Lego is a line of plastic construction toys that are manufactured by The Lego Group, a held company based in Billund, Denmark. The company's flagship product, consists of colourful interlocking plastic bricks accompanying an array of gears, figurines called minifigures, various other parts. Lego pieces can be assembled and connected in many ways to construct objects, including vehicles and working robots. Anything constructed can be taken apart again, the pieces reused to make new things; the Lego Group began manufacturing the interlocking toy bricks in 1949. Movies, games and six Legoland amusement parks have been developed under the brand; as of July 2015, 600 billion Lego parts had been produced. In February 2015, Lego replaced Ferrari as Brand Finance's "world's most powerful brand"; the Lego Group began in the workshop of Ole Kirk Christiansen, a carpenter from Billund, who began making wooden toys in 1932. In 1934, his company came to be called "Lego", derived from the Danish phrase leg godt, which means "play well".
In 1947, Lego expanded to begin producing plastic toys. In 1949 Lego began producing, among other new products, an early version of the now familiar interlocking bricks, calling them "Automatic Binding Bricks"; these bricks were based on the Kiddicraft Self-Locking Bricks, patented in the United Kingdom in 1939 and released in 1947. Lego had received a sample of the Kiddicraft bricks from the supplier of an injection-molding machine that it purchased; the bricks manufactured from cellulose acetate, were a development of the traditional stackable wooden blocks of the time. The Lego Group's motto is det bedste er ikke for godt which means "only the best is the best"; this motto, still used today, was created by Christiansen to encourage his employees never to skimp on quality, a value he believed in strongly. By 1951 plastic toys accounted for half of the Lego company's output though the Danish trade magazine Legetøjs-Tidende, visiting the Lego factory in Billund in the early 1950s, felt that plastic would never be able to replace traditional wooden toys.
Although a common sentiment, Lego toys seem to have become a significant exception to the dislike of plastic in children's toys, due in part to the high standards set by Ole Kirk. By 1954, Christiansen's son, had become the junior managing director of the Lego Group, it was his conversation with an overseas buyer. Godtfred saw the immense potential in Lego bricks to become a system for creative play, but the bricks still had some problems from a technical standpoint: their locking ability was limited and they were not versatile. In 1958, the modern brick design was developed; the modern Lego brick design was patented on 28 January 1958. The Lego Group's Duplo product line was introduced in 1969 and is a range of simple blocks whose lengths measure twice the width and depth of standard Lego blocks and are aimed towards younger children. In 1978, Lego produced the first minifigures. In May 2011, Space Shuttle Endeavour mission STS-134 brought 13 Lego kits to the International Space Station, where astronauts built models to see how they would react in microgravity, as a part of the Lego Bricks in Space program.
In May 2013, the largest model created was displayed in New York City and was made of over 5 million bricks. Other records include a 4 km railway. In February 2015, Lego replaced Ferrari as the "world's most powerful brand." Lego's popularity is demonstrated by its wide representation and usage in many forms of cultural works, including books and art work. It has been used in the classroom as a teaching tool. In the US, Lego Education North America is a joint venture between Pitsco, Inc. and the educational division of the Lego Group. In 1998, Lego bricks were one of the original inductees into the National Toy Hall of Fame at The Strong in Rochester, New York. Lego pieces of all varieties constitute a universal system. Despite variation in the design and the purposes of individual pieces over the years, each piece remains compatible in some way with existing pieces. Lego bricks from 1958 still interlock with those made in the current time, Lego sets for young children are compatible with those made for teenagers.
Six bricks of 2 × 4 studs can be combined in 915,103,765 ways. Each Lego piece must be manufactured to an exacting degree of precision; when two pieces are engaged they must fit yet be disassembled. The machines that manufacture Lego bricks have tolerances as small as 10 micrometres. Primary concept and development work takes place at the Billund headquarters, where the company employs 120 designers; the company has smaller design offices in the UK, Spain and Japan which are tasked with developing products aimed at these markets. The average development period for a new product is around twelve months, split into three stages; the first stage is to identify market trends and developments, including contact by the designers directly with the market. The second stage is the design and development of the product based upon the results of the first stage; as of September 2008 the design teams use 3D modelling software to generate CAD drawings from initial design sketches. The designs are prototyped using an in-house stereolithography machine.
A nitrile is any organic compound that has a −C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, including methyl cyanoacrylate, used in super glue, nitrile rubber, a nitrile-containing polymer used in latex-free laboratory and medical gloves. Nitrile rubber is widely used as automotive and other seals since it is resistant to fuels and oils. Organic compounds containing multiple nitrile groups are known as cyanocarbons. Inorganic compounds containing the − C ≡ N group cyanides instead. Though both nitriles and cyanides can be derived from cyanide salts, most nitriles are not nearly as toxic; the N−C−C geometry is linear in nitriles, reflecting the sp hybridization of the triply bonded carbon. The C−N distance is short at 1.16 Å, consistent with a triple bond. Nitriles are polar; as liquids, they have high relative permittivities in the 30s. The first compound of the homolog row of nitriles, the nitrile of formic acid, hydrogen cyanide was first synthesized by C. W. Scheele in 1782.
In 1811 J. L. Gay-Lussac was able to prepare the toxic and volatile pure acid. Around 1832 benzonitrile, the nitrile of benzoic acid, was prepared by Friedrich Wöhler and Justus von Liebig, but due to minimal yield of the synthesis neither physical nor chemical properties were determined nor a structure suggested. In 1834 Théophile-Jules Pelouze synthesized propionitrile, suggesting it to be an ether of propionic alcohol and hydrocyanic acid; the synthesis of benzonitrile by Hermann Fehling in 1844 by heating ammonium benzoate was the first method yielding enough of the substance for chemical research. Fehling determined the structure by comparing his results to the known synthesis of hydrogen cyanide by heating ammonium formate, he coined the name "nitrile" for the newfound substance, which became the name for this group of compounds. Industrially, the main methods for producing nitriles are hydrocyanation. Both routes are green in the sense. In ammoxidation, a hydrocarbon is oxidized in the presence of ammonia.
This conversion is practiced on a large scale for acrylonitrile: CH3CH=CH2 + 3⁄2 O2 + NH3 → NCCH=CH2 + 3 H2OIn the production of acrylonitrile, a side product is acetonitrile. On an industrial scale, several derivatives of benzonitrile, phthalonitrile, as well as Isobutyronitrile are prepared by ammoxidation; the process is assumed to proceed via the imine. Hydrocyanation is an industrial method for producing nitriles from hydrogen cyanide and alkenes; the process requires homogeneous catalysts. An example of hydrocyanation is the production of adiponitrile, a precursor to nylon-6,6 from 1,3-butadiene: CH2=CH−CH=CH2 + 2 HCN → NC4CN Two salt metathesis reactions are popular for laboratory scale reactions. In the Kolbe nitrile synthesis, alkyl halides undergo nucleophilic aliphatic substitution with alkali metal cyanides. Aryl nitriles are prepared in the Rosenmund-von Braun synthesis; the cyanohydrins are a special class of nitriles. Classically they result from the addition of alkali metal cyanides to aldehydes in the cyanohydrin reaction.
Because of the polarity of the organic carbonyl, this reaction requires no catalyst, unlike the hydrocyanation of alkenes. O-Silyl cyanohydrins are generated by the addition trimethylsilyl cyanide in the presence of a catalyst. Cyanohydrins are prepared by transcyanohydrin reactions starting, for example, with acetone cyanohydrin as a source of HCN. Nitriles can be prepared by the dehydration of primary amides. In the presence of ethyl dichlorophosphate and DBU benzamide converts to benzonitrile: Other reagents that are common used for this purpose include P4O10, SOCl2. Two intermediates in this reaction are amide tautomer A and its phosphate adduct B. In a related dehydration, secondary amides give nitriles by the von Braun amide degradation. In this case, one C-N bond is cleaved; the dehydration of aldoximes affords nitriles. Typical reagents for this transformation are triethylamine/sulfur dioxide, zeolites, or sulfuryl chloride. Exploiting this approach is the One-pot synthesis of nitriles from aldehyde with hydroxylamine in the presence of sodium sulfate.
From aryl carboxylic acids Aromatic nitriles are prepared in the laboratory from the aniline via diazonium compounds. This is the Sandmeyer reaction, it requires transition metal cyanides. ArN+2 + CuCN → ArCN + N2 + Cu+ A commercial source for the cyanide group is diethylaluminum cyanide Et2AlCN which can be prepared from triethylaluminium and HCN, it has been used in nucleophilic addition to ketones. For an example of its use see: Kuwajima Taxol total synthesis cyanide ions facilitate the coupling of dibromides. Reaction of α,α′-dibromoadipic acid with sodium cyanide in ethanol yields the cyano cyclobutane: In the so-called Franchimont Reaction an α-bromocarboxylic acid is dimerized after hydrolysis of the cyanogroup and decarboxylationAromatic nitriles can be prepared from base hydrolysis of trichloromethyl aryl ketimines in the Houben-Fischer synthesis Nitriles can be obtained from primary amines via oxidation. Common methods include the use of potassium persulfate, Trichloroisocyanuric acid, or anodic electrosynthesis.
Α-Amino acids form nitriles and carbon dioxide via various means of oxidative decarboxylation. Henry Drysdale Dakin discovered this oxidation in 1916. Nitrile groups in organic compounds can undergo a variety of reactions depending on the reactants or conditio
Synthetic resins are industrially produced resins viscous substances that convert into rigid polymers by the process of curing. In order to undergo curing, resins contain reactive end groups, such as acrylates or epoxides; some synthetic resins have properties similar to natural plant resins. Synthetic resins are of several classes; some are manufactured by esterification of organic compounds. Some are thermosetting plastics in which the term "resin" is loosely applied to the reactant or product, or both. "Resin" may be applied to one of two monomers in a copolymer, the other being called a "hardener", as in epoxy resins. For thermosetting plastics that require only one monomer, the monomer compound is the "resin". For example, liquid methyl methacrylate is called the "resin" or "casting resin" while in the liquid state, before it polymerizes and "sets". After setting, the resulting PMMA is renamed acrylic glass, or "acrylic".. The classic variety is epoxy resin, manufactured through polymerization-polyaddition or polycondensation reactions, used as a thermoset polymer for adhesives and composites.
Epoxy resin is two times stronger than concrete and waterproof. Accordingly, it has been in use for industrial flooring purposes since the 1960s. Since 2000, however and polyurethane resins are used in interiors as well in Western Europe. Synthetic casting "resin" for embedding display objects in Plexiglas/Lucite is methyl methacrylate liquid, into which a polymerization catalyst is added and mixed, causing it to "set"; the polymerization creates a block of PMMA plastic which holds the display object in a transparent block. Another synthetic polymer, sometimes called by the same general category, is acetal resin. By contrast with the other synthetics, however, it has a simple chain structure with the repeat unit of form −−. Ion exchange resins are used in water purification and catalysis of organic reactions. See AT-10 resin, melamine resin. Certain ion exchange resins are used pharmaceutically as bile acid sequestrants as hypolipidemic agents, although they may be used for purposes other than lowering cholesterol.
Solvent Impregnated Resins are porous resin particles which contain an additional liquid extractant inside the porous matrix. The contained extractant is supposed to enhance the capacity of the resin particles. A large category of resins, which constitutes 75% of resins used, is that of the unsaturated polyester resins; the production of PVC entails the production of "vinyl chloride resins", which differ in the degree of polymerization. Health hazards associated with synthetic resins are less of a concern than the hazards associated with the cured products, which are more in contact with consumers. Issues of interest include the effects of solvent carriers. Dental restorative materials based on bis-GMA-containing resins can break down into or be contaminated with the related compound bisphenol A, a potential endocrine disrupter. However, no negative health effects of bis-GMA use in dental resins have been found. Resin casting
In condensed matter physics and materials science, an amorphous or non-crystalline solid is a solid that lacks the long-range order, characteristic of a crystal. In some older books, the term has been used synonymously with glass. Nowadays, "glassy solid" or "amorphous solid" is considered to be the overarching concept, glass the more special case: Glass is an amorphous solid that exhibits a glass transition. Polymers are amorphous. Other types of amorphous solids include gels, thin films, nanostructured materials such as glass doors and windows. Amorphous materials have an internal structure made of interconnected structural blocks; these blocks can be similar to the basic structural units found in the corresponding crystalline phase of the same compound. Whether a material is liquid or solid depends on the connectivity between its elementary building blocks so that solids are characterized by a high degree of connectivity whereas structural blocks in fluids have lower connectivity. In pharmaceutical industry, the amorphous drugs were shown to have higher bioavailability than their crystalline counterparts due to the high solubility of amorphous phase.
Moreover, certain compounds can undergo precipitation in their amorphous form in vivo, they can decrease each other's bioavailability if administered together. Amorphous materials have some shortrange order at the atomic length scale due to the nature of chemical bonding. Furthermore, in small crystals a large fraction of the atoms are the crystal; the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales. Amorphous phases are important constituents of thin films, which are solid layers of a few nanometres to some tens of micrometres thickness deposited upon a substrate. So-called structure zone models were developed to describe the micro structure and ceramics of thin films as a function of the homologous temperature Th, the ratio of deposition temperature over melting temperature. According to these models, a necessary condition for the occurrence of amorphous phases is that Th has to be smaller than 0.3, the deposition temperature must be below 30% of the melting temperature.
For higher values, the surface diffusion of deposited atomic species would allow for the formation of crystallites with long range atomic order. Regarding their applications, amorphous metallic layers played an important role in the discussion of a suspected superconductivity in amorphous metals. Today, optical coatings made from TiO2, SiO2, Ta2O5 etc. and combinations of them in most cases consist of amorphous phases of these compounds. Much research is carried out into thin amorphous films as a gas separating membrane layer; the technologically most important thin amorphous film is represented by few nm thin SiO2 layers serving as isolator above the conducting channel of a metal-oxide semiconductor field-effect transistor. Hydrogenated amorphous silicon, a-Si:H in short, is of technical significance for thin-film solar cells. In case of a-Si:H the missing long-range order between silicon atoms is induced by the presence by hydrogen in the percent range; the occurrence of amorphous phases turned out as a phenomenon of particular interest for studying thin-film growth.
Remarkably, the growth of polycrystalline films is used and preceded by an initial amorphous layer, the thickness of which may amount to only a few nm. The most investigated example is represented by thin multicrystalline silicon films, where such as the unoriented molecule. An initial amorphous layer was observed in many studies. Wedge-shaped polycrystals were identified by transmission electron microscopy to grow out of the amorphous phase only after the latter has exceeded a certain thickness, the precise value of which depends on deposition temperature, background pressure and various other process parameters; the phenomenon has been interpreted in the framework of Ostwald's rule of stages that predicts the formation of phases to proceed with increasing condensation time towards increasing stability. Experimental studies of the phenomenon require a defined state of the substrate surface and its contaminant density etc. upon which the thin film is deposited. R. Zallen; the Physics of Amorphous Solids.
Wiley Interscience. S. R. Elliot; the Physics of Amorphous Materials. Longman. N. Cusack; the Physics of Structurally Disordered Matter: An Introduction. IOP Publishing. N. H. March. A. Street. P. Tosi, eds.. Amorphous Solids and the Liquid State. Springer. D. A. Adler. B. Schwartz. C. Steele, eds.. Physical Properties of Amorphous Materials. Springer. A. Inoue. Amorphous and Nanocrystalline Materials. Springer. Journal of non-crystalline solids
Natural rubber called India rubber or caoutchouc, as produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds, plus water. Thailand and Indonesia are two of the leading rubber producers. Forms of polyisoprene that are used as natural rubbers are classified as elastomers. Rubber is harvested in the form of the latex from the rubber tree or others; the latex is a sticky, milky colloid drawn off by making incisions in the bark and collecting the fluid in vessels in a process called "tapping". The latex is refined into rubber ready for commercial processing. In major areas, latex is allowed to coagulate in the collection cup; the coagulated lumps are processed into dry forms for marketing. Natural rubber is used extensively in many applications and products, either alone or in combination with other materials. In most of its useful forms, it has a large stretch ratio and high resilience, is waterproof; the major commercial source of natural rubber latex is the Pará rubber tree, a member of the spurge family, Euphorbiaceae.
This species is preferred. A properly managed tree responds to wounding by producing more latex for several years. Congo rubber a major source of rubber, came from vines in the genus Landolphia. Dandelion milk contains latex; the latex exhibits the same quality as the natural rubber from rubber trees. In the wild types of dandelion, latex content varies greatly. In Nazi Germany, research projects tried to use dandelions as a base for rubber production, but failed. In 2013, by inhibiting one key enzyme and using modern cultivation methods and optimization techniques, scientists in the Fraunhofer Institute for Molecular Biology and Applied Ecology in Germany developed a cultivar, suitable for commercial production of natural rubber. In collaboration with Continental Tires, IME began a pilot facility. Many other plants produce forms of latex rich in isoprene polymers, though not all produce usable forms of polymer as as the Pará; some of them require more elaborate processing to produce anything like usable rubber, most are more difficult to tap.
Some produce other desirable materials, for example chicle from Manilkara species. Others that have been commercially exploited, or at least showed promise as rubber sources, include the rubber fig, Panama rubber tree, various spurges, the related Scorzonera tau-saghyz, various Taraxacum species, including common dandelion and Russian dandelion, most for its hypoallergenic properties, guayule; the term gum rubber is sometimes applied to the tree-obtained version of natural rubber in order to distinguish it from the synthetic version. The first use of rubber was by the indigenous cultures of Mesoamerica; the earliest archeological evidence of the use of natural latex from the Hevea tree comes from the Olmec culture, in which rubber was first used for making balls for the Mesoamerican ballgame. Rubber was used by the Maya and Aztec cultures – in addition to making balls Aztecs used rubber for other purposes such as making containers and to make textiles waterproof by impregnating them with the latex sap.
The Pará rubber tree is indigenous to South America. Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736. In 1751, he presented a paper by François Fresneau to the Académie that described many of rubber's properties; this has been referred to as the first scientific paper on rubber. In England, Joseph Priestley, in 1770, observed that a piece of the material was good for rubbing off pencil marks on paper, hence the name "rubber", it made its way around England. In 1764 François Fresnau discovered. Giovanni Fabbroni is credited with the discovery of naphtha as a rubber solvent in 1779. South America remained the main source of latex rubber used during much of the 19th century; the rubber trade was controlled by business interests but no laws expressly prohibited the export of seeds or plants. In 1876, Henry Wickham smuggled 70,000 Pará rubber tree seeds from Brazil and delivered them to Kew Gardens, England. Only 2,400 of these germinated.
Seedlings were sent to India, British Ceylon, Dutch East Indies and British Malaya. Malaya was to become the biggest producer of rubber. In the early 1900s, the Congo Free State in Africa was a significant source of natural rubber latex gathered by forced labor. King Leopold II's colonial state brutally enforced production quotas. Tactics to enforce the rubber quotas included removing the hands of victims to prove they had been killed. Soldiers came back from raids with baskets full of chopped-off hands. Villages that resisted were razed to encourage better compliance locally. See Atrocities in the Congo Free State for more information on the rubber trade in the Congo Free State in the late 1800s and early 1900s. Liberia and Nigeria started production. In India, commercial cultivation was introduced by British planters, although the experimental efforts to grow rubber on a commercial scale were initiated as early as 1873 at the Calcutta Botanical Gardens; the first commercial Hevea plantations were established at Thattekadu in Kerala in 1902.
In years the plantation expanded to Karnataka, Tamil Nadu and the Andaman and Nicobar Islands of India. India today is the
In chemistry, an alcohol is any organic compound in which the hydroxyl functional group is bound to a carbon. The term alcohol referred to the primary alcohol ethanol, used as a drug and is the main alcohol present in alcoholic beverages. An important class of alcohols, of which methanol and ethanol are the simplest members, includes all compounds for which the general formula is CnH2n+1OH, it is these simple monoalcohols. The suffix -ol appears in the IUPAC chemical name of all substances where the hydroxyl group is the functional group with the highest priority; when a higher priority group is present in the compound, the prefix hydroxy- is used in its IUPAC name. The suffix -ol in non-IUPAC names typically indicates that the substance is an alcohol. However, many substances that contain hydroxyl functional groups have names which include neither the suffix -ol, nor the prefix hydroxy-. Alcohol distillation originated in India. During 2000 BCE, people of India used. Alcohol distillation was known to Islamic chemists as early as the eighth century.
The Arab chemist, al-Kindi, unambiguously described the distillation of wine in a treatise titled as "The Book of the chemistry of Perfume and Distillations". The Persian physician, alchemist and philosopher Rhazes is credited with the discovery of ethanol; the word "alcohol" is from a powder used as an eyeliner. Al- is the Arabic definite article, equivalent to the in English. Alcohol was used for the fine powder produced by the sublimation of the natural mineral stibnite to form antimony trisulfide Sb2S3, it was considered to be the essence or "spirit" of this mineral. It was used as an antiseptic and cosmetic; the meaning of alcohol was extended to distilled substances in general, narrowed to ethanol, when "spirits" was a synonym for hard liquor. Bartholomew Traheron, in his 1543 translation of John of Vigo, introduces the word as a term used by "barbarous" authors for "fine powder." Vigo wrote: "the barbarous auctours use alcohol, or alcofoll, for moost fine poudre."The 1657 Lexicon Chymicum, by William Johnson glosses the word as "antimonium sive stibium."
By extension, the word came to refer to any fluid obtained by distillation, including "alcohol of wine," the distilled essence of wine. Libavius in Alchymia refers to "vini alcohol vel vinum alcalisatum". Johnson glosses alcohol vini as "quando omnis superfluitas vini a vino separatur, ita ut accensum ardeat donec totum consumatur, nihilque fæcum aut phlegmatis in fundo remaneat." The word's meaning became restricted to "spirit of wine" in the 18th century and was extended to the class of substances so-called as "alcohols" in modern chemistry after 1850. The term ethanol was invented 1892, combining the word ethane with the "-ol" ending of "alcohol". IUPAC nomenclature is used in scientific publications and where precise identification of the substance is important in cases where the relative complexity of the molecule does not make such a systematic name unwieldy. In naming simple alcohols, the name of the alkane chain loses the terminal e and adds the suffix -ol, e.g. as in "ethanol" from the alkane chain name "ethane".
When necessary, the position of the hydroxyl group is indicated by a number between the alkane name and the -ol: propan-1-ol for CH3CH2CH2OH, propan-2-ol for CH3CHCH3. If a higher priority group is present the prefix hydroxy-is used, e.g. as in 1-hydroxy-2-propanone. In cases where the OH functional group is bonded to an sp2 carbon on an aromatic ring the molecule is known as a phenol, is named using the IUPAC rules for naming phenols. In other less formal contexts, an alcohol is called with the name of the corresponding alkyl group followed by the word "alcohol", e.g. methyl alcohol, ethyl alcohol. Propyl alcohol may be n-propyl alcohol or isopropyl alcohol, depending on whether the hydroxyl group is bonded to the end or middle carbon on the straight propane chain; as described under systematic naming, if another group on the molecule takes priority, the alcohol moiety is indicated using the "hydroxy-" prefix. Alcohols are classified into primary and tertiary, based upon the number of carbon atoms connected to the carbon atom that bears the hydroxyl functional group.
The primary alcohols have general formulas RCH2OH. The simplest primary alcohol is methanol, for which R=H, the next is ethanol, for which R=CH3, the methyl group. Secondary alcohols are those of the form RR'CHOH, the simplest of, 2-propanol. For the tertiary alcohols the general form is RR'R"COH; the simplest example is tert-butanol, for which each of R, R', R" is CH3. In these shorthands, R, R', R" represent substituents, alkyl or other attached organic groups. In archaic nomenclature, alcohols can be named as derivatives of methanol using "-carbinol" as the ending. For instance, 3COH can be named trimethylcarbinol. Alcohols have a long history of myriad uses. For simple mono-alcohols, the focus on this article, the following are most important industrial alcohols: methanol for the production of formaldehyde and as a fuel additive ethanol for alcoholic beverages, fuel additive, solvent 1-propanol, 1-butanol, isobutyl alcohol for use as a solvent a
An acid is a molecule or ion capable of donating a hydron, or, capable of forming a covalent bond with an electron pair. The first category of acids is the proton donors or Brønsted acids. In the special case of aqueous solutions, proton donors form the hydronium ion H3O+ and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents. A Brønsted or Arrhenius acid contains a hydrogen atom bonded to a chemical structure, still energetically favorable after loss of H+. Aqueous Arrhenius acids have characteristic properties which provide a practical description of an acid. Acids form aqueous solutions with a sour taste, can turn blue litmus red, react with bases and certain metals to form salts; the word acid is derived from the Latin acidus/acēre meaning sour. An aqueous solution of an acid has a pH less than 7 and is colloquially referred to as'acid', while the strict definition refers only to the solute. A lower pH means a higher acidity, thus a higher concentration of positive hydrogen ions in the solution.
Chemicals or substances having the property of an acid are said to be acidic. Common aqueous acids include hydrochloric acid, acetic acid, sulfuric acid, citric acid; as these examples show, acids can be solutions or pure substances, can be derived from acids that are solids, liquids, or gases. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid; the second category of acids are Lewis acids. An example is boron trifluoride, whose boron atom has a vacant orbital which can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia. Lewis considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly or by releasing protons into the solution, which accept electron pairs. However, hydrogen chloride, acetic acid, most other Brønsted-Lowry acids cannot form a covalent bond with an electron pair and are therefore not Lewis acids.
Conversely, many Lewis acids are not Brønsted-Lowry acids. In modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists always refer to a Lewis acid explicitly as a Lewis acid. Modern definitions are concerned with the fundamental chemical reactions common to all acids. Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water, so the Arrhenius and Brønsted-Lowry definitions are the most relevant; the Brønsted-Lowry definition is the most used definition. Hydronium ions are acids according to all three definitions. Although alcohols and amines can be Brønsted-Lowry acids, they can function as Lewis bases due to the lone pairs of electrons on their oxygen and nitrogen atoms; the Swedish chemist Svante Arrhenius attributed the properties of acidity to hydrogen ions or protons in 1884. An Arrhenius acid is a substance that, when added to water, increases the concentration of H+ ions in the water. Note that chemists write H+ and refer to the hydrogen ion when describing acid-base reactions but the free hydrogen nucleus, a proton, does not exist alone in water, it exists as the hydronium ion, H3O+.
Thus, an Arrhenius acid can be described as a substance that increases the concentration of hydronium ions when added to water. Examples include molecular substances such as acetic acid. An Arrhenius base, on the other hand, is a substance which increases the concentration of hydroxide ions when dissolved in water; this decreases the concentration of hydronium because the ions react to form H2O molecules: H3O+ + OH− ⇌ H2O + H2ODue to this equilibrium, any increase in the concentration of hydronium is accompanied by a decrease in the concentration of hydroxide. Thus, an Arrhenius acid could be said to be one that decreases hydroxide concentration, while an Arrhenius base increases it. In an acidic solution, the concentration of hydronium ions is greater than 10−7 moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acidic solutions thus have a pH of less than 7. While the Arrhenius concept is useful for describing many reactions, it is quite limited in its scope.
In 1923 chemists Johannes Nicolaus Brønsted and Thomas Martin Lowry independently recognized that acid-base reactions involve the transfer of a proton. A Brønsted-Lowry acid is a species. Brønsted-Lowry acid-base theory has several advantages over Arrhenius theory. Consider the following reactions of acetic acid, the organic acid that gives vinegar its characteristic taste: CH3COOH + H2O ⇌ CH3COO− + H3O+ CH3COOH + NH3 ⇌ CH3COO− + NH+4Both theories describe the first reaction: CH3COOH acts as an Arrhenius acid because it acts as a source of H3O+ when dissolved in water, it acts as a Brønsted acid by donating a proton to water. In the second example CH3COOH undergoes the same transformation, in this case donating a proton to ammonia, but does not relate to the Arrhenius definition of an acid because the reaction does not produce hydronium. CH3COOH is