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.
Properties
Acids in water solutions exhibit the following common properties: they taste sour; turn litmus litmus, organic dye usually used in the laboratory as an indicator of acidity or alkalinity (see acids and bases ). Naturally pink in color, it turns blue in alkali solutions and red in acids.
..... Click the link for more information. 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 salt, chemical compound (other than water) formed by a chemical reaction between an acid and a base (see acids and bases ).
Characteristics and Classification of Salts
..... Click the link for more information. are formed; this process, called neutralization neutralization, chemical reaction, according to the Arrhenius theory of acids and bases , in which a water solution of acid is mixed with a water solution of base to form a salt and water; this reaction is complete only if the resulting solution has neither acidic
..... Click the link for more information. , is complete only if the resulting solution has neither acidic nor basic properties.
Classification
Acids and bases can be classified as organic or inorganic. Some of the more common organic acids are: citric acid citric acid or 2-hydroxy-1,2,3-propanetricarboxylic acid, HO2CCH2C(OH)(CO2H)CH2CO2
..... Click the link for more information. , carbonic acid carbonic acid, H2CO3, a weak dibasic acid (see acids and bases ) formed when carbon dioxide dissolves in water; it exists only in solution.
..... Click the link for more information. , hydrogen cyanide hydrogen cyanide, HCN, colorless, volatile, and extremely poisonous chemical compound whose vapors have a bitter almond odor. It melts at −14°C; and boils at 26°C;. It is miscible in all proportions with water or ethanol and is soluble in ether.
..... Click the link for more information. , salicylic acid, lactic acid lactic acid, CH3CHOHCO2H, a colorless liquid organic acid. It is miscible with water or ethanol. Lactic acid is a fermentation product of lactose (milk sugar); it is present in sour milk, koumiss, leban, yogurt, and cottage cheese.
..... Click the link for more information. , and tartaric acid tartaric acid, HO2CCHOHCHOHCO2H, white crystalline dicarboxylic acid. It occurs as three distinct isomers , the dextro-, levo-, and meso- forms.
..... Click the link for more information. . Some examples of organic bases are: pyridine pyridine (pĭr`ĭdēn) or azine
..... Click the link for more information. and ethylamine. Some of the common inorganic acids are: hydrogen sulfide hydrogen sulfide, chemical compound, H2S, a colorless, extremely poisonous gas that has a very disagreeable odor, much like that of rotten eggs. It is slightly soluble in water and is soluble in carbon disulfide.
..... Click the link for more information. , phosphoric acid phosphoric acid, any one of three chemical compounds made up of phosphorus , oxygen, and hydrogen (see acids and bases ). The most common, orthophosphoric acid, H3PO4, is usually simply called phosphoric acid.
..... Click the link for more information. , hydrogen chloride hydrogen chloride, chemical compound, HCl, a colorless, poisonous gas with an unpleasant, acrid odor. It is very soluble in water and readily soluble in alcohol and ether. It fumes in moist air. It is not flammable, and the liquid is a poor conductor of electricity.
..... Click the link for more information. , and sulfuric acid sulfuric acid, chemical compound, H2SO4, colorless, odorless, extremely corrosive, oily liquid. It is sometimes called oil of vitriol.
Concentrated Sulfuric Acid
When heated, the pure 100% acid loses sulfur trioxide gas, SO
..... Click the link for more information. . Some common inorganic bases are: sodium hydroxide sodium hydroxide, chemical compound, NaOH, a white crystalline substance that readily absorbs carbon dioxide and moisture from the air. It is very soluble in water, alcohol, and glycerin. It is a caustic and a strong base (see acids and bases ).
..... Click the link for more information. , sodium carbonate sodium carbonate, chemical compound, Na2CO3, soluble in water and very slightly soluble in alcohol. Pure sodium carbonate is a white, odorless powder that absorbs moisture from the air, has an alkaline taste, and forms a strongly alkaline water
..... Click the link for more information. , sodium bicarbonate sodium bicarbonate or sodium hydrogen carbonate, chemical compound, NaHCO3, a white crystalline or granular powder, commonly known as bicarbonate of soda or baking soda. It is soluble in water and very slightly soluble in alcohol.
..... Click the link for more information. , calcium hydroxide calcium hydroxide, Ca(OH)2, colorless crystal or white powder. It is prepared by reacting calcium oxide (lime) with water, a process called slaking, and is also known as hydrated lime or slaked lime.
..... Click the link for more information. , and calcium carbonate calcium carbonate, CaCO3, white chemical compound that is the most common nonsiliceous mineral. It occurs in two crystal forms: calcite, which is hexagonal, and aragonite, which is rhombohedral.
..... Click the link for more information. .
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 pH, range of numbers expressing the relative acidity or alkalinity of a solution. In general, pH values range from 0 to 14. The pH of a neutral solution, i.e., one which is neither acidic nor alkaline, is 7.
..... Click the link for more information. (see separate article). Strong acids and strong bases make very good electrolytes (see electrolysis electrolysis (ĭlĕktrŏl`əsĭs)
..... Click the link for more information. ), i.e., their solutions readily conduct electricity. Weak acids and weak bases make poor electrolytes.
See buffer buffer, solution that can keep its relative acidity or alkalinity constant, i.e., keep its pH constant, despite the addition of strong acids or strong bases.
..... Click the link for more information. ; catalyst catalyst, substance that can cause a change in the rate of a chemical reaction without itself being consumed in the reaction; the changing of the reaction rate by use of a catalyst is called catalysis.
..... Click the link for more information. ; indicators, acid-base indicators, acid-base, organic compounds that, in aqueous solution, exhibit color changes indicative of the acidity or basicity of the solution. Common indicators include p-nitrophenol, which is colorless from pH 1 to 5 and yellow from p
..... Click the link for more information. ; titration titration (tītrā`shən)
..... Click the link for more information. .
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 dissociation, in chemistry, separation of a substance into atoms or ions. Thermal dissociation occurs at high temperatures. For example, hydrogen molecules (H2
..... Click the link for more information. ), 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 amphoterism (ăm'fətĕr`ĭzm)
..... Click the link for more information. ). 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.