Nutrition is the science that interprets the interaction of nutrients and other substances in food in relation to maintenance, reproduction and disease of an organism. It includes food intake, assimilation, biosynthesis and excretion; the diet of an organism is what it eats, determined by the availability and palatability of foods. For humans, a healthy diet includes preparation of food and storage methods that preserve nutrients from oxidation, heat or leaching, that reduce risk of foodborne illnesses. In humans, an unhealthy diet can cause deficiency-related diseases such as blindness, scurvy, preterm birth and cretinism, or nutrient excess health-threatening conditions such as obesity and metabolic syndrome. Undernutrition can lead to wasting in acute cases, the stunting of marasmus in chronic cases of malnutrition; the first recorded dietary advice, carved into a Babylonian stone tablet in about 2500 BC, cautioned those with pain inside to avoid eating onions for three days. Scurvy found to be a vitamin C deficiency, was first described in 1500 BC in the Ebers Papyrus.
According to Walter Gratzer, the study of nutrition began during the 6th century BC. In China, the concept of qi developed, a spirit or "wind" similar to what Western Europeans called pneuma. Food was classified into "hot" and "cold" in China, India and Persia. Humours developed first in China alongside qi. Ho the Physician concluded that diseases are caused by deficiencies of elements, he classified diseases as well as prescribed diets. About the same time in Italy, Alcmaeon of Croton wrote of the importance of equilibrium between what goes in and what goes out, warned that imbalance would result in disease marked by obesity or emaciation; the first recorded nutritional experiment with human subjects is found in the Bible's Book of Daniel. Daniel and his friends were captured by the king of Babylon during an invasion of Israel. Selected as court servants, they were to share in the king's fine foods and wine, but they objected, preferring vegetables and water in accordance with their Jewish dietary restrictions.
The king's chief steward reluctantly agreed to a trial. Daniel and his friends received their diet for ten days and were compared to the king's men. Appearing healthier, they were allowed to continue with their diet. Around 475 BC, Anaxagoras stated that food is absorbed by the human body and, contains "homeomerics", suggesting the existence of nutrients. Around 400 BC, who recognized and was concerned with obesity, which may have been common in southern Europe at the time, said, "Let food be your medicine and medicine be your food." The works that are still attributed to him, Corpus Hippocraticum, called for moderation and emphasized exercise. Salt and other spices were prescribed for various ailments in various preparations for example mixed with vinegar. In the 2nd century BC, Cato the Elder believed that cabbage could cure digestive diseases, ulcers and intoxication. Living about the turn of the millennium, Aulus Celsus, an ancient Roman doctor, believed in "strong" and "weak" foods. One mustn't overlook the doctrines of Galen: In use from his life in the 1st century AD until the 17th century, it was heresy to disagree with him for 1500 years.
Galen was physician to gladiators in Pergamon, in Rome, physician to Marcus Aurelius and the three emperors who succeeded him. Most of Galen's teachings were gathered and enhanced in the late 11th century by Benedictine monks at the School of Salerno in Regimen sanitatis Salernitanum, which still had users in the 17th century. Galen believed in the bodily humours of Hippocrates, he taught that pneuma is the source of life. Four elements combine into "complexion"; the states are made up of pairs of attributes, which are made of four humours: blood, green bile, black bile. Galen thought that for a person to have gout, kidney stones, or arthritis was scandalous, which Gratzer likens to Samuel Butler's Erehwon where sickness is a crime. In the 1500s, Paracelsus was the first to criticize Galen publicly. In the 16th century and artist Leonardo da Vinci compared metabolism to a burning candle. Leonardo did not publish his works on this subject, but he was not afraid of thinking for himself and he disagreed with Galen.
16th century works of Andreas Vesalius, sometimes called the father of modern human anatomy, overturned Galen's ideas. He was followed by piercing thought amalgamated with the era's mysticism and religion sometimes fueled by the mechanics of Newton and Galileo. Jan Baptist van Helmont, who discovered several gases such as carbon dioxide, performed the first quantitative experiment. Robert Boyle advanced chemistry. Sanctorius measured body weight. Physician Herman Boerhaave modeled the digestive process. Physiologist Albrecht von Haller worked out the difference between muscles. Sometimes forgotten during his life, James Lind, a physician in the British navy, performed the first scientific nutrition experiment in 1747. Lind discovered that lime juice saved sailors, at sea for years from scurvy, a deadly an
Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst, not consumed in the catalyzed reaction and can continue to act repeatedly. Because of this, only small amounts of catalyst are required to alter the reaction rate in principle. In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative reaction pathway with a lower activation energy than the non-catalyzed mechanism. In catalyzed mechanisms, the catalyst reacts to form a temporary intermediate, which regenerates the original catalyst in a cyclic process. A substance which provides a mechanism with a higher activation energy does not decrease the rate because the reaction can still occur by the non-catalyzed route. An added substance which does reduce the reaction rate is not considered a catalyst but a reaction inhibitor. Catalysts may be classified as either heterogeneous. A homogeneous catalyst is one whose molecules are dispersed in the same phase as the reactant's molecules.
A heterogeneous catalyst is one whose molecules are not in the same phase as the reactant's, which are gases or liquids that are adsorbed onto the surface of the solid catalyst. Enzymes and other biocatalysts are considered as a third category. In the presence of a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. A catalyst may participate in multiple chemical transformations; the effect of a catalyst may vary due to the presence of other substances known as inhibitors or poisons or promoters. Catalyzed reactions have a lower activation energy than the corresponding uncatalyzed reaction, resulting in a higher reaction rate at the same temperature and for the same reactant concentrations. However, the detailed mechanics of catalysis is complex. Catalysts may bind to the reagents to polarize bonds, e.g. acid catalysts for reactions of carbonyl compounds, or form specific intermediates that are not produced such as osmate esters in osmium tetroxide-catalyzed dihydroxylation of alkenes, or cause dissociation of reagents to reactive forms, such as chemisorbed hydrogen in catalytic hydrogenation.
Kinetically, catalytic reactions are typical chemical reactions. The catalyst participates in this slowest step, rates are limited by amount of catalyst and its "activity". In heterogeneous catalysis, the diffusion of reagents to the surface and diffusion of products from the surface can be rate determining. A nanomaterial-based catalyst is an example of a heterogeneous catalyst. Analogous events associated with substrate binding and product dissociation apply to homogeneous catalysts. Although catalysts are not consumed by the reaction itself, they may be inhibited, deactivated, or destroyed by secondary processes. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric side products. Additionally, heterogeneous catalysts can dissolve into the solution in a solid–liquid system or sublimate in a solid–gas system; the production of most industrially important chemicals involves catalysis. Most biochemically significant processes are catalysed.
Research into catalysis is a major field in applied science and involves many areas of chemistry, notably organometallic chemistry and materials science. Catalysis is relevant to many aspects of environmental science, e.g. the catalytic converter in automobiles and the dynamics of the ozone hole. Catalytic reactions are preferred in environmentally friendly green chemistry due to the reduced amount of waste generated, as opposed to stoichiometric reactions in which all reactants are consumed and more side products are formed. Many transition metals and transition metal complexes are used in catalysis as well. Catalysts called. A catalyst works by providing an alternative reaction pathway to the reaction product; the rate of the reaction is increased as this alternative route has a lower activation energy than the reaction route not mediated by the catalyst. The disproportionation of hydrogen peroxide creates oxygen, as shown below. 2 H2O2 → 2 H2O + O2This reaction is preferable in the sense that the reaction products are more stable than the starting material, though the uncatalysed reaction is slow.
In fact, the decomposition of hydrogen peroxide is so slow that hydrogen peroxide solutions are commercially available. This reaction is affected by catalysts such as manganese dioxide, or the enzyme peroxidase in organisms. Upon the addition of a small amount of manganese dioxide, the hydrogen peroxide reacts rapidly; this effect is seen by the effervescence of oxygen. The manganese dioxide is not consumed in the reaction, thus may be recovered unchanged, re-used indefinitely. Accordingly, manganese dioxide catalyses this reaction. Catalytic activity is denoted by the symbol z and measured in mol/s, a unit, called katal and defined the SI unit for catalytic activity since 1999. Catalytic activity is not a kind of reaction rate, but a property of the catalyst under certain conditions, in relation to a specific chemical reaction. Catalytic activity of one katal of a catalyst means one mole of that catalyst will catalyse 1 mole of the reactant to product in one second. A catalyst may and will have different catalytic activity for di
An allyl group is a substituent with the structural formula H2C=CH−CH2R, where R is the rest of the molecule. It consists of a methylene bridge attached to a vinyl group; the name is derived from the Latin word for Allium sativum. In 1844, Theodor Wertheim isolated an allyl derivative from garlic oil and named it "Schwefelallyl"; the term allyl applies to many compounds related to H2C=CH−CH2, some of which are of practical or of everyday importance, for example, allyl chloride. A site adjacent to the unsaturated carbon atom is called allylic site. A group attached at this site is sometimes described as allylic. Thus, CH2=CHCH2OH "has an allylic hydroxyl group". Allylic C−H bonds are about 15% weaker than the C−H bonds in ordinary sp3 carbon centers and are thus more reactive; this heightened reactivity has many practical consequences. The industrial production of acrylonitrile by ammoxidation of propene exploits the easy oxidation of the allylic C−H centers: 2CH3−CH=CH2 + 2NH3 + 3O2 → 2CH2=CH−C≡N + 6H2OUnsaturated fats spoil by rancidification involving attack at allylic C−H centers.
Benzylic and allylic are related in terms of structure, bond strength, reactivity. Other reactions that tend to occur with allylic compounds are allylic oxidations, ene reactions, the Tsuji–Trost reaction. Benzylic groups are related to allyl groups. A CH2 group connected to two vinyl groups is said to be doubly allylic; the bond dissociation energy of C−H bonds on a doubly allylic centre is about 10% less than the bond dissociation energy of a C−H bond, allylic. The weakened C−H bonds reflect the high stability of the resulting pentadienyl radicals. Compounds containing the C=C−CH2−C=C linkages, e.g. linoleic acid derivatives, are prone to autoxidation, which can lead to polymerization or form semisolids. This reactivity pattern is fundamental to the film-forming behavior of the "drying oils", which are components of oil paints and varnishes; the term homoallylic refers to the position on a carbon skeleton next to an allylic position. In but-3-enyl chloride CH2=CHCH2CH2Cl, the chloride is homoallylic because it is bonded to the homoallylic site.
The allyl group is encountered in organic chemistry. Allylic radicals and cations are discussed as intermediates in reactions. All feature three contiguous sp²-hybridized carbon centers and all derive stability from resonance; each species can be presented by two resonance structures with the charge or unpaired electron distributed at both 1,3 positions. In terms of MO theory, the MO diagram has three molecular orbitals: the first one bonding, the second one non-bonding and the higher energy orbital is antibonding. Allylation is any chemical reaction. Allylation refers to the addition of an allyl anion equivalent to an organic electrophile: Carbonyl allylation is a type of organic reaction in which an activated allyl group is added to carbonyl group producing an allylic tertiary alcohol. A typical allylation of an aldehyde is represented by the following two-step process that begins with allylation followed by hydrolysis of the intermediate: RCHO + CH2=CHCH2M → CH2=CHCH2RCH CH2=CHCH2RCH + H2O → CH2=CHCH2RCH + MOHA popular reagent for asymmetric allylation is the "Brown reagent", allyldiisopinocampheylborane.
The introduction of allylic groups into molecular frameworks generates many opportunities for downstream diversification. A common method to introduce allyl moieties into organic molecules is through 1,2-allylation of carbonyl groups; the homoallylic alcohol products can undergo a variety of diversity-generating reactions such as ozonolysis and olefin metathesis. Allylmetal reagents such as allylboranes and allylindium compounds are used by organic chemists to introduce allyl groups. Allylstannanes are stable reagents in the allylmetal family, have been employed in a variety of complex organic syntheses. In fact, allylstannane addition is one of the most common methods for producing polypropionates and other oxygenated molecules with a contiguous arrays of stereocenters. Ley and coworkers used an allylstannane to allylate a threose-derived aldehyde en route to the macrolide antascomicin B, which structurally resembles FK506 and rapamycin, is a potent binder of FKBP12. Allylboration is often used to add allyl groups in a 1,2 fashion to aldehydes and ketones.
Thanks to decades of research, there is now a wide variety of organoboron reagents available to the synthetic chemist, including organoboranes that predictably yield products in high diastereo- and enantioselectivity. If a one-pot metal insertion and allylation procedure is required, indium- mediated allylation is an attractive option for generating homoallylic alcohols directly from allyl halides and carbonyl compounds. In general, the method is called the Barbier reaction, can employ a variety of metals such as magnesium, zinc and tin; the reaction is used as a milder form of the Grignard addition reaction, can tolerate aqueous solvents Organotantalum reagents are useful for conjugate addition to enones. Of particular interest is the ability of certain organotantalum reagents to promote the conjugate allylation of enones. Although the direct allylation of carbonyl groups is prevalent throughout the literature, little has been reported on the conjugate allylation of enones. Prior to Shibata and Baba's report, only three methods existed to selectively allylate enones, via: Hosomi Sakurai reaction, allylbarium reagents, allylcopper reagents.
Transmetalation of allyltin, alkynyltin, α-stannyl esters, allenyltin compounds with TaCl5 a
Organic chemistry is a subdiscipline of chemistry that studies the structure and reactions of organic compounds, which contain carbon in covalent bonding. Study of structure determines their chemical formula. Study of properties includes physical and chemical properties, evaluation of chemical reactivity to understand their behavior; the study of organic reactions includes the chemical synthesis of natural products and polymers, study of individual organic molecules in the laboratory and via theoretical study. The range of chemicals studied in organic chemistry includes hydrocarbons as well as compounds based on carbon, but containing other elements oxygen, sulfur and the halogens. Organometallic chemistry is the study of compounds containing carbon–metal bonds. In addition, contemporary research focuses on organic chemistry involving other organometallics including the lanthanides, but the transition metals zinc, palladium, cobalt and chromium. Organic compounds constitute the majority of known chemicals.
The bonding patterns of carbon, with its valence of four—formal single and triple bonds, plus structures with delocalized electrons—make the array of organic compounds structurally diverse, their range of applications enormous. They form the basis of, or are constituents of, many commercial products including pharmaceuticals; the study of organic chemistry overlaps organometallic chemistry and biochemistry, but with medicinal chemistry, polymer chemistry, materials science. Before the nineteenth century, chemists believed that compounds obtained from living organisms were endowed with a vital force that distinguished them from inorganic compounds. According to the concept of vitalism, organic matter was endowed with a "vital force". During the first half of the nineteenth century, some of the first systematic studies of organic compounds were reported. Around 1816 Michel Chevreul started a study of soaps made from various alkalis, he separated the different acids. Since these were all individual compounds, he demonstrated that it was possible to make a chemical change in various fats, producing new compounds, without "vital force".
In 1828 Friedrich Wöhler produced the organic chemical urea, a constituent of urine, from inorganic starting materials, in what is now called the Wöhler synthesis. Although Wöhler himself was cautious about claiming he had disproved vitalism, this was the first time a substance thought to be organic was synthesized in the laboratory without biological starting materials; the event is now accepted as indeed disproving the doctrine of vitalism. In 1856 William Henry Perkin, while trying to manufacture quinine accidentally produced the organic dye now known as Perkin's mauve, his discovery, made known through its financial success increased interest in organic chemistry. A crucial breakthrough for organic chemistry was the concept of chemical structure, developed independently in 1858 by both Friedrich August Kekulé and Archibald Scott Couper. Both researchers suggested that tetravalent carbon atoms could link to each other to form a carbon lattice, that the detailed patterns of atomic bonding could be discerned by skillful interpretations of appropriate chemical reactions.
The era of the pharmaceutical industry began in the last decade of the 19th century when the manufacturing of acetylsalicylic acid—more referred to as aspirin—in Germany was started by Bayer. By 1910 Paul Ehrlich and his laboratory group began developing arsenic-based arsphenamine, as the first effective medicinal treatment of syphilis, thereby initiated the medical practice of chemotherapy. Ehrlich popularized the concepts of "magic bullet" drugs and of systematically improving drug therapies, his laboratory made decisive contributions to developing antiserum for diphtheria and standardizing therapeutic serums. Early examples of organic reactions and applications were found because of a combination of luck and preparation for unexpected observations; the latter half of the 19th century however witnessed systematic studies of organic compounds. The development of synthetic indigo is illustrative; the production of indigo from plant sources dropped from 19,000 tons in 1897 to 1,000 tons by 1914 thanks to the synthetic methods developed by Adolf von Baeyer.
In 2002, 17,000 tons of synthetic indigo were produced from petrochemicals. In the early part of the 20th century and enzymes were shown to be large organic molecules, petroleum was shown to be of biological origin; the multiple-step synthesis of complex organic compounds is called total synthesis. Total synthesis of complex natural compounds increased in complexity to terpineol. For example, cholesterol-related compounds have opened ways to synthesize complex human hormones and their modified derivatives. Since the start of the 20th century, complexity of total syntheses has been increased to include molecules of high complexity such as lysergic acid and vitamin B12; the discovery of petroleum and the development of the petrochemical industry spurred the development of organic chemistry. Converting individual petroleum compounds into different types of compounds by various chemical processes led to organic reactions enabling a broad range of
The Serer people are a West African ethnic group. They are the third largest ethnic group in Senegal making up 15% of the Senegalese population, they are found in northern Gambia and southern Mauritania. The Serer people originated in the Senegal River valley at the border of Senegal and Mauritania, moved south in the 11th and 12th century again in the 15th and 16th centuries as their villages were invaded and they were subjected to religious pressures, they have had a sedentary settled culture and have been known for their farming expertise and transhumant stock-raising. The Serer people have been noted as a matrilineal ethnic group that long resisted the expansion of Islam, fought against jihads in the 19th century opposed the French colonial rule. In the 20th century, most of them converted to Islam, but some are Christians or follow their traditional religion; the Serer society, like other ethnic groups in Senegal, has had social stratification featuring endogamous castes and slaves. The Serer people are referred to as Sérère, Serere, Kegueme and sometimes wrongly "Serre".
The Serer people are found in contemporary Senegal in the west-central part of the country, running from the southern edge of Dakar to the Gambian border. In The Gambia, they occupy parts of old "Nuimi" and "Baddibu" as well as the Gambian "Kombo"; the Serer-Noon occupy the ancient area of Thiès in modern-day Senegal. The Serer-Ndut are found in southern north west of ancient Thiès; the Serer-Njeghen occupy old Baol. The Serer people are diverse and though they spread throughout the Senegambia region, they are more numerous in places like old Baol, Saloum and in the Gambia, a colony of the Kingdom of Saloum. Senegal: 1.84 million The Gambia: 31,900 Mauritania: 3,500The Serer occupy the Sine and Saloum areas. The Serer people include the Seex, Serer-Noon, Serer-Ndut, Serer-Njeghene, Serer-Safene, Serer-Niominka, Serer-Palor, the Serer-Laalaa; each group speaks a Cangin language. "Serer" is the standard English spelling. "Seereer" or "Sereer" reflects the Serer pronunciation of the name and are used by Senegalese Serer historians or scholars.
The meaning of the word "Serer" is uncertain. Issa Laye Thiaw views it as pre-Islamic and suggests four possible derivations:1. From the Serer Wolof word reer meaning'misplaced', i.e. doubting the truth of Islam. 2. From the Serer Wolof expression seer reer meaning "to find something hidden or lost." 3. From "the Arabic word seereer meaning sahir magician or one who practices magic". 4. From a Pulaar word meaning separation, divorce, or break, again referring to rejecting Islam. Professor Cheikh Anta Diop citing the work of the 19th century French archeologist and Egyptologist, Paul Pierret, states that the word Serer means "he who traces the temple." Diop went on to write: "That would be consistent with their present religious position: they are one of the rare Senegalese populations who still reject Islam. Their route is marked by the upright stones found at about the same latitude from Ethiopia all the way to the Sine-Salum, their present habitat." Professor Dennis Galvan writes that "The oral historical record, written accounts by early Arab and European explorers, physical anthropological evidence suggest that the various Serer peoples migrated south from the Fuuta Tooro region beginning around the eleventh century, when Islam first came across the Sahara."
Over generations these people Pulaar speaking herders migrated through Wolof areas and entered the Siin and Saluum river valleys. This lengthy period of Wolof-Serer contact has left us unsure of the origins of shared "terminology, political structures, practices."Professor Étienne Van de Walle gave a later date, writing that "The formation of the Sereer ethnicity goes back to the thirteenth century, when a group came from the Senegal River valley in the north fleeing Islam, near Niakhar met another group of Mandinka origin, called the Gelwar, who were coming from the southeast. The actual Sereer ethnic group is a mixture of the two groups, this may explain their complex bilinear kinship system", their own oral traditions recite legends on they being part of, or related to the Toucouleur people in the Senegal River valley area. Serer people resisted Islamization and Wolofization from the 11th century during the Almoravid movement, migrated south where they intermixed with the Diola people, they violently resisted the 19th century jihads and Marabout movement to convert Senegambia to Islam.
After the Ghana Empire was sacked as certain kingdoms gained their independence, Abu-Bakr Ibn-Umar, leader of the Almoravids launched a jihad into the region. According to Serer oral history, a Serer bowman named Amar Godomat shot and killed Abu-Bakr Ibn-Umar with an arrow; the last kings of Sine and Saloum were Maad a Sinig Mahecor Joof and Maad Saloum Fode N'Gouye Joof respectively. They both died in 1969. After their deaths, the Serer Kingdoms of Sine and Saloum
A dimer is an oligomer consisting of two monomers joined by bonds that can be either strong or weak, covalent or intermolecular. The term homodimer is used when the two molecules are heterodimer when they are not; the reverse of dimerisation is called dissociation. When two oppositely charged ions associate into dimers, they are referred to as Bjerrum pairs. Carboxylic acids form dimers by hydrogen bonding of the acidic hydrogen and the carbonyl oxygen when anhydrous. For example, acetic acid forms a dimer in the gas phase, where the monomer units are held together by hydrogen bonds. Under special conditions, most OH-containing molecules form dimers. Borane occurs as the dimer diborane, due to the high Lewis acidity of the boron center. Excimers and exciplexes are excited structures with a short lifetime. For example, noble gases do not form stable dimers, but do form the excimers Ar2*, Kr2* and Xe2* under high pressure and electrical stimulation. Molecular dimers are formed by the reaction of two identical compounds e.g.: 2A → A-A.
In this example, monomer "A" is said to dimerise to give the dimer "A-A". An example is a diaminocarbene, which dimerise to give a tetraaminoethylene: 2 C2 → 2C=C2Carbenes are reactive and form bonds. Dicyclopentadiene is an asymmetrical dimer of two cyclopentadiene molecules that have reacted in a Diels-Alder reaction to give the product. Upon heating, it "cracks" to give identical monomers: C10H12 → 2 C5H6Many nonmetallic elements occur as dimers: hydrogen, oxygen, the halogens, i.e. fluorine, chlorine and iodine. Noble gases can form dimers linked for example dihelium or diargon. Mercury occurs as a mercury cation, formally a dimeric ion. Other metals may form a proportion of dimers in their vapour. Known metallic dimers include Li2, Na2, K2, Rb2 and Cs2. Many small organic molecules, most notably formaldehyde form dimers; the dimer of formaldehyde is dioxetane. In the context of polymers, "dimer" refers to the degree of polymerization 2, regardless of the stoichiometry or condensation reactions.
This is applicable to disaccharides. For example, cellobiose is a dimer of glucose though the formation reaction produces water: 2C6H12O6 → C12H22O11 + H2OHere, the dimer has a stoichiometry different from the pair of monomers. Amino acids can form dimers, which are called dipeptides. An example is glycylglycine. Other examples are carnosine. Pyrimidine dimers are formed by a photochemical reaction from pyrimidine DNA bases; this cross-linking causes DNA mutations, causing skin cancers. Monomer Trimer Polymer Protein dimer "IUPAC "Gold Book" definition". Retrieved 2009-04-30
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