soap

soap a cleansing agent. It cleanses by lowering the surface tension of water, by emulsifying grease, and by absorbing dirt into the foam. Ancient peoples are believed to have employed wood ashes and water for washing and to have relieved the resulting irritation with grease or oil. In the 1st cent. AD, Pliny described a soap of tallow and wood ashes used by Germanic tribes to brighten their hair. A soap factory and bars of scented soap were excavated at Pompeii. Soap fell into disuse after the fall of Rome but was revived in Italy probably in the 8th cent. and reached France c.1200; Marseilles became noted as a soapmaking center. Although soap was known in England in the 14th cent., the first English patent to a soapmaker was issued in the 17th cent. The industry was handicapped in England from 1712 to 1853 by a heavy tax on soap. In the American colonies soap factories appeared at an early date, and many housewives made soap from waste fats and lye (obtained by leaching wood ashes). The manufacture of soap was stimulated by Chevreul's discovery of oleic and stearic acids in the early 19th cent. and by Leblanc's method (1791) of preparing soda from salt. Chemically, soaps are metallic salts of fatty acids . The manufacture of soap is based on a chemical reaction (saponification) in which an alkali acts upon a fat to form a metal salt (soap) and an alcohol (glycerol). A number of methods may be employed to make soap, but all are based on the same principle of operation. Fats and oils (often blended) are heated in a large vessel, then enough alkali to react with all the fat is stirred in. Salt is added, and the soap then forms a light curd that floats to the surface. Glycerol, a valuable byproduct, can be distilled from the liquid residue. To produce a purer soap, the curds are washed with salt solution, water is later added, and the solution is allowed to settle; the upper of the two layers thus formed is the pure soap, called settled soap. It is thoroughly churned, poured into huge frames, cut with wires, shaped, and stamped. Hard-milled soap is run over chilled rollers and is scraped off as chips which are rolled into ribbons, cut, and shaped. Soap is marketed also as chips, flakes, and beads and in powdered form. Soap powders, as distinguished from powdered soap, contain builders that assist in rough cleaning. Soaps differ according to the lathering properties of the fat or oils and according to the alkali employed. When sodium hydroxide is used as the alkali, hard soaps are formed; potassium hydroxide yields soft soaps. Aluminum, calcium, magnesium, lead, or other metals are used in place of sodium or potassium for soaps used in industry as paint driers, ointments, and lubricating greases and in waterproofing. Fillers are added to many soaps to increase lathering, cleansing, and water-softening properties; the sodium salt of rosin is commonly used in yellow laundry soap to increase lathering. Soap substitutes include saponin-containing plants such as soapwort and shagbark and the modern soapless detergents (usually sulfonated alcohols), which may be used in hard water and even in saltwater without forming curds

aspartic acid

aspartic acid , organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l -stereoisomer participates in the biosynthesis of proteins. Its acidic side chain adds a negative charge and hence a greater degree of water-solubility to proteins in neutral solution and has been shown to be near the active sites of some enzymes (see pepsin ). Aspartic acid is not essential to the human diet. It was discovered in protein in 1868.

benzoic acid

benzoic acid , C 6 H 5 CO 2 H, crystalline solid organic acid that melts at 122°C and boils at 249°C. It is the simplest aromatic carboxylic acid (see aryl group and carboxyl group ). In addition to being synthesized from a variety of organic compounds (e.g., benzyl alcohol, benzaldehyde, toluene, and phthalic acid), it may be obtained from resins, notably gum benzoin . It is used largely for making its salts and esters, most notably sodium benzoate, which is widely used as a preservative in foods and beverages and as a mild antiseptic in mouthwashes and toothpastes.

acid-base indicators

acid-base indicators 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 p H 1 to 5 and yellow from p H 5 to 9; methyl orange, yellow in basic and neutral solutions and reddish below p H 3.7; phenolphthalein, colorless in acid and neutral solutions, pink at about p H 8.5, and purplish at p H 10; and litmus . Most indicators are also used in large amounts for dyeing; small quantities are nonetheless invaluable for use as indicators in chemical laboratories

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. Properties 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. Classification 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 p H (see separate article). 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, H 2 O, 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, H 3 O + (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, BF 3 , can be considered a Lewis acid and ethyl alcohol can be considered a Lewis base.

amino acid

amino acid , any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins. They are characterized by the presence of a carboxyl group (COOH) and an amino group (NH 2 ) attached to the same carbon at the end of the compound. The 20 amino acids commonly found in animals are alanine , arginine , asparagine , aspartic acid , cysteine , glutamic acid , glutamine , glycine , histidine , isoleucine , leucine , lysine , methionine , phenylalanine , proline , serine , threonine , tryptophan , tyrosine , and valine . More than 100 less common amino acids also occur in biological systems, particularly in plants. Every amino acid except glycine can occur as either of two optically active stereoisomers, d or l ; the more common isomer in nature is the l -form. When the carboxyl carbon atom of one amino acid covalently binds to the amino nitrogen atom of another amino acid with the release of a water molecule, a peptide bond is formed. Amino acids are released in the intestinal tract by the digestion of food proteins and are then carried in the bloodstream to the body cells, where they are used for growth, maintenance, and repair. Cellular catabolism breaks amino acids down into smaller fragments. Many of the amino acids necessary in metabolism can be synthesized in the human or animal body when needed; these are called nonessential. Others cannot be synthesized in sufficient quantities; these are termed essential and must be provided in the diet.

butyric acid

butyric acid or butanoic acid , CH 3 CH 2 CH 2 CO 2 H, viscous, foul-smelling, liquid carboxylic acid; m.p. about -5°C; b.p. 163.5°C. It is miscible with water, ethanol, and ether. It is a low molecular weight fatty acid that is present in butter as an ester of glycerol; the odor of rancid butter is due largely to the presence of free butyric acid. Butyric acid is used in the manufacture of plastics. Isobutyric acid, or 2-methylpropanoic acid, (CH 3 ) 2 CHCO 2 H, is a geometric isomer of the butyric acid described above; it has different physical properties but similar chemical properties

hydrogen sulfide(H2SO4)

hydrogen sulfide chemical compound, H 2 S, 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. Dissolved in water, it forms a very weak dibasic acid that is sometimes called hydrosulfuric acid. Hydrogen sulfide is flammable; in an excess of air it burns to form sulfur dioxide and water, but if not enough oxygen is present, it forms elemental sulfur and water. Hydrogen sulfide is found naturally in volcanic gases and in some mineral waters. It is often formed during decay of animal matter. It is a part of many unrefined carbonaceous fuels, e.g., natural gas, crude oil, and coal; it is obtained as a byproduct of refining such fuels. It may be made by reacting hydrogen gas with molten sulfur or with sulfur vapors, or by treating a metal sulfide (e.g., ferrous sulfide, FeS) with an acid. Hydrogen sulfide reacts with most metal ions to form sulfides; the sulfides of some metals are insoluble in water and have characteristic colors that help to identify the metal during chemical analysis. Hydrogen sulfide also reacts directly with silver metal, forming a dull, gray-black tarnish of silver sulfide (Ag 2 S).

chemical engineering

Academic discipline and industrial activity concerned with developing processes and designing and operating plants to change materials' physical or chemical states. With roots in the inorganic and coal-based chemical industries of western Europe and the oil-refining industry in North America, it was spurred by the need to supply chemicals and products during the two World Wars. The field includes research, design, construction, operation, sales, and management activities. Chemical engineers must master chemistry (including the nature of chemical reactions, the effects of temperature and pressure on equilibrium, and the effects of catalysts on reaction rates), physics, and mathematics. The engineering aspect, involving fluid flow ( deformation and flow) and heat and mass transfer, is broken down into “unit operations,” including vaporization, distillation, absorption, filtration, extraction, crystallization, agitation and mixing, drying, and size reduction; each is described mathematically, and its principles apply to any material. Chemical engineers work not only in the chemical and oil industries but also in such processing industries as foods, paper, textiles, plastics, nuclear, and biotechnology.For more information on chemical engineering, visit Britannica.com.

chemical analysis

chemical analysis the study of the chemical composition and structure of substances. More broadly, it may be considered the corpus of all techniques whereby any exact chemical information is obtained. There are two branches in analytical chemistry: qualitative analysis and quantitative analysis. Qualitative analysis is the determination of those elements and compounds that are present in a sample of unknown material. Quantitative analysis is the determination of the amount by weight of each element or compound present. The procedures by which these aims may be achieved include testing for the chemical reaction of a putative constituent with an admixed reagent or for some well-defined physical property of the putative constituent. Classical methods include use of the analytical balance, gas manometer, buret, and visual inspection of color change. Gas and paper chromatography are particularly important modern methods. Physical techniques such as use of the mass spectrometer are also employed. For samples in the gaseous state, optical spectroscopy provides the best technique for determining which atomic and molecular species are present.

chemical reaction

chemical reaction process by which one or more substances may be transformed into one or more new substances. Energy is released or is absorbed, but no loss in total molecular weight occurs. When, for example, water is decomposed, its molecules, each of which consists of one atom of oxygen and two of hydrogen, are broken down; the hydrogen atoms then combine in pairs to form hydrogen molecules and the oxygen atoms to form oxygen molecules. In a chemical reaction, substances lose their characteristic properties. Water, for example, a liquid which neither burns nor supports combustion, is decomposed to yield flammable hydrogen and combustion-supporting oxygen. In some reactions heat is given off (exothermic reactions), and in others heat is absorbed (endothermic reactions). Furthermore, the new substances formed differ from the original substances in the energy they contain. Chemical reactions are classified according to the kind of change that takes place. When a compound, which consists of two or more elements or groups of elements, is broken down into its constituents, the reaction is called simple decomposition. When two compounds react with one another to form two new compounds, the reaction is called double decomposition. In so-called replacement reactions the place of one of the elements in a compound is taken by another element reacting with the compound. When elements combine to form a compound, the reaction is termed chemical combination. Oxidation and reduction reactions are extremely important. Reversible reactions are those in which the chemical change taking place may be paralleled by another change back to the original substances. The rates at which chemical reactions proceed depend upon various factors, e.g., upon temperature, pressure, and the concentration of the substances involved and, sometimes, upon the use of a chemical called a catalyst . In some chemical reactions, such as that of photographic film, light is an important factor. The changes taking place in a chemical reaction are represented by a chemical equation . An element's activity, i.e., its tendency to enter into compounds, varies from one element to another.

chemical bond

chemical bond mechanism whereby atoms combine to form molecules . There is a chemical bond between two atoms or groups of atoms when the forces acting between them are strong enough to lead to the formation of an aggregate with sufficient stability to be regarded as an independent species. The number of bonds an atom forms corresponds to its valence . The amount of energy required to break a bond and produce neutral atoms is called the bond energy. All bonds arise from the attraction of unlike charges according to Coulomb's law; however, depending on the atoms involved, this force manifests itself in quite different ways. The principal types of chemical bond are the ionic, covalent, metallic, and hydrogen bonds. The ionic and covalent bonds are idealized cases, however; most bonds are of an intermediate type. The Ionic Bond The ionic bond results from the attraction of oppositely charged ions. The atoms of metallic elements, e.g., those of sodium, lose their outer electrons easily, while the atoms of nonmetals, e.g., those of chlorine, tend to gain electrons. The highly stable ions that result retain their individual structures as they approach one another to form a stable molecule or crystal. In an ionic crystal like sodium chloride, no discrete diatomic molecules exist; rather, the crystal is composed of independent Na + and Cl - ions, each of which is attracted to neighboring ions of the opposite charge. Thus the entire crystal is a single giant molecule. The Covalent Bond A single covalent bond is created when two atoms share a pair of electrons. There is no net charge on either atom; the attractive force is produced by interaction of the electron pair with the nuclei of both atoms. If the atoms share more than two electrons, double and triple bonds are formed, because each shared pair produces its own bond. By sharing their electrons, both atoms are able to achieve a highly stable electron configuration corresponding to that of an inert gas . For example, in methane (CH 4 ), carbon shares an electron pair with each hydrogen atom; the total number of electrons shared by carbon is eight, which corresponds to the number of electrons in the outer shell of neon; each hydrogen shares two electrons, which corresponds to the electron configuration of helium. In most covalent bonds, each atom contributes one electron to the shared pair. In certain cases, however, both electrons come from the same atom. As a result, the bond has a partly ionic character and is called a coordinate link. Actually, the only purely covalent bond is that between two identical atoms. Covalent bonds are of particular importance in organic chemistry because of the ability of the carbon atom to form four covalent bonds. These bonds are oriented in definite directions in space, giving rise to the complex geometry of organic molecules. If all four bonds are single, as in methane, the shape of the molecule is that of a tetrahedron. The importance of shared electron pairs was first realized by the American chemist G. N. Lewis (1916), who pointed out that very few stable molecules exist in which the total number of electrons is odd. His octet rule allows chemists to predict the most probable bond structure and charge distribution for molecules and ions. With the advent of quantum mechanics, it was realized that the electrons in a shared pair must have opposite spin, as required by the Pauli exclusion principle . The molecular orbital theory was developed to predict the exact distribution of the electron density in various molecular structures. The American chemist Linus Pauling introduced the concept of resonance to explain how stability is achieved when more than one reasonable molecular structure is possible: the actual molecule is a coherent mixture of the two structures. Metallic and Hydrogen Bonds Unlike the ionic and covalent bonds, which are found in a great variety of molecules, the metallic and hydrogen bonds are highly specialized. The metallic bond is responsible for the crystalline structure of pure metals. This bond cannot be ionic because all the atoms are identical, nor can it be covalent, in the ordinary sense, because there are too few valence electrons to be shared in pairs among neighboring atoms. Instead, the valence electrons are shared collectively by all the atoms in the crystal. The electrons behave like a free gas moving within the lattice of fixed, positive ionic cores. The extreme mobility of the electrons in a metal explains its high thermal and electrical conductivity. Hydrogen bonding is a strong electrostatic attraction between two independent polar molecules, i.e., molecules in which the charges are unevenly distributed, usually containing nitrogen, oxygen, or fluorine. These elements have strong electron-attracting power, and the hydrogen atom serves as a bridge between them. The hydrogen bond, which plays an important role in molecular biology, is much weaker than the ionic or covalent bonds. It is responsible for the structure of ice.

polycarbonates

polycarbonates group of clear, thermoplastic polymers used mainly as molding compounds (see plastic ). Polycarbonates are prepared by the reaction of an aromatic difunctional phenol with either phosgene or an aromatic or aliphatic carbonate. The commercially important polycarbonates use 2,2-bis (4-hydroxyphenol)-propane (bisphenol A) and diphenyl carbonate. This polymer is a clear plastic with a slight yellow discoloration. It has excellent electrical properties and a high impact strength.

helium(He)

helium , gaseous chemical element; symbol He; at. no. 2; at. wt. 4.0026; m.p. below -272°C at 26 atmospheres pressure; b.p. -268.934°C at 1 atmosphere pressure; density 0.1785 grams per liter at STP ; valence usually 0. Spectroscopic evidence for the presence of helium in the sun was first obtained during a solar eclipse in 1868. A bright yellow emission line was observed and was later shown to correspond to no known element; the new element was named by J. N. Lockyer and E. Frankland from helios [Gr.,=sun]. Helium was isolated (1895) from a sample of the uranium mineral cleveite by Sir William Ramsay. Properties and Isotopes Helium is less dense than any other known gas except hydrogen and is about one seventh as dense as air. Extremely unreactive, it is an inert gas in Group 18 of the periodic table . Natural helium is a mixture of two stable isotopes, helium-3 and helium-4. In helium obtained from natural gas about one atom in 10 million is helium-3. The unstable isotopes helium-5, helium-6, and helium-8 have been synthesized. The alpha particles that are emitted from certain radioactive substances are identical to helium-4 nuclei (two protons and two neutrons). Helium-4 is unusual in that it forms two different kinds of liquids. When it is cooled below 4.22°K (its boiling point at atmospheric pressure) it condenses to liquid helium-I, which behaves as an ordinary liquid. When liquid helium-I is cooled below about 2.18°K (at atmospheric pressure), liquid helium-II is formed. Liquid helium-II has a number of unusual properties. It is sometimes called a superfluid because it has extremely low viscosity. It also has extremely high heat conductivity and expands on cooling. It cannot be contained in an open beaker since a thin film of it creeps up the side, over the lip, and flows down the outside. The study of these phenomena is a part of low-temperature physics. When helium-3 is liquefied and cooled it does not exhibit the properties of liquid helium-II; this difference in properties between helium-3 and helium-4 can be explained in terms of quantum mechanics. Natural Occurrence and Preparation Helium is rare and costly. Wells in Texas (where the Federal Helium Reserve was established in 1925 near Amarillo), Oklahoma, and Kansas are the principal world source. Crude helium is separated by liquefying the other gases present in the natural gas; it is then either further purified or stored for later purification and use. Some helium is extracted directly from the atmosphere; the gas is also found in certain uranium minerals and in some mineral waters, but not in economic quantities. It has been estimated that helium makes up only about 0.000001% of the combined weight of the earth's atmosphere and crust; it is most concentrated in the exosphere, which is the outermost region of the atmosphere, 600-1500 mi (960-2400 km) above the earth's surface. Helium is abundant in outer space; it makes up about 23% of the mass of the visible universe. It is the end product of energy-releasing fusion processes in stars (see interstellar matter ). Uses Helium's noncombustibility and buoyancy (second only to hydrogen) make it the most suitable gas for balloons and other lighter-than-air craft. A mixture of helium and oxygen is often supplied as a breathing mixture for deep-sea divers and caisson workers and is used in decompression chambers; because helium is less soluble in human blood than nitrogen, its use reduces the risk of caisson disease, or the "bends." Helium can also be used wherever an unreactive atmosphere is needed, e.g., in electric arc welding, in growing crystals of silicon and germanium for semiconductors, and in refining titanium and zirconium metals. It is also used to pressurize the fuel tanks of liquid-fueled rockets. Liquid helium is essential for many low temperature applications (see low-temperature physics ).

bromine (Br)

bromine [Gr.,=stench], volatile, liquid chemical element; symbol Br; at. no. 35; at. wt. 79.904; m.p. -7.2°C; b.p. 58.78°C; sp. gr. of liquid 3.12 at 20°C; density of vapor 7.14 grams per liter at STP ; valence -1, +1, +3, +5, or +7. At ordinary temperatures bromine is a brownish-red liquid that gives off a similarly colored vapor with an offensive, suffocating odor. It is a member of the halogen family in Group 17 of the periodic table . It is the only nonmetallic element that is liquid under ordinary conditions. It is soluble in water to some extent; the aqueous solution, called bromine water, acts as an oxidizing agent. It is also soluble in alcohol, ether, and carbon disulfide. Bromine is less active chemically than chlorine or fluorine but is more active than iodine . It forms compounds similar to those of the other halogens (see bromide ). Oxides of bromine are unstable, but two acids, hypobromous acid, HBrO, and bromic acid, HBrO 3 , are known with their salts. Hydrobromic acid is the aqueous solution of hydrogen bromide, HBr. Bromine does not occur uncombined in nature but is found in combination with other elements, notably sodium, potassium, magnesium, and silver. In compounds it is present in seawater, in mineral springs, and in common salt deposits, e.g., those at Stassfurt, Germany. It occurs in the United States, principally in Michigan, Ohio, and West Virginia. Bromine for commercial purposes is obtained by treating brines (from salt wells or seawater) with chlorine, which displaces the bromine. It is important in the preparation of organic compounds, such as ethylene dibromide, which is used in conjunction with an antiknock compound in gasoline. Bromine has a powerful corrosive action on the skin, destroying the tissue, and the vapor is strongly irritating to the eyes and the membranes of the nose and throat. The element was discovered in seawater by Antoine Jérôme Balard in 1826.

parabola

parabola , plane curve consisting of all points equidistant from a given fixed point (focus) and a given fixed line (directrix) (see illustration) . It is the conic section cut by a plane parallel to one of the elements of the cone. The axis of a parabola is the line through the focus perpendicular to the directrix. The vertex is the point at which the axis intersects the curve. The latus rectum is the chord through the focus perpendicular to the axis. Examples of this curve are the path of a projectile and the shape of the cross section of a parallel beam reflector.

calcium carbonate (ca)

calcium carbonate CaCO 3 , 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. Calcium carbonate is largely insoluble in water but is quite soluble in water containing dissolved carbon dioxide, combining with it to form the bicarbonate Ca(HCO 3 ) 2 . Such reactions on limestone (which is mainly composed of calcite) account for the formation of stalactites and stalagmites in caves. Iceland spar is a pure form of calcium carbonate and exhibits birefringence, or double refraction .

Cs (cesium)

CS chemical compound (orthochlorobenzalmalonitrile) used in riot control and, by the military, as a harassing agent. The compound is dispersed as an aerosol or as a finely divided powder. Exposure to CS causes intense pain in the eyes and upper respiratory tract; the pain spreads to the lungs and gives the sensation of suffocation. In humid weather CS may cause severe blistering of the skin. Heavy exposure to the compound may cause serious lung damage, resulting in death. Nonetheless, CS is less toxic than many other tear gases . CS was first synthesized in the 1920s by Ben Corson and Roger Stoughton; the compound's name is derived from their initials.

barium(Ba)

barium [Gr.,=heavy], metallic chemical element; symbol Ba; at. no. 56; at. wt. 137.33; m.p. 725°C; b.p. 1,640°C; sp. gr. 3.5 at 20°C; valence +2. Barium is a soft, silver-white, chemically active, poisonous metal with a face-centered cubic crystalline structure. It is an alkaline-earth metal in Group 2 of the periodic table . Its principal ore is barite (barium sulfate); it also occurs in the mineral witherite (barium carbonate). The pure metal is obtained by the electrolysis of fused barium salts or, industrially, by the reduction of barium oxide with aluminum. Barium is often used in barium-nickel alloys for spark-plug electrodes and in vacuum tubes as a drying and oxygen-removing agent. Barium oxidizes in air, and it reacts vigorously with water to form the hydroxide, liberating hydrogen. In moist air it may spontaneously ignite. It burns in air to form the peroxide, which produces hydrogen peroxide when treated with water. Barium reacts with almost all of the nonmetals; all of its water-soluble and acid-soluble compounds are poisonous. Barium carbonate is used in glass, as a pottery glaze, and as a rat poison. Chrome yellow (barium chromate) is used as a paint pigment and in safety matches. The chlorate and nitrate are used in pyrotechnics to provide a green color. Barium oxide strongly absorbs carbon dioxide and water; it is used as a drying agent. Barium chloride is used in medicinal preparations and as a water softener. Barium sulfide phosphoresces after exposure to light; it is sometimes used as a paint pigment. Barite, the sulfate ore, has many industrial uses. Because barium sulfate is virtually insoluble in water and acids, it can be used to coat the alimentary tract to increase the contrast for X-ray photography without being absorbed by the body and poisoning the subject. Barium salts give a characteristic green color in the flame test . Barium metal was first isolated in 1808 by Sir Humphry Davy by electrolysis

chromium (cr)

chromium [Gr.,=color], metallic chemical element; symbol Cr; at. no. 24; at. wt. 51.996; m.p. about 1,857°C; b.p. 2,672°C; sp. gr. about 7.2 at 20°C; valence +2, +3, +6. Chromium is a silver-gray, lustrous, brittle, hard metal that can be highly polished. It is found in Group 6 of the periodic table . It does not tarnish in air, but burns when heated, forming the green chromic oxide. When combined with oxygen, besides yielding chromic oxide, which is used as a pigment, it forms chromic anhydride (the red trioxide and anhydride of chromic acid). With other metallic elements, e.g., lead and potassium, together with oxygen, it forms the chromates and dichromates. These compounds are salts of chromic acid and are used as pigments in paints, in dyeing, and in the tanning of leather. Chrome yellow, a pigment, consists largely of lead chromate. Other chrome colors are black, red, orange, and green. In the chrome process for tanning leather, a dichromate is used, and chromium hydroxide, a basic compound of chromium, hydrogen, and oxygen, is precipitated and held in the leather. The hydroxide is used also as a mordant in dyeing cloth. A mixture of potassium dichromate and sulfuric acid is used as a powerful agent for cleaning laboratory glassware. Chromium is a comparatively rare element, never occurring by itself in nature but always in compounds. Its chief source is the mineral chromite, which is composed of iron, chromium, and oxygen and is found principally in the nations of the former Soviet Union, South Africa, Zimbabwe, Turkey, and the Philippines. The element, in the form of chromic oxide, gives the greenish tint to the emerald and the aquamarine. Metallic chromium is prepared by reduction of the oxide by aluminum or by carbon. It is used in plating other metals because of its hardness and nontarnishing properties. In alloys with other metals it contributes hardness, strength, and heat resistance. Its most important use is in the steel industry, where it is a constituent of several alloy steels, e.g., chromium steel or chrome steel. Stainless steel contains from 11% to 18% chromium. An alloy of nickel and chromium, often called Nichrome, is widely used as a heating element in electric toasters, coffeepots, and other appliances. Stellite is an extremely hard alloy of cobalt, chromium, and tungsten, with small amounts of iron, silicon, and carbon; it is used in metal cutting tools and for wear-resistant surfaces. A similar alloy, with molybdenum instead of tungsten, is used in surgical tools since it does not react with body fluids. Chromium was discovered in 1797 by L. N. Vauquelin.

plaster of Paris

Quick-setting gypsum plaster consisting of a fine white powder, calcium sulfate hemihydrate, which hardens when moistened and allowed to dry. It is made by heating gypsum to 250–360°F (120–180°C). Used since ancient times, plaster of paris is so called because of its preparation from the abundant gypsum found in Paris. It is used to make molds and casts for ceramics and sculptures, to precast and hold ornamental plasterwork on ceilings and cornices, and for orthopedic bandages (casts). In medieval and Renaissance times, gesso (plaster of paris mixed with glue) was applied to wood panels, plaster, stone, or canvas to provide the ground for tempera and oil painting.For more information on plaster of paris, visit Britannica.com

Iron(Fe)

iron metallic chemical element; symbol Fe [Lat. ferrum ]; at. no. 26; at. wt. 55.845; m.p. about 1,535°C; b.p. about 2,750°C; sp. gr. 7.87 at 20°C; valence +2, +3, +4, or +6. Iron is biologically significant. Because iron is a component of hemoglobin, a red oxygen-carrying pigment of the red blood cells of vertebrates, iron compounds are important in nutrition; one cause of anemia is iron deficiency. For the history of the use of iron, see Iron Age . Properties Iron is a lustrous, ductile, malleable, silver-gray metal found in Group 8 of the periodic table . It is known to exist in four distinct crystalline forms (see allotropy ). The most common is the α-form, which is stable below about 770°C, and has a body-centered cubic crystalline structure; it is often called ferrite. Iron is attracted by a magnet and is itself easily magnetized (see magnetism ). It is a good conductor of heat and electricity. It displaces hydrogen from hydrochloric or dilute sulfuric acid, but becomes passive (loses its normal chemical activity) when treated with cold nitric acid. Compounds Iron forms such compounds as oxides, hydroxides, halides, acetates, carbonates, sulfides, nitrates, sulfates, and a number of complex ions. It is chemically active and forms two major series of chemical compounds, the bivalent iron (II), or ferrous, compounds and the trivalent iron (III), or ferric, compounds. Ferrous sulfate heptahydrate, FeSO 4 ·7H 2 O, sometimes called green vitriol, is a compound formed by the reaction of dilute sulfuric acid (formerly called oil of vitriol) with metallic iron; it is used in the manufacture of ink, in dyeing, and as a disinfectant. Ferric chloride hexahydrate, FeCl 3 ·6H 2 O, is a yellow-brown crystalline compound used as a mordant in dyeing and as an etching compound. Ferric oxide, Fe 2 O 3 , is a reddish-brown powder used as a paint pigment and in abrasive rouges. Prussian blue, KFe 2 (CN) 6 , is a pigment containing the ferrocyanide complex ion. Iron rusts readily in moist air, forming a complex mixture of compounds that is mostly a ferrous-ferric oxide with the composition Fe 3 O 4 . Natural Occurrence Iron is an abundant element in the universe; it is found in many stars, including the sun. Iron is the fourth most abundant element in the earth's crust, of which it constitutes about 5% by weight, and is believed to be the major component of the earth's core. Iron is found distributed in the soil in low concentrations and is found dissolved in groundwaters and the ocean to a limited extent. It is rarely found uncombined in nature except in meteorites, but iron ores and minerals are abundant and widely distributed. The principal ores of iron are hematite (ferric oxide, Fe 2 O 3 ) and limonite (ferric oxide trihydrate, Fe 2 O 3 ·3H 2 O). Other ores include siderite (ferrous carbonate, FeCO 3 ), taconite (an iron silicate), and magnetite (ferrous-ferric oxide, Fe 3 O 4 ), which often occurs as a white sand. Iron pyrite (iron disulfide, FeS 2 ) is a crystalline gold-colored mineral known as fool's gold. Chromite is a chromium ore that contains iron. Lodestone is a form of magnetite that exhibits natural magnetic properties. Production and Refining Iron is produced in the United States chiefly from oxide ores. For many years rich hematite ores were produced by open-pit mining in the Mesabi Range near Lake Superior. However, these ores have been largely depleted, and iron is now produced from low-grade ores that are treated to improve their quality; this process is called beneficiation. Iron ores are refined in the blast furnace . The product of the blast furnace is called pig iron and contains about 4% carbon and small amounts of manganese, silicon, phosphorus, and sulfur. About 95% of this iron is processed further to make steel , often by the open-hearth process or the Bessemer process , but more recently in the United States and other countries by the basic oxygen process or by an electric arc furnace. The balance is cast in sand molds into blocks called pigs. It is further processed in iron foundries (see casting ). Cast Iron Cast iron is made when pig iron is remelted in small cupola furnaces (similar to the blast furnace in design and operation) and poured into molds to make castings. It usually contains 2% to 6% carbon. Scrap iron or steel is often added to vary the composition. Cast iron is used extensively to make machine parts, engine cylinder blocks, stoves, pipes, steam radiators, and many other products. Gray cast iron, or gray iron, is produced when the iron in the mold is cooled slowly. Part of the carbon separates out in plates in the form of graphite but remains physically mixed in the iron. Gray iron is brittle but soft and easily machined. White cast iron, or white iron, which is harder and more brittle, is made by cooling the molten iron rapidly. The carbon remains distributed throughout the iron as cementite (iron carbide, Fe 3 C). A malleable cast iron can be made by annealing white iron castings in a special furnace. Some of the carbon separates from the cementite; it is much more finely divided than in gray iron. A ductile iron may be prepared by adding magnesium to the molten pig iron; when the iron is cast the carbon forms tiny spherical nodules around the magnesium. Ductile iron is strong, shock resistant, and easily machined. Wrought Iron Wrought iron is commercially purified iron. In the Aston process, pig iron is refined in a Bessemer converter and then poured into molten iron silicate slag. The resulting semisolid mass is passed between rollers that squeeze out most of the slag. The wrought iron has a fibrous structure with threads of slag running through it; it is tough, malleable, ductile, corrosion resistant, and melts only at high temperatures. It is used to make rivets, bolts, pipes, chains, and anchors, and is also used for ornamental ironwork .

Neon(Ne)

neon [Gr.,=new], gaseous chemical element; symbol Ne; at. no. 10; at. wt. 20.179; m.p. -248.67°C; b.p. -246.048°C; density 0.8999 grams per liter at STP ; valence 0. Neon is a colorless, odorless, and tasteless gas. It is one of the inert gases in Group 18 of the periodic table ; it does not form compounds in the normal chemical sense. A small amount of neon in a partially evacuated glass tube emits a bright reddish-orange glow while conducting electricity. Neon is a rare gas present in the atmosphere to a very limited extent. It is obtained as a byproduct in the production of liquid air. The greatest commercial use of neon is in advertising signs (see lighting ). It is also used in high-intensity beacons, in some electron tubes, in Geiger counters, in automotive ignition timing lights, and in high-voltage warning indicators. It is used for particle detection in high-energy physics research. Neon finds use in lasers both as a light-emitting agent and as a coolant. Liquid neon is a particularly good cryogenic refrigerant since it will absorb more heat without vaporizing than an equal volume of liquid helium or liquid hydrogen. Neon was discovered in 1898 by William Ramsay and M. W. Travers.

Neon(Ne)

neon [Gr.,=new], gaseous chemical element; symbol Ne; at. no. 10; at. wt. 20.179; m.p. -248.67°C; b.p. -246.048°C; density 0.8999 grams per liter at STP ; valence 0. Neon is a colorless, odorless, and tasteless gas. It is one of the inert gases in Group 18 of the periodic table ; it does not form compounds in the normal chemical sense. A small amount of neon in a partially evacuated glass tube emits a bright reddish-orange glow while conducting electricity. Neon is a rare gas present in the atmosphere to a very limited extent. It is obtained as a byproduct in the production of liquid air. The greatest commercial use of neon is in advertising signs (see lighting ). It is also used in high-intensity beacons, in some electron tubes, in Geiger counters, in automotive ignition timing lights, and in high-voltage warning indicators. It is used for particle detection in high-energy physics research. Neon finds use in lasers both as a light-emitting agent and as a coolant. Liquid neon is a particularly good cryogenic refrigerant since it will absorb more heat without vaporizing than an equal volume of liquid helium or liquid hydrogen. Neon was discovered in 1898 by William Ramsay and M. W. Travers.

carbon dioxide

carbon dioxide chemical compound, CO 2 , a colorless, odorless, tasteless gas that is about one and one-half times as dense as air under ordinary conditions of temperature and pressure. It does not burn, and under normal conditions it is stable, inert and nontoxic. It will however support combustion of magnesium to give magnesium oxide and carbon. Although it is not a poison, it can cause death by suffocation if inhaled in large amounts. It is a fairly stable compound but decomposes at very high temperatures into carbon and oxygen. It is fairly soluble in water, one volume of it dissolving in an equal volume of water at room temperature and pressure; the resultant weakly acidic aqueous solution is called carbonic acid . The gas is easily liquefied by compression and cooling. If liquid carbon dioxide is quickly decompressed it rapidly expands and some of it evaporates, removing enough heat so that the rest of it cools into solid carbon dioxide "snow." A standard test for the presence of carbon dioxide is its reaction with limewater (a saturated water solution of calcium hydroxide ) to form a milky-white precipitate of calcium hydroxide. Carbon dioxide occurs in nature both free and in combination (e.g., in carbonates ). It is part of the atmosphere , making up about 1% of the volume of dry air. Because it is a product of combustion of carbonaceous fuels (e.g., coal, coke, fuel oil, gasoline, and cooking gas), there is usually more of it in city air than in country air. The natural balance of carbon dioxide in the atmosphere is growing from its stable level of 0.13% to a predicted 0.14% by the year 2000. It is anticipated that this extra carbon dioxide will fuel the greenhouse effect, warm the atmosphere, and further disrupt the natural carbon dioxide cycle (see global warming ). In various parts of the world—notably in Italy, Java, and Yellowstone National Park in the United States—carbon dioxide is formed underground and issues from fissures in the earth. Natural mineral waters such as Vichy water sparkle (effervesce) because excess carbon dioxide that dissolved in them under pressure collects in bubbles and escapes when the pressure is released. The chokedamp (see damp ) of mines, pits, and old, unused wells is largely carbon dioxide. Carbon dioxide is a raw material for photosynthesis in green plants and is a product of animal respiration . It is also a product of the decay of organic matter. Carbon dioxide has varied commercial uses. Its greatest use as a chemical is in the production of carbonated beverages; it provides the sparkle in carbonated beverages such as soda water. Formed by the action of yeast or baking powder, carbon dioxide causes the rising of bread dough. The compound is also used in water softening, in the manufacture of aspirin and lead paint pigments, and in the Solvay process for the preparation of sodium carbonate. In some fire extinguishers carbon dioxide is expelled through a nozzle and settles on the flame, smothering it. It also has numerous nonchemical uses. It is used as a pressurizing medium and propellant, e.g., in aerosol cans of food, in fire extinguishers, in target pistols, and for inflating life rafts. Because it is relatively inert, it is used to provide a nonreactive atmosphere, e.g., for packaging foods, such as coffee, that can be spoiled by oxidation during storage. Solid carbon dioxide, known as dry ice, is used as a refrigerating agent. There are three principal commercial sources for carbon dioxide. High-purity carbon dioxide is produced from some wells. The gas is obtained as a byproduct of chemical manufacture, as in the fermentation of grain to make alcohol and the burning of limestone to make lime. It is also manufactured directly by burning carbonaceous fuels. For commercial use it is available as a liquid under high pressure in steel cylinders, as a low-temperature liquid at lower pressures, and as the solid dry ice.

carbon black

carbon black mixture of partially burned hydrocarbons. Carbon black is produced by partial combustion of natural gas . It is used as a black pigment for inks and paints, and is used in large amounts by the tire industry in the production of vulcanized rubber. Lampblack resembles carbon black, but is produced by burning liquid hydrocarbons, e.g., kerosene; it is often somewhat oily, is duller than carbon black, and may have a bluish undertone. It is sometimes used in making contact brushes for electrical apparatus.

carbon

carbon [Lat.,=charcoal], nonmetallic chemical element; symbol C; at. no. 6; at. wt. 12.011; m.p. about 3,550°C; graphite sublimes about 3,375°C; b.p. 4,827°C; sp. gr. 1.8-2.1 (amorphous), 1.9-2.3 (graphite), 3.15-3.53 (diamond); valence +2, +3, +4, or -4. Properties and Isotopes Carbon is found free in nature in at least four distinct forms (see allotropy ). One form, graphite , is a very soft, dark gray or black, lustrous material with either a hexagonal or rhombohedral crystalline structure. Diamond , a second crystalline form, is the hardest substance known. In a third form, the so-called amorphous carbon, the element occurs partly free and partly combined with other elements; charcoal , coal , coke , lampblack, peat , and lignite are some sources of amorphous carbon. A fourth form contains the fullerenes , stable molecules consisting of carbon atoms that arrange themselves into 12 pentagonal faces and any number greater than 1 of hexagonal faces. The most prominent of the fullerenes is buckminsterfullerene , a spheroidal molecule, resembling a soccer ball, consisting of 60 carbon atoms. A fifth form, "white" carbon, is believed to exist. Carbon has the capacity to act chemically both as a metal and as a nonmetal. It is a constituent of all organic matter. Carbon has 13 known isotopes, which have from 2 to 14 neutrons in the nucleus and mass numbers from 8 to 20. Carbon-12 was chosen by IUPAC in 1961 as the basis for atomic weights ; it is assigned an atomic mass of exactly 12 atomic mass units. Carbon-13 absorbs radio waves and is used in nuclear magnetic resonance spectrometry to study organic compounds. Carbon-14, which has a half-life of 5,730 years, is a naturally occurring isotope that can also be produced in a nuclear reactor. It is used extensively as a research tool in tracer studies; a compound synthesized with carbon-14 is said to be "tagged" and can be traced through a chemical or biochemical reaction. Carbon-14 has been used in the study of such problems as utilization of foods in animal nutrition, catalytic petroleum processes, photosynthesis, and the mechanism of aging in steel. It is also used for determining the age of archaeological specimens (see dating ). Compounds There are more carbon compounds than there are compounds of all other elements combined. The study of carbon compounds, both natural and synthetic, is called organic chemistry. Plastics , foods, textiles , and many other common substances contain carbon. Hydrocarbon fuels (e.g., natural gas), marsh gas, and the gases resulting from the combustion of fuels (e.g., carbon monoxide and carbon dioxide) are compounds of carbon. With oxygen and a metallic element, carbon forms many important carbonates, such as calcium carbonate (limestone) and sodium carbonate (soda). Certain active metals react with it to make industrially important carbides, such as silicon carbide (an abrasive known as carborundum), calcium carbide, used for producing acetylene gas, and tungsten carbide, an extremely hard substance used for rock drills and metalworking tools. Natural Occurrence and Uses Carbon has been known to humans in its various forms since ancient times. Although carbon makes up only .032% of the earth's crust, it is very widely distributed and forms a vast number of compounds. Carbon exists in the stars; a series of thermonuclear reactions called the carbon cycle (see nucleosynthesis ) is a source of energy for some stars. Carbon in the form of diamonds has been found in meteorites. Coke is used as a fuel in the production of iron. Carbon electrodes are widely used in electrical apparatus. The "lead" of the ordinary pencil is graphite mixed with clay. The successful linking in the 1940s of carbon with silicon has led to the development of a vast number of new substances known collectively as the silicones . Biological Importance All living organisms contain carbon; the human body is about 18% carbon by weight. In green plants carbon dioxide and water are combined to form simple sugars ( carbohydrates ); light from the sun provides the energy for this process ( photosynthesis ). The energy from the sun is stored in the chemical bonds of the sugar molecule. Anabolism, the synthesis of complex compounds (such as fats , proteins , and nucleic acids ) from simpler substances, involves the utilization of energy stored by photosynthesis. Catabolism is the release of stored energy by the oxidative destruction of organic compounds; water and carbon dioxide are two byproducts of catabolism. This continuing synthesis and degradation involving carbon dioxide is known as the biological carbon cycle .

cobalt

cobalt metallic chemical element; symbol Co; at. no. 27; at. wt. 58.9332; m.p. 1,495°C; b.p. about 2,870°C; sp. gr. 8.9 at 20°C; valence +2 or +3. Cobalt is a silver-white, lustrous, hard, brittle metal. It is a member of Group 9 of the periodic table . Like iron, it can be magnetized. It is similar to iron and nickel in its physical properties. The element is active chemically, forming many compounds, e.g., the series of cobaltous and cobaltic salts and the complex cobalt ammines derived from cobaltic salts and ammonia. Cobalt yellow, green, and blue are pigments of high quality that contain cobalt; another blue pigment, smalt, is made by powdering a fused mixture of cobalt oxide, potassium carbonate, and sand; these pigments are often used for coloring glass and ceramics. Cobalt chloride, used as an invisible ink, is almost colorless in dilute solution when applied to paper. Upon heating it undergoes dehydration and turns blue, becoming colorless again when the heat is removed and water is taken up. The element rarely occurs uncombined in nature but is often found in meteoric metal. It is a constituent of the minerals cobaltite and smaltite and of other ores, usually in association with other metals. Pure cobalt metal is prepared by reduction of its compounds by aluminum (the Goldschmidt process), by carbon, or by hydrogen. It is a component of several alloys, including the high-speed steels carboloy and stellite, from which very hard cutting tools are made. It is a component of some stainless steels, and of high-temperature alloys for use in jet engines. Alnico, an alloy of cobalt, aluminum, nickel, and other metals, is used to make high-strength, permanent magnets. As an element in the diet of sheep, cobalt prevents a disease called swayback and improves the quality of the wool. A radioactive isotope, cobalt-60 (with gamma ray emission 25 times that of radium), is prepared by neutron bombardment. It is used for cancer therapy and in industry for detecting flaws in metal parts. See hydrogen bomb . Cobalt was discovered in 1735 by Georg Brandt, a Swedish chemist.

hydrocarbon

hydrocarbon , any organic compound composed solely of the elements hydrogen and carbon. The hydrocarbons differ both in the total number of carbon and hydrogen atoms in their molecules and in the proportion of hydrogen to carbon. The hydrocarbons can be divided into various homologous series. Each member of such a series shows a definite relationship in its structural formula to the members preceding and following it, and there is generally some regularity in changes in physical properties of successive members of a series. The alkanes are a homologous series of saturated aliphatic hydrocarbons. The first and simplest member of this series is methane, CH 4 ; the series is sometimes called the methane series. Each successive member of a homologous series of hydrocarbons has one more carbon and two more hydrogen atoms in its molecule than the preceding member. The second alkane is ethane, C 2 H 6 , and the third is propane, C 3 H 8 . Alkanes have the general formula C n H 2n+2 (where n is an integer greater than or equal to 1). Generally, hydrocarbons of low molecular weight, e.g., methane, ethane, and propane, are gases; those of intermediate molecular weight, e.g., hexane, heptane, and octane, are liquids; and those of high molecular weight, e.g., eicosane (C 20 H 42 ) and polyethylene, are solids. Paraffin is a mixture of high-molecular-weight alkanes; the alkanes are sometimes called the paraffin series. Other homologous series of hydrocarbons include the alkenes and the alkynes . The various alkyl derivatives of benzene are sometimes referred to as the benzene series. Many common natural substances, e.g., natural gas, petroleum, and asphalt, are complex mixtures of hydrocarbons. The coal tar obtained from coal by coking is also a mixture of hydrocarbons. Natural gas, petroleum, and coal tar are important sources of many hydrocarbons. These complex mixtures can be refined into simpler mixtures or pure substances by fractional distillation. During the refining of petroleum, one kind of hydrocarbon is often converted to another, more useful kind by cracking. Useful hydrocarbon mixtures include cooking gas, gasoline, naphtha, benzine, kerosene, paraffin, and lubricating oils. Many hydrocarbons are useful as fuels; they burn in air to form carbon dioxide and water. The hydrocarbons differ in chemical activity. The alkanes are unaffected by many common reagents, while the alkenes and alkynes are much more reactive, as a result of the presence of unsaturation (i.e., a carbon-carbon double or triple bond) in their molecules. Many important compounds are derived from hydrocarbons, either by substitution or replacement by some other chemical group or element of one or more of the hydrogen atoms of the hydrocarbon molecule, or by the addition of some element or group to a double or triple bond (in an unsaturated hydrocarbon). Such derivatives include alcohols, aldehydes, ethers, carboxylic acids, and halocarbons.

hydrogen

hydrogen [Gr.,=water forming], gaseous chemical element; symbol H; at. no. 1; at. wt. 1.00794; m.p. -259.14°C; b.p. -252.87°C; density 0.08988 grams per liter at STP; valence usually +1. The Isotopes and Forms Atmospheric hydrogen is a mixture of three isotopes . The most common is called protium (mass no. 1, atomic mass 1.007822); the protium nucleus (protium ion) is a proton. A second isotope of hydrogen is deuterium (mass no. 2, atomic mass 2.0140), the so-called heavy hydrogen, often represented in chemical formulas by the symbol D. The deuterium nucleus, or ion, is called the deuteron; it consists of a proton plus a neutron. The two isotopes are found in atmospheric hydrogen in the proportion of about 1 atom of deuterium to every 6,700 atoms of protium. Protium and deuterium differ slightly in their chemical and physical properties; for example, the boiling point of deuterium is about 3°C lower than protium. The properties of compounds they form differ depending on the ratio of the two isotopes present. Deuterium oxide (D 2 O), the so-called heavy water, is present in ordinary water; the concentration of deuterium oxide is increased by electrolysis of the water. The melting point (3.79°C), boiling point (101.4°C), and specific gravity (1.107 at 25°C) of deuterium oxide are higher than those of ordinary water. Deuterium oxide is used as a moderator in nuclear reactors. Deuterium is also of importance because of the wide use it has found in scientific research; for example, chemical reaction mechanisms have been studied by the use of deuterium atoms as tracers (i.e., deuterium is substituted for atoms of ordinary hydrogen in compounds), making it possible to follow the course of individual molecules in a reaction. Tritium (mass no. 3, atomic mass 3.016), a third hydrogen isotope, is a radioactive gas with a half-life of about 12 1/4 years; it is often represented in chemical formulas by the symbol T. It is produced in nuclear reactors and occurs to a very limited extent in atmospheric hydrogen. It is used in the hydrogen bomb, in luminous paints, and as a tracer. The tritium nucleus, or ion, is called the triton; it consists of a proton plus two neutrons. Tritium oxide (T 2 O) has a melting point (4.49°C) higher than that of deuterium oxide. Besides being a mixture of three isotopes, hydrogen is a mixture of two forms, an ortho form and a para form, which differ in their electronic and nuclear spins. At room temperature atmospheric hydrogen is about 3/4 ortho -hydrogen and 1/4 para -hydrogen. The two forms differ slightly in their physical properties. Properties Under ordinary conditions hydrogen is a colorless, odorless, tasteless gas that is only slightly soluble in water; it is the least dense gas known. It is the first element in Group 1 of the periodic table . Ordinary hydrogen gas is made up of diatomic molecules (H 2 ) that react with oxygen to form water (H 2 O) and hydrogen peroxide (H 2 O 2 ), usually as a result of combustion. A jet of hydrogen burns in air with a very hot blue flame. The flame produced by a mixture of oxygen and hydrogen gases (as in the oxyhydrogen blowpipe) is extremely hot and is used in welding and to melt quartz and certain glasses. Hydrogen gas must be used with caution because it is highly flammable; it forms easily ignited explosive mixtures with oxygen or with air (because of the oxygen in the air). At high temperatures hydrogen is a chemically active mixture of monohydrogen (atomic hydrogen) and the normal diatomic hydrogen (see allotropy ). Hydrogen has a great affinity for oxygen and is a powerful reducing agent (see oxidation and reduction ). It reacts with nitrogen to form ammonia. With the halogens it forms compounds (hydrogen halides) that are strongly acidic in water solution. With sulfur it forms hydrogen sulfide (H 2 S), a colorless gas with an odor like rotten eggs; with sulfur and oxygen it forms sulfuric acid . It combines with several metals to form metal hydrides such as calcium hydride. Combined with carbon (and usually other elements) it is a constituent of a great many organic compounds, such as hydrocarbons , carbohydrates , fats, oils, proteins, and organic acids and bases. It is theoretically possible for hydrogen to exhibit the properties of a metal, such as electrical conductivity. Although researchers have been able to squeeze hydrogen into liquid and crystalline solid states through applications of intense heat, cold, and pressure, the metallic form eluded them until 1996. By compressing liquid hydrogen to nearly 2 million atmospheres pressure and a temperature of 4,400°K, a team at the Lawrence Livermore National Laboratory created metallic hydrogen for a millionth of a second. While there is no practical application for the accomplishment, proof of the existence of a metallic form of hydrogen may have implications for theories of how Jupiter's magnetic field is produced. Sources and Commercial Preparation While hydrogen is only about one part per million in the atmosphere, it is the most abundant element in the universe. It is believed that hydrogen makes up about three quarters of the mass of the universe, or over 90% of the molecules. It is found in the sun and in other stars, where it is the major fuel in the fusion reactions (see nucleosynthesis ) from which stars derive their energy. Hydrogen is prepared commercially by catalytic reaction of steam with hydrocarbons, by the reaction of steam with hot coke (carbon), by the electrolysis of water, and by the reaction of mineral acids on metals. Millions of cubic feet of hydrogen gas are produced daily in the United States alone. Uses Hydrogen was formerly used for filling balloons, airships, and other lighter-than-air craft, a dangerous practice because of hydrogen's explosive flammability; there were disastrous fires, e.g., the immolation of the German airship Hindenburg at its mooring at Lakehurst, N.J., in 1937. Helium is preferable for use in lighter-than-air craft since it is not flammable. Hydrogen is used in the Haber process for the fixation of atmospheric nitrogen, in the production of methanol, and in hydrogenation of fats and oils. It is also important in low-temperature research. It can be liquefied under pressure and cooled; when the pressure is released, rapid evaporation takes place and some of the hydrogen solidifies. Discovery of Hydrogen and Its Isotopes Although hydrogen was prepared many years earlier, it was first recognized as a substance distinct from other flammable gases in 1766 by Henry Cavendish , who is credited with its discovery; it was named by A. L. Lavoisier in 1783. Deuterium was discovered by H. C. Urey , F. G. Brickwedde, and G. M. Murphy in 1932, although its existence had been suspected for some years. Deuterium oxide was also discovered by Urey and was first obtained in nearly pure form by G. N. Lewis. Tritium was synthesized by Ernest Rutherford, L. E. Oliphant, and Paul Harteck in 1935.

silicon

silicon nonmetallic chemical element; symbol Si; at. no. 14; at. wt. 28.0855; m.p. 1,410°C; b.p. 2,355°C; sp. gr. 2.33 at 25°C; valence usually +4. Silicon is the element directly below carbon and above germanium in Group 14 of the periodic table . It is more metallic in its properties than carbon; in many ways it resembles germanium. Silicon has two allotropic forms, a brown amorphous form, and a dark crystalline form. The crystalline form has a structure like diamond and the physical properties given above. Silicon forms compounds with metals (silicides) and with nonmetals. With carbon it forms silicon carbide ; with oxygen a dioxide, silica ; with oxygen and metals, silicates . With hydrogen it forms several hydrides or silanes, the simplest being monosilane, SiH 4 , a colorless gas. It also forms compounds with the halogens, sulfur, and nitrogen and forms numerous organo-silicon compounds. Silicon is the second most abundant element of the earth's crust; it makes up about 28% of the crust by weight. Oxygen, most abundant, makes up about 47%. Aluminum, third in abundance, makes up about 8%. Silicon is widely distributed, occurring in silica and silicates, but never uncombined. Silicon is obtained commercially by heating sand and coke in an electric furnace. It is used in the steel industry in an alloy known as ferrosilicon, and also to form other alloys, such as those with aluminum, copper, and manganese; in these alloys it contributes hardness and corrosion resistance. A purified silicon is used in the preparation of silicones . Silicon of very high purity is prepared by thermal decomposition of silanes; it is used in transistors and other semiconductor devices. Silica is widely used in the production of glass . Silicates in the form of clay are used in pottery, brick, tile, and other ceramics. Silicon is found in many plants and animals; it is a major component of the test (cell wall) of diatoms. Silicosis is a lung disease caused by inhaling silica dust. Discovery of the element is usually credited to J. J. Berzelius, who in 1824 prepared fairly pure amorphous silicon.