to yield hydroxide ions, OH . For example, hydrochloric acid, HCl, dissociates in water to yield the required hydrogen ions, H+, and also chloride ions, Cl . The base sodium hydroxide, NaOH, dissociates in water to yield the required hydroxide ions, OH, and also sodium ions, Na+.

The Brönsted-Lowry Theory

Some substances act as acids or bases when they are dissolved in solvents other than water, such as liquid ammonia. The Brönsted-Lowry theory, named for the Danish chemist Johannes Brönsted and the British chemist Thomas Lowry, provides a more general definition of acids and bases that can be used to deal both with solutions that contain no water and solutions that contain water. It defines an acid as a proton donor and a base as a proton acceptor. In the Brönsted-Lowry theory, water, H2O, can be considered an acid or a base since it can lose a proton to form a hydroxide ion, OH, or accept a proton to form a hydronium ion, H3O+ (see amphoterism). When an acid loses a proton, the remaining species can be a proton acceptor and is called the conjugate base of the acid. Similarly when a base accepts a proton, the resulting species can be a proton donor and is called the conjugate acid of that base. For example, when a water molecule loses a proton to form a hydroxide ion, the hydroxide ion can be considered the conjugate base of the acid, water. When a water molecule accepts a proton to form a hydronium ion, the hydronium ion can be considered the conjugate acid of the base, water.

The Lewis Theory

Another theory that provides a very broad definition of acids and bases has been put forth by the American chemist Gilbert Lewis. The Lewis theory defines an acid as a compound that can accept a pair of electrons and a base as a compound that can donate a pair of electrons. Boron trifluoride, BF3, can be considered a Lewis acid and ethyl alcohol can be considered a Lewis base.
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The lowest and most visible part of a building, often treated with distinctive materials, such as rustication; also pertains to the lowest part of a column or pier that rests on a pedestal, plinth or stylobate.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



in electrical and radio engineering, a component of an electron or ion device, such as an incandescent lamp, a fluorescent lamp, or an electron tube. The base is used to install the device in, for example, a socket and to provide the connection between the elements within the device, such as the filaments or the electrodes, and the external electric circuit.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


Any chemical species, ionic or molecular, capable of accepting or receiving a proton (hydrogen ion) from another substance; the other substance acts as an acid in giving of the proton. Also known as Brønsted base.
(chemical engineering)
The primary substance in solution in crude oil, and remaining after distillation.
(computer science)
The region that lies between an emitter and a collector of a transistor and into which minority carriers are injected.
The part of an electron tube that has the pins, leads, or other terminals to which external connections are made either directly or through a socket.
The plastic, ceramic, or other insulating board that supports a printed wiring pattern.
Foundation or part upon which an object or instrument rests.
(graphic arts)
A transparent plastic film on which a photographic emulsion is applied.
A side or face upon which the altitude of a geometric configuration is thought of as being constructed.
For a logarithm, the number of which the logarithm is the exponent.
For a number system, the number whose powers determine place value.
For a topological space, a collection of sets, unions of which form all the open sets of the space.
Station or installation from which military forces operate and from which supplies are obtained.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


bases, 3
1. The lowest (and often widest) visible part of a building, often distinctively treated. A base is distinguished from a foundation or footing in being visible rather than buried.
2. A low, thickened section of a wall; a wall base. Also see socle.
3. Lower part of a column or pier, wider than the shaft, and resting on a plinth, pedestal, podium, or stylobate. Also see Asiatic base, Attic base.
4. A baseboard; skirting.
5. A preparation for a finished surface, as for flooring, stucco, paint, etc.; a surface to which the base coat of plaster is applied. Also see backing, ground.
6. In paint, either the medium or the main chemical ingredient.
7. In asphaltic or portland cement concrete paving, the prepared bottom course of crushed stone or gravel upon which subsequent courses are laid; serves to distribute localized wheel loads over a larger subbase and hence to improve load-bearing capacity.
8. The lowest point of any vertical pipe.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


i. A location where aircraft with all facilities are based. Refers to an air force base, naval air station, and fixed base operator.
ii. In air traffic control phraseology and when transmitted on the radio, it means turning onto the base leg of the circuit pattern. The base leg, as it is known, is flown at right angles to the runway, just before turning to the final direction for landing.
iii. In photogrammetry, the distance at the scale of the stereoscopic model between adjacent perspective centers as reconstructed in the plotting instrument. Also called an air base.
iv. In military radio telephony usage, it means the airfield from which the mission was launched, as in “return to base.”
v. The same as base course.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved


1. a chemical compound that combines with an acid to form a salt and water. A solution of a base in water turns litmus paper blue, produces hydroxyl ions, and has a pH greater than 7. Bases are metal oxides or hydroxides or amines
2. Biochem. any of the nitrogen-containing constituents of nucleic acids: adenine, thymine (in DNA), uracil (in RNA), guanine, or cytosine
3. the inorganic material on which the dye is absorbed in lake pigments; carrier
4. Biology
a. the part of an organ nearest to its point of attachment
b. the point of attachment of an organ or part
5. Architect
a. the lowest division of a building or structure
b. the lower part of a column or pier
6. the lower side or face of a geometric construction
7. Maths
a. the number of distinct single-digit numbers in a counting system, and so the number represented as 10 in a place-value system
b. (of a logarithm or exponential) the number whose powers are expressed
c. (of a mathematical structure) a substructure from which the given system can be generated
d. the initial instance from which a generalization is proven by mathematical induction
8. Logic Maths the initial element of a recursive definition, that defines the first element of the infinite sequence generated thereby
9. Electronics the region in a transistor between the emitter and collector
10. Photog the glass, paper, or cellulose-ester film that supports the sensitized emulsion with which it is coated
11. a starting or finishing point in any of various games
12. Baseball any of the four corners of the diamond, which runners have to reach in order to score


1. English history
a. (of land tenure) held by villein or other ignoble service
b. holding land by villein or other ignoble service
2. Music an obsolete spelling of bass
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


This article is provided by FOLDOC - Free Online Dictionary of Computing (


(1) A starting or reference point.

(2) In a bipolar transistor, the elements that act as a switch. In NMOS and PMOS transistors, which make up CMOS circuits, the base is called the "gate." See transistor.

(3) A multiplier in a numbering system. In a decimal system, each digit position is worth 10x the position to its right. In binary, each digit position is worth 2x the position to its right.

(4) (Base) The database program in an office suite. See OpenOffice and LibreOffice.
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