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An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

Any of several leguminous plants, or their seeds, long utilized as food by humans or livestock. There are more than 40,000 varieties of beans, with only a small number of these being mass-produced for human or animal consumption. In particular,…

Any member of the grass family (Poaceae or Gramineae) that produces edible grains usable as food by humans and livestock. Cereals are important cultivated crops. Common cereals include rice, wheat, barley, oats, maize (corn), sorghum, rye, and…

The response of corals to environmental stress in which they eject the single-cell algae that live within the transparent coral tissue, making the white coral skeleton visible. Corals (Fig. 1) are animals that live in a partnership (symbiosis) with…

A species translocated from one region or continent to another outside its normal range, particularly when the translocation is the result of human activity (intentional or otherwise). Many thousands of species, including animals, plants, fungi,…

A theory expressing the idea that life on Earth passed through an era when biological processes were carried out predominantly by ribonucleic acids, without the involvement of deoxyribonucleic acids or proteins. The RNA World theory (or hypothesis)…

A chemical compound, (CH3)2CO; the first member of the homologous series of aliphatic ketones. Acetone (see illustration) is a colorless, volatile, and flammable liquid. Acetone occurs naturally in plants and animals. Its physical properties include…

In the Brönsted classification, an acid is any chemical species, ionic or molecular, capable of donating a proton (hydrogen ion) to another substance; the other substance acts as a base in accepting the proton. For these two interrelated classes of…

One of a large group of organic substances possessing the halocarbonyl group in which X stands for fluorine, chlorine, bromine, or iodine. The terms acyl and aroyl halides refer to aliphatic or aromatic derivatives, respectively.The great inherent…

A discipline that combines the means to know the location and heading of an aircraft with respect to some map or coordinate system, with guidance which means steering the aircraft along some desired path toward a destination. The construction of the…

Any compound having highly basic properties, strong acrid taste, and ability to neutralize acids. Aqueous solutions of alkalies are high in hydroxyl ions, have a pH above 7, and turn litmus paper from red to blue. Caustic alkalies include sodium…

The elements of group 1 in the periodic table. Of the alkali metals, lithium differs most from the rest of the group, and tends to resemble the alkaline-earth metals (group 2 of the periodic table) in many ways. In this respect lithium behaves as do…

Usually calcium, strontium, and barium, the heaviest members of group 2 of the periodic table (excepting radium). Other members of the group are beryllium, magnesium, and radium, sometimes included among the alkaline-earth metals. Beryllium…

A natural process in which harmful or beneficial effects are caused by secondary metabolites that spread from a donor organism to a recipient and are produced by plants, algae, bacteria, and fungi. Allelopathy is a key ecological process and has…

A discipline within the field of animal behavior that focuses on the reception and use of signals. Animal communication could well include all of animal behavior because a liberal definition of the term signal could encompass all stimuli perceived…

Vegetables that are prevalent in Asian cuisines, but less well known in Western diets. Asian vegetables (Fig. 1), also referred to as Asian greens or oriental vegetables, are very important in Asian countries, but are not as popular in North America…

A white, crystalline compound, 1-aspartyl-1-phenylalanine methyl ester (APM), used as an artificial sweetener. Aspartame is composed of two amino acids: aspartic acid and phenylalanine [structure (1)]. Amino acid Artificial sweetener Aspartame…

A nonsteroidal anti-inflammatory drug used to control pain, fever, and inflammation. Aspirin is one of the most prevalent medicines in the world and is used to reduce fever and treat minor aches, pains, and inflammation. Furthermore, it is the only…

An analytical technique for determining the composition of a sample in terms of the chemical elements present and their quantities or concentrations. Unlike other methods of elemental analysis, however, the sample is decomposed into its constituent…

A fermented beverage produced from grain. Beer is a generic term used to describe alcoholic beverages made from cereal grains, especially barley (Fig. 1), in the form of malt. Ale, lager, porter, and stout are different kinds of beer made by…

The study of behavior change in response to drugs, chemicals, or other environmental conditions or agents. Behavioral toxicology investigates the effect of toxic exposure on organismal behavior. The toxic exposure can involve various chemicals,…

An organic acid, C6H5COOH; also known as benzene carboxylic acid. Benzoic acid is a crystalline solid, with the structure below. Benzoic acid has a melting point of 122°C, and sublimes at 100°C. Benzoic acid is only slightly soluble in water but is…

A chemical element, chemical symbol Be, atomic number 4, and atomic weight 9.0122. Beryllium is the first element in the group called alkaline-earth metals on the periodic table (Fig. 1). Alkaline-earth metals Atomic number Chemical element Periodic…

The destruction of organic compounds by microorganisms. Biodegradation is a process (carried out by microorganisms) that decomposes organic material into its constituent elements or new compounds. Microorganisms, particularly bacteria (and fungi to…

A microbial (often bacterial) community that is enveloped by an adhesive substance (the glycocalyx) at the interface of a liquid and a surface. Biofilms (see illustration) are defined as highly structured communities of microorganisms that are…

The study of the cycling of chemicals between organisms and the surface environment of the Earth. The chemicals either can be taken up by organisms and used for growth and synthesis of living matter or can be processed to obtain energy. The chemical…

The use of parasitoids, predators, and pathogens to reduce injurious pest insect populations and consequently the damage that they cause. Biological insect control (or insect biocontrol) is the regulation of insect populations by biological means.…

In glass blowing, a long straight tube on which molten glass is gathered and worked, partly by blowing into the tube. The blowpipe (see illustration) is spun to shape the glass object further by centrifugal force, or by a tool, in which case the…

A chemical element, Cf, atomic number 98, the ninth member of the actinide series of elements. Its discovery and production have been based upon artificial nuclear transmutation of radioactive isotopes of lighter elements. All isotopes of…

Mercury(I) chloride, Hg2Cl2, a covalent compound which is insoluble in water. The substance sublimes when heated. The formula weight is 472.086 and the specific gravity is 7.16 at 20°C (68°F). The material is a white, impalpable powder consisting of…

A functional group found in organic compounds in which a carbon atom is doubly bonded to an oxygen atom. Depending upon the nature of the other groups attached to carbon, the most common compounds containing the carbonyl group are aldehydes (X…

A cluster compound containing both carbon (C) and boron (B) atoms as well as hydrogen (H) atoms external to the framework of the cluster. A cluster compound is one with insufficient electrons to allow for classical two-center two-electron bonds…

One of a large family of organic substances widely distributed in nature, and characterized by the presence of one or more carboxyl groups (COOH). These groups typically yield protons in aqueous solution.Acid and baseIn the type formula,…

Catenanes are compounds that are made of interlocking macrocycles. The interlocking rings are said to be mechanically rather than chemically bound. Catenanes are named according to the number of interlocking rings. The simplest catenane, containing…

A chemical element, Ce, atomic number 58, atomic weight 140.12. It is the most abundant metallic element of the rare-earth group in the periodic table. The naturally occurring element is made up of the isotopes 136Ce, 138Ce, 140Ce, and 142Ce. A…

A chemical element, Cs, with an atomic number of 55 and an atomic weight of 132.905, the heaviest of the alkali metals in group 1 of the periodic table (except for francium, the radioactive member of the alkali metal family). Cesium is a soft,…

Charged particles can be accelerated to high velocities by electromagnetic fields. They are then able to travel through matter (termed an absorber), interacting with it, losing energy, and causing various effects important in many applications. In…

A branch of physical chemistry that seeks to explain time-dependent phenomena, such as energy transfer and chemical reaction, in terms of the detailed motion of the nuclei and electrons that constitute the system. As such, chemical dynamics seeks…

The study of ecological interactions mediated by chemicals that organisms produce. Chemical ecology seeks to integrate chemistry and biology in order to provide information about the chemical interactions among organisms and their environment (Fig.…

Atoms with the same atomic number; that is, the same number of protons in their atomic nucleus. Some common elements are oxygen, hydrogen, iron, copper, gold, silver, nitrogen, chlorine, and uranium (see illustration). Most of the elements are…

A useful but obsolescent term for the energy available from elements and compounds when they react, as in a combustion reaction. In precise terminology, there is no such thing as chemical energy, since all energy is stored in matter as either…

In a dynamic or kinetic sense, chemical equilibrium is a condition in which a chemical reaction is occurring at equal rates in its forward and reverse directions, so that the concentrations of the reactant and product substances do not change with…

A branch of physical chemistry that measures the rates of chemical reactions, describes them in terms of elementary steps, and understands them in terms of the fundamental interactions between molecules. Reaction kinetics Although the ultimate state…

A system of principles and rules used to name chemical compounds. Various nomenclature systems are used. Hence, a compound may have several acceptable names. Which name is preferred depends on the context. Each nomenclature type has several dialects…

A system of symbols and notation for the chemical elements and the combinations of these elements which form numerous chemical compounds. This system of symbols, devised since about 1800 by chemists, consists of letters, numerals, and marks that are…

The application of thermodynamic principles to systems involving physical and chemical transformations is carried out in order to (1) develop quantitative relationships among the identifiable forms of energy and their conjugate variables, (2)…

The type of luminescence wherein a chemical reaction supplies the energy responsible for the emission of light (ultraviolet, visible, or infrared) in excess of that of a blackbody (thermal radiation) at the same temperature and within the same…

A chemical discipline that uses mathematical and statistical methods to design or select optimal measurement procedures and experiments and to provide maximum chemical information by analyzing chemical data. Chemometrics is actually a collection of…

A chemical element, Cr, atomic number 24, and atomic weight 51.996, which is the weighted average for several isotopes weighing 50 (4.31%), 52 (83.76%), 53 (9.55%), and 54 (2.38%). The orbital arrangement of the electrons is 1s2, 2s2, 2p6, 3s2, 3p6,…

A simplified chemical reaction method recognized in 2001 by a group led by Nobel Laureate Prof. K. Barry Sharpless at the Scripps Research Institute. “Click” is meant to signify that joining molecular pieces is as easy as clicking together the two…

Products derived from the seeds of the tropical tree Theobroma cacao. The cacao tree (Fig. 1) grows within a narrow belt along both sides of the Equator. There are three basic varieties: Criollo, which was native to Central and South America;…

Keeping perishable products at low temperatures to extend shelf life. Cold storage vastly retards the natural deterioration of perishable products at higher temperatures. Time and temperature are the key factors that determine how well foods (see…

Periodic arrays of suspended colloidal particles. Common colloidal suspensions (colloids) such as milk, blood, or latex are polydisperse; that is, the suspended particles have a distribution of sizes and shapes. However, suspensions of particles of…

A method in which very large numbers of chemical entities are synthesized by condensing a small number of reagents togther in all combinations defined by a small set of reactions. The main objective of combinatorial chemistry is synthesis of arrays…

A method for preparing a large number of chemical compounds, commonly known as a combinatorial library, which are then screened to identify compounds having a desired function, such as a particular biological or catalytic activity. Combinatorial…

A substance composed of two or more elements that do not vary in composition from sample to sample, and that have fixed and definite physical properties, such as density and refractive index. The elements in compounds cannot be separated by simple…

A branch of theoretical chemistry that uses a digital computer to model systems of chemical interest. In this discipline, the essential physics that determines molecular behavior is captured in mathematical models and programmed for digital…

Numerical systems defining the quantitative relations of the components of mixtures. In solutions the concentration is expressed as the mass, volume, or number of moles of solute present in proportion to the amount of solvent or of total solution.…

The study of the energies and structures of con­formations of organic molecules and their chemi­cal and physical properties. Organic molecules are not static entities. The constituent atoms vibrate and groups rotate about the bond…

A higher-order bonding interaction between electron orbitals on three or more contiguous atoms in a molecule, which leads to characteristic changes in physical properties and chemical reactivity. One participant in this interaction can be the…

Management of the human use of natural resources to provide the maximum benefit to current generations while maintaining capacity to meet the needs of future generations. Conservation includes both the protection and rational use of natural…

A synthetic method which avoids the premature termination of growing radicals and produces polymers with precise compositions, architectures, and functionalities.Free-radical polymerization is used extensively in industry for the synthesis of a…

A field which, in its broadest usage, is acid-base chemistry as defined by G. N. Lewis. However, the term coordination chemistry is generally used to describe the chemistry of metals and metal ions in their interactions with other molecules or ions.…

A group of chemical compounds in which all or a part of the molecular bonding is of the coordinate covalent type.This article summarizes the different types of compounds that are known and discusses their nomenclature, structure, stereochemistry,…

Element 112 was discovered in 1996. It should be a heavy homolog of the elements mercury, cadmium, and zinc, and is expected to be the last element in the 6d shell. Cadmium Half-life Mercury (element) ZincA long-standing claim to having synthesized…

A macromolecule in which two or more types of monomer are incorporated into the polymer chain. This contrasts with a homopolymer, which consists of only one type of monomer. Copolymerization offers the ability to alter the properties of a…

Pathological conditions affecting the health of coral and causing its physiological dysfunction. Broadly defined, coral disease (Fig. 1) is any impairment to coral health resulting in physiological disturbance and breakdown. It usually involves…

A measure of the three-dimensional structure of live coral communities on reefs. Coral reef structural complexity is an intrinsic element of coral reef ecosystems and quantitates the surface area of coral in relation to its linear area. The…

The characteristic wavelength dependence of the optical rotatory dispersion curve or the circular dichroism curve or both in the vicinity of an absorption band.When an initially plane-polarized light wave traverses an optically active medium, two…

A large, tawny brown wild cat, Puma concolor (family Felidae), once widespread over most of the Americas; also known as catamount, mountain lion, painter, and puma. The cougar (Puma concolor; see illustration) is a carnivorous member of the family…

Electrolysis cell in which a product is obtained with 100% efficiency as a result of an electrochemical reaction. The quantity of electricity, that is, the number of coulombs of electricity (Q), can be determined very accurately by weighing the…

The unusual physical properties displayed by substances near their critical points. The study of critical phenomena of different substances is directed toward a common theory. Critical points Ideally, if a certain amount of water (H2O) is sealed…

The formation of a solid from a solution, melt, vapor, or a different solid phase. Crystallization from solution is an important industrial operation because of the large number of materials marketed as crystalline particles. Fractional…

A chemical element, Cm, in the actinide series, with an atomic number of 96. Curium does not exist in the terrestrial environment, but may be produced artificially. The chemical properties of curium are so similar to those of the typical rare earths…

A term used to refer to the free acid, H2NCN, or commonly to the calcium salt of this acid, CaCN2, which is properly called calcium cyanamide. Calcium cyanamide is manufactured by the cyanamide process, in which nitrogen gas is passed through finely…

A compound containing the OCN group, typically a salt or ester of cyanic acid (HOCN). The cyanate ion is ambidentate, that is, it has two reactive sites, because it can bind through the oxygen (O) or the nitrogen (N). Cyanate is commonly N-bonded…

A compound containing the —CN group, for example, potassium cyanide, KCN; calcium cyanide, Ca(CN)2; and hydrocyanic (or prussic) acid, HCN. Chemically, the simple inorganic cyanides resemble chlorides in many ways. Organic compounds containing this…

A derivative of hydrocarbon in which all of the hydrogen atoms are replaced by the C N group. Only two cyanocarbons, dicyanoacetylene and dicyanobutadiyne, were known before 1958. Since then, tetracyanomethane, hexacyanoethane, tetracyanoethylene,…

A colorless, highly toxic gas having the molecular formula C2N2. Cyanogen belongs to a class of compounds known as pseudohalogens, because of the similarity of their chemical behavior to that of the halogens. Liquid cyanogen boils at −21.17°C…

Any of a group of cyclic oligomers of glucose in the normal C-1 conformation in which the individual glucose units are connected by 1,4 bonds. They are called Schardinger dextrins (after the discoverer), cyclodextrins (to emphasize their cyclic…

A molecule composed of two building blocks: an aromatic ring (most frequently a benzene ring) and an aliphatic unit forming a bridge between two (or more) positions of the aromatic ring. Structure ( 1 ), an [n](1,3)cyclophane, provides a general…

Equipment used in the production and processing of milk and milk products, including milking machines, cream separators, coolers, pasteurizers, homogenizers, butter-making equipment, evaporators and dryers, and related items of equipment. Dairy…

The total pressure of a mixture of gases is the sum of the partial pressures of each gas in the mixture. The law was established by John Dalton (1766–1844). In his original formulation, the partial pressure of a gas is the pressure of the gas if it…

The eighteenth of the synthetic transuranium elements, symbol Ds, atomic number 110. Darm-stadtium should be a heavy homolog of the elements platinum, palladium, and nickel. It is the eighth element in the 6d shell. The various isotopes of…

A form of navigation that determines position of a craft by advancing a previous position to a new one on the basis of assumed distance and direction moved. The name probably stems from the early practice of determining speed by throwing overboard a…

The electrode potential at which the electrolysis current begins to increase appreciably. Decomposition potentials are used as an approximate characteristic of industrial electrode processes.Electrochemical process ElectrolysisDecomposition…

The law that a given chemical compound always contains the same elements in the same fixed proportions by weight. Thus, whatever its source, silver chloride always contains 108 g (3.81 oz) of silver to every 35.45 g (1.251 oz) of chlorine. If a…

The absorption of atmospheric water vapor by a crystalline solid until the crystal eventually dissolves into a saturated solution. This behavior is well known for certain salts such as hydrated calcium chloride, CaCl2 · 6H2O, and zinc chloride,…

A phenomenon in which the most loosely held bonding electrons of some molecules serve to bind not two but several atoms. This contrasts with localization, a characteristic of ordinary single bonds, as in the normal paraffins, for example, methane…

A large molecule having a well-defined three-dimensional structure. Dendritic macromolecules play a crucial role in the chemistry of living systems. In contrast to the high level of structural precision that characterizes many biologically active…

A substance used to withdraw moisture from other materials. Although the removal of large quantities of water is done by evaporation, aided by moving air currents and by elevated temperature, the last traces of moisture are often held very tightly…

A process in which atomic and molecular species residing on the surface of a solid leave the surface and enter the surrounding gas or vacuum. In stimulated desorption studies, species residing on a surface are made to desorb by incident electrons or…

The isotope of the element hydrogen with atomic weight 2.0144 and symbols 2H or D. Considerations of nuclear stability and a discrepancy between the chemical and physical atomic weights of hydrogen led to the prediction of a stable isotope of…

A process of selective diffusion through a membrane by dissolved solutes in liquid solution. As dialysis is usually carried out, the membrane permits the diffusion of low-molecular-weight solutes (crystalloids) but prevents the passage of colloidal…

Very thin coatings that are applied to other materials to provide them with special physical or chemical properties unique to diamond. While crystalline diamond is extremely expensive, many of the useful properties of diamond can be obtained in a…

The process by which an aromatic primary amine is converted to a diazonium compound. The preparation and reactions of diazonium salts were discovered in 1858 and were the basis of the synthetic dye industry and the development of other industrial…

The 1,4-addition of an alkene (the dienophile) to a conjugated diene. The reaction, also known as the diene synthesis, is one of the most valuable and versatile methods for the preparation of compounds containing a six-membered ring, and proceeds…

A versatile solvent, abbreviated DMSO, used industrially and in chemical laboratories as a medium for carrying out chemical reactions. Its uses have been extended to that of a chemical reagent with DMSO as a reactant undergoing a chemical change.…

Equipment used for determining the direction from which a received signal is being radiated. Direction finders serve a variety of applications. Direction finders can be used for navigation purposes to determine the heading of a vehicle relative to a…

An internationally standardized navigation system which allows an aircraft to measure its distance from a selected ground-based beacon. Such beacons are used throughout the world by all airliners, most of the military aircraft of the West, and a…