Many researchers do not regard multiple chemical sensitivity as a medically valid syndrome, believing that the depression that frequently accompanies it is an indication that the symptoms are psychological in origin. Others note that descriptions of the syndrome are largely anecdotal and not proven scientifically, or that the imprecisely defined syndrome is easily abused as a diagnosis, pointing to what they feel is an exaggerated number of worker's compensation cases involving MCS. Nevertheless, many sufferers do seem to improve when they eliminate contact with the chemicals known to trigger their condition; in extreme cases this may mean confinement to specially treated living quarters.

Poison gas was first used during World War I, when the Germans released (Apr., 1915) chlorine gas against the Allies. The Germans also introduced mustard gas later in the war. Afterward, the major powers continued to stockpile gases for possible future use and several actually used it: the British in Afghanistan, the French and Spanish in Africa, the Italians in Ethiopia, and the Japanese in China. Lethal gases were not employed in combat during World War II, but the Germans did use gases for mass murder during the Holocaust. The Germans also invented and stockpiled the first nerve gas. It is odorless and colorless and attacks the body muscles, including the involuntary muscles. It is the most lethal and insidious weapon of chemical warfare. Since World War II, chemical weapons are known to have been used by Egypt in Yemen (during the 1962-67 civil war) and by Iraq against Iran during the Iran-Traq War and against Kurdish rebels.

Besides lethal gases, which attack the skin, blood, nervous, or respiratory system and require hospitalization of the victim, there are also nonlethal incapacitating agents, which, like tear gas, cause temporary physical disability. Such agents have often been employed in riot control, espionage, and warfare. Various forms of herbicides and defoliants are also used to destroy crops or vegetation, as Agent Orange was used by the United States during the Vietnam War.

The potential effectiveness of chemical warfare is increasing with improved methods of dissemination, such as artillery shells, grenades, missiles, and aircraft and submarine spray guns. Some protection against chemical weapons is possible using suits, sealed vehicles, and shelters. Such countermeasures usually protect against nuclear fallout and biological warfare as well. Lethal chemical weapons are held by many nations and they continue to be used. The danger of the proliferation of chemical and biological weapons remains despite arms control because they are relatively easy to manufacture and deploy.

Efforts to control chemical and biological weapons began in the late 19th cent. The Geneva Protocol of 1925, which went into force in 1928, condemned the use of chemical weapons but did not ban the development and stockpiling of chemical weapons. The United States did not ratify the protocol until 1974. In 1990, with the end of the cold war, the United States and the Soviet Union agreed to cut their arsenals by 80% in an effort to create a climate of change that would discourage smaller nations from stockpiling and using such lethal weapons. In 1993 a international treaty banning the production, stockpiling (both by 2007), and use of chemical weapons and calling for the establishment of an independent organization to verify compliance was adopted. The agreement, which became effective in 1997, has been signed and ratified by 160 nations. The treaty is enforced by the Organization for the Prohibition of Chemical Weapons, which is based in The Hague. The alleged Iraqi retention, after the Persian Gulf War cease-fire, of chemical weapons and other weapons of mass destruction was the main pretext for the 2003 U.S.-British invasion of Iraq.

History

Chemical industries can be traced back to Middle Eastern artisans, who refined alkali and limestone for the production of glass as early as 7,000 B.C., to the Phoenicians who produced soap in the 6th cent. B.C., and to the Chinese who developed black powder, a primitive explosive around the 10th cent. A.D. In the Middle Ages, alchemists produced small amounts of chemicals and by 1635 the Pilgrims in Massachusetts were producing saltpeter for gunpowder and chemicals for tanning. But, large-scale chemical industries first developed in 19th cent. In 1823, British entrepreneur James Muspratt started mass producing soda ash (needed for soap and glass) using a process developed by Nicolas Leblanc in 1790. Advances in organic chemistry in the last half of the 19th cent. allowed companies to produce synthetic dyes from coal tar for the textile industry as early as the 1850s.

In the 1890s, German companies began mass producing sulfuric acid and, at about the same time, chemical companies began using the electrolytic method, which required large amounts of electricity and salt, to create caustic soda and chlorine. Man-made fibers changed the textile industry when rayon (made from wood fibers) was introduced in 1914; the introduction of synthetic fertilizers by the American Cyanamid Company in 1909 led to a green revolution in agriculture that dramatically improved crop yields. Advances in the manufacture of plastics led to the invention of celluloid in 1869 and the creation of such products as nylon by Du Pont in 1928. Research in organic chemistry in the 1910s allowed companies in the 1920s and 30s to begin producing chemicals for oil. Today, petrochemicals made from oil are the industry's largest sector. Synthetic rubber came into existence during World War II, when the war cut off supplies of rubber from Asia.

Since the 1950s growing concern about toxic waste produced by chemical industries has led to increased government regulation and the establishment of the Environmental Protection Agency (1972). The leakage of toxic chemicals at the Union Carbide plant in Bhopal, India (1984), was the worst industrial disaster in history and heightened public concern about lax environmental regulations for chemical companies in developing countries. Beginning in the 1980s, U.S. corporations faced expanding competition from foreign producers, including some Third World oil producers who have set up their own oil refining and petrochemical industries. In 1997 the U.S. chemical industry produced about $389 billion worth of products and employed 1,032,000 workers. It exported about $71 billion worth of chemicals.

Basic Notation Used in Equations

The chemical equation 2H₂+O₂→2H₂O represents the reaction of hydrogen and oxygen to form water. The arrow points in the direction of the reaction—from the reactants (substances that react) toward the product or products. In this case the reactants are hydrogen (written H₂ because each molecule consists of two atoms of hydrogen) and oxygen (written O₂ because each molecule consists of two atoms of oxygen) and the product is water. The coefficient 2 before the H₂ indicates that two molecules of hydrogen take part in the reaction, and the 2 before the H₂O indicates that two molecules of water are produced. When no number is written, as in front of the O₂, a one is assumed; one molecule of oxygen takes part in the reaction. The equation shows that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water. Because of the relationship between molecules and the mole, the equation also shows that two moles of hydrogen react with one mole of oxygen to form two moles of water. The same sort of relationship holds with the gram-formula weight.

Methodology for Writing an Equation

There are three steps involved in writing a chemical equation. The first step is to decide which substances are the reactants and which are the products. For example, natural gas (cooking gas) burns in air, providing heat and producing no visible products. The natural gas is principally methane, and the portion of the air that reacts (supports combustion) is oxygen. These are the reactants. Products of the reaction are heat and two invisible gases, carbon dioxide and water vapor. We can now write the word equation methane + oxygen → carbon dioxide + water vapor + heat. The next step is to determine the correct formula for each substance and substitute it for the name. The equation now becomes CH₄+O₂→CO₂+H₂O. (A notation for heat is often omitted.)

The final step is to balance this equation. As the equation is now written, three oxygen atoms are produced from two, and four hydrogen atoms become only two. This cannot occur, since atoms are not created or destroyed in chemical reactions. The equation is already balanced for carbon, since there is one carbon atom on the reactant side and one carbon atom on the product side. There are four hydrogen atoms in the methane molecule on the reactant side, so there must be four hydrogen atoms in water molecules on the product side (since water is the only product containing hydrogen); thus there must be two water molecules, each containing two hydrogen atoms. The equation can now be written CH₄+O₂→CO₂+2H₂O. It is not yet balanced, since there are only two oxygen atoms shown as reactants and four as products. The equation is completely balanced by showing two oxygen molecules (four atoms) as reactants: CH₄+2O₂→CO₂+2H₂O.

Additional Symbols Used in Chemical Equations

There are a number of other symbols used in chemical equations. A symbol written above or below the reaction arrow indicates special reaction conditions. For example, when mercuric oxide is heated it decomposes into mercury metal and oxygen gas; this reaction is shown by the equation 2HgO Δ⃗ 2Hg + O₂↑. The Greek letter delta under the arrow represents the heating. The upward-pointing arrow after the O₂ indicates that this product is gaseous and escapes. When a precipitate is formed by a reaction, the substance that precipitates is often followed by a downward-pointing arrow, e.g., AgNO₃ + NaCl H₂O͢; AgCl↓ + NaNO₃. The H₂O above the arrow shows that the reaction takes place in the presence of water—in this case, in water solution. The formulas AgNO₃, NaCl, and NaNO₃ do not represent molecules, since these substances are almost completely ionized in water solution (see ion).

When chemical equilibrium occurs in a reaction, the double arrow is used instead of the single arrow. For example, liquid water dissociates to form hydronium ions (H₃O⁺) and hydroxide ions (OH^-). These ions exist in equilibrium with water molecules. The equation is 2H₂O &rlhar2H2O; H₃O⁺ + OH^-. The sign = is sometimes used in place of the double arrow.

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 (CH₄), 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.

Condition in the course of a reversible chemical reaction in which no net change in the amounts of reactants and products occurs: Products are reverting to reactants at the same rate as reactants are forming products. For practical purposes, the reaction under those conditions is completed. Expressed in terms of the law of mass action, the reaction rate to form products is equal to the reaction rate to re-form reactants. The ratio of the reaction rate constants (i.e., of the amounts of reactants and products, each raised to the proper power), defines the equilibrium constant. Changing the conditions of temperature or pressure changes the reaction's equilibrium; a high temperature or pressure may be used to “push” a reaction that at ordinary conditions makes little product. See also H.-L. Le Châtelier.

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One of the 117 presently known kinds of substances that constitute all matter at and above the level of atoms (the smallest units of any element). All atoms of an element are identical in nuclear charge (number of protons) and number of electrons (see atomic number), but their mass (atomic weight) may differ if they have different numbers of neutrons (see isotope). Each permanently named element has a one- or two-letter chemical symbol. Elements combine to form a wide variety of compounds. All elements with atomic numbers greater than 83 (bismuth), and some isotopes of lighter elements, are unstable and radioactive (see radioactivity). The transuranium elements, with atomic numbers greater than 92 (see uranium), artificially created by bombardment of other elements with neutrons or other particles, were discovered beginning in 1940. The most common elements (by weight) in Earth's crust are oxygen, 49percnt; silicon, 26percnt; aluminum, 8percnt; and iron, 5percnt. Of the known elements, 11 (hydrogen, nitrogen, oxygen, fluorine, chlorine, and the six noble gases) are gases under ordinary conditions, two (bromine and mercury) are liquids (two more, cesium and gallium, melt at about or just above room temperature), and the rest are solids. Seealso periodic table.

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Use of lethal or incapacitating chemical weapons in war, and the methods of combating such agents. Chemical weapons include choking agents such as the chlorine and phosgene gas employed first by the Germans and later by the Allies in World War I; blood agents such as hydrogen cyanide or cyanogen gas, which block red blood cells from taking up oxygen; blister agents such as sulfur gas and Lewisite, also dispensed as a gas, which burn and blister the skin; and nerve agents such as Tabun, Sarin, Soman, and VX, which block the transmission of nerve impulses to the muscles, heart, and diaphragm. The horrific casualties suffered in World War I led to the 1925 Geneva Protocol, which made it illegal to employ chemical weapons but did not ban their production. Chemical weapons were used a number of times afterward, most notably by Italy in Ethiopia (1935–36), by Japan in China (1938–42), by Egypt in Yemen (1966–67), and by Iran and Iraq against each other (1984–88). During the Cold War the Soviet Union and U.S. built up enormous chemical arsenals; these were dismantled under the terms of the 1993 Chemical Weapons Convention, which prohibits all development, production, acquisition, stockpiling, or transfer of such weapons. Not all countries have signed the convention, and many are suspected of pursuing clandestine chemical programs. Many military forces have adopted various defensive measures, including chemical sensors, protective garments and gas masks, decontaminants, and injectable antidotes, and some have reserved the option of retaliating in kind to any chemical attack. In 1995 a religious cult killed 12 civilians and injured thousands more with Sarin gas in Tokyo; this pointed out the power of chemical agents as weapons of terror as well as the difficulty of protecting civilian populations. Seealso biological warfare.

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Notation of one or two letters derived from the scientific names of the chemical elements (e.g., S for sulfur, Cl for chlorine, Zn for zinc). Some hark back to Latin names: Au (aurum) for gold, Pb (plumbum) for lead. Others are named for people or places (e.g. einsteinium, Es, for Einstein). The present symbols express the system set out by the atomic theory of matter. John Dalton first used symbols to designate single atoms of elements, not indefinite amounts, and Jons Jacob Berzelius gave many of the current names. Chemical formulas of compounds are written as combinations of the elements' symbols, with numbers indicating their atomic proportions, using various conventions for ordering and grouping. Thus, sodium chloride is written as NaCl and sulfuric acid as H₂SO₄.

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Any chemical process in which substances are changed into different ones, with different properties, as distinct from changing position or form (phase). Chemical reactions involve the rupture or rearrangement of the bonds holding atoms together (see bonding), never atomic nuclei. The total mass and number of atoms of all reactants equals those of all products, and energy is almost always consumed or liberated (see heat of reaction). The speed of reactions varies (see reaction rate). Understanding their mechanisms lets chemists alter reaction conditions to optimize the rate or the amount of a given product; the reversibility of the reaction and the presence of competing reactions and intermediate products complicate these studies. Reactions can be syntheses, decompositions, or rearrangements, or they can be additions, eliminations, or substitutions. Examples include oxidation-reduction, polymerization, ionization (see ion), combustion (burning), hydrolysis, and acid-base reactions.

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Subdivision of hydrology that deals with the chemical characteristics of the water on and beneath the surface of the Earth. Water in all forms is affected chemically by the materials with which it comes into contact, and it can dissolve many elements in significant quantities. Chemical hydrology is concerned with the processes involved and thus includes study of phenomena such as the transport of salts from land to sea (by erosion of rocks and surface runoff) and from sea to land (by evaporation, cloud formation, and precipitation) and the age and origin of groundwater in desert regions and of ice sheets and glaciers.

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Expression of the composition or structure of a chemical compound. Formulas for molecules use chemical symbols with subscript numbers to show the number of atoms of each element: O₂ for molecular oxygen, O₃ for ozone, CH₄ for methane, C₆H₆ for benzene. Parentheses may enclose atoms that act as a group. General formulas show the proportions of atoms in members of a class (e.g., C_math.nH_{2math.n+ 2} for alkanes). If the substance does not exist as molecules (see ionic bond), empirical formulas show the relative proportions of the constituents (e.g., NaCl for sodium chloride). Structural formulas show bonds (see bonding) between atoms in a molecule as short lines between symbols; they are particularly useful for showing how isomers differ. A projection formula also indicates the three-dimensional arrangement of the atoms (see Fischer projection; stereochemistry).

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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 (see 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.

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Physical and/or psychological dependency on a psychoactive (mind-altering) substance (e.g., alcohol, narcotics, nicotine), defined as continued use despite knowing that the substance causes harm. Physical dependency results when the body builds up a tolerance to a drug, needing increasing doses to achieve the desired effects and to prevent withdrawal symptoms. Psychological dependency may have more to do with one's psychological makeup; some people may have a genetic tendency to addiction. The most common addictions are to alcohol (see alcoholism), barbiturates, tranquilizers, and amphetamines, as well as to the stimulants nicotine and caffeine. Initial treatment (detoxification) should be conducted with medical supervision. Individual and group psychotherapy are critical elements. Alcoholics Anonymous and similar support groups can increase the success rate of other efforts. The ability to admit addiction and the will to change are necessary first steps.

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Any of the interactions that account for the association of atoms into molecules, ions, crystals, metals, and other stable species. When atoms' nuclei and electrons interact, they tend to distribute themselves so that the total energy is lowest; if the energy of a group arrangement is lower than the sum of the components' energies, they bond. The physics and mathematics of bonding were developed as part of quantum mechanics. The number of bonds an atom can form—its valence—equals the number of electrons it contributes or receives. Covalent bonds form molecules; atoms bond to specific other atoms by sharing an electron pair between them. If the sharing is even, the molecule is not polar; if it is uneven, the molecule is an electric dipole. Ionic bonds are the extreme of uneven sharing; certain atoms give up electrons, becoming cations. Other atoms take up the electrons and become anions. All the ions are held together in a crystal by electrostatic forces. In crystalline metals, a diffuse electron sharing bonds the atoms (metallic bonding). Other types include hydrogen bonding; bonds in aromatic compounds; coordinate covalent bonds; multicentre bonds, exemplified by boranes (boron hydrides), in which more than two atoms share electron pairs; and the bonds in coordination complexes (see transition element), still poorly understood. Seealso van der Waals forces.

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