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The United Nations has proclaimed 2019 the International Year of the Periodic Table of Chemical Elements (IYPT2019). The same year also marks the 150th anniversary of the publication of Russian chemist Dmitri Mendeleev’s periodic table, containing…

Either the taking up of energy from radiation by the medium through which radiation is passing, or the taking up of matter in bulk by other matter. A simple example of the absorption of energy is how sunlight warms a tree's leaves (see…

A powdered, granular, or pelleted form of amorphous carbon characterized by very large surface area per unit volume because of an enormous number of fine pores. Activated carbon is capable of collecting gases, liquids, or dissolved substances on the…

A process in which atoms or molecules move from a bulk phase (that is, solid, liquid, or gas) onto a solid or liquid surface. An example is purification by adsorption where impurities are filtered from liquids or gases by their adsorption onto the…

One of a class of organic chemical compounds represented by the general formula RCHO. Aldehydes are closely related to ketones, since both types of compounds contain the carbonyl group In the case of ketones, the carbonyl group is attached to…

A chemical element, Al, with an atomic number of 13, and an atomic weight of 26.982. Pure aluminum is soft and lacks strength, but it can be alloyed with other elements to increase strength and impart a number of useful properties. Alloys of…

A substance that, when present at a lower concentration than that of the oxidizable substrate, significantly inhibits or delays oxidative processes, while being itself oxidized. For antioxidants, such as ascorbic acid (Fig. 1), antioxidative…

The application of chemistry to the technical examination, authentication, and preservation of cultural property. Chemists working in museums engage in a broad range of investigations, most frequently studying the chemical composition and structure…

The class of animals consisting of the birds. Birds (Fig. 1) are vertebrate animals comprising the class Aves. (In some classification schemes, the term Neornithes may also be used when referring to modern birds.) Primarily adapted for flying, the…

An instrument for weighing. The word “balance,” derived from the Latin bilanx (“having two pans”), is often used interchangeably with “scale” or “scales” (Old English, meaning dishes or plates). However, balance is the preferred term for an…

An electrochemical device that stores chemical energy which can be converted into electrical energy, thereby providing a direct-current voltage source. Although the term “battery” is properly applied to a group of two or more electrochemical cells…

A method for quantitatively determining the concentration, biological activity, or potency of a substance by its effect on the growth of a suitable animal, plant, or microorganism under controlled conditions. A bioassay (biological assay) is a…

A discipline concerned with the application of ethics to biological problems, especially in the field of medicine. Bioethics is a broad term that encompasses the environmental, ethical, legal, and social implications of agriculture, life sciences,…

A biochemical reaction involving the transfer of a negatively charged electron from one organic compound to another organic compound or to oxygen. Biological oxidation is an energy-producing reaction in living cells, and it is coupled with a…

A law of gases which states that at constant temperature the volume of a gas varies inversely with its pressure. This law, formulated by Robert Boyle (1627–1691), can also be stated thus: The product of the volume of a gas times the pressure exerted…

A crystalline methylxanthine alkaloid that acts as a stimulant to the central nervous system. Chemically, caffeine is 1,3,7-trimethylxanthine, and has the structural formula shown below. Caffeine is found in nuts, seeds, or leaves in over 60 plant…

A class of the phylum Porifera, including exclusively marine sponges with a skeleton composed of spicules of calcium carbonate to which can be added a solid basal calcitic skeleton. The sponges that constitute the phylum Porifera are assigned to…

A chemical element, Ca, of atomic number 20, atomic weight 40.078. Calcium—the fifth most abundant element in the Earth's crust and the third among metals—is a member of the alkaline-earth metals of group 2 of the periodic table. It is the most…

A colorless or white crystalline solid, with the formula C10H16O. Camphor has an aromatic odor and sublimes at room temperature. As a result, it has found use in the flavor and fragrance industry. Camphor, a bicyclic monoterpene ketone, exists in…

A chemical element, C, with an atomic number of 6 and an atomic weight of 12.01115. Carbon is unique in chemistry because it forms a vast number of compounds, larger than the sum total of all other elements combined. By far the largest group of…

A colorless, odorless gas at atmospheric temperature and pressure, CO2, with a molecular weight of 44.009 g/mol. Under normal conditions, it is stable, inert, and nontoxic (see illustration). Ball-and-stick model of carbon dioxide. Red = oxygen.…

Molecular-scale tubes of graphitic carbon with outstanding properties. They are among the stiffest and strongest fibers known, and have remarkable electronic properties. In addition, carbon nanotubes are lightweight, elastic, thermally conductive,…

The phenomenon in which a relatively small amount of foreign material, called a catalyst, speeds up the rate of a chemical reaction without itself being consumed. A catalyst is a material, and not light or heat. It increases a reaction rate. For…

A mechanical method of separating immiscible liquids or solids from liquids by application of centrifugal force. This force can be very great, and separations which proceed slowly by gravity can be speeded up enormously in centrifugal equipment.…

An aquatic mammalian order comprising approximately 90 living species of whales, dolphins, and porpoises and their fossil relatives. Like all other mammals, members of the order Cetacea (cetaceans) nurse their young with milk produced by the mother,…

A chemical reaction in which many molecules undergo chemical reaction after one molecule becomes activated. In ordinary chemical reactions, every molecule that reacts must first become activated by collision with other rapidly moving molecules. The…

A thermodynamic law, also known as Gay-Lussac's law, which states that at constant pressure the volume of a fixed mass or quantity of gas varies directly with the absolute temperature. As such, Charles' law, V = kT, describes how gases expand when…

The force that holds atoms together in molecules and solids. Chemical bonds are very strong. To break one bond in each molecule in a mole of material typically requires an energy of many tens of kilocalories.It is convenient to classify chemical…

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 method used in chemistry to purify substances or to isolate them from other substances, for either preparative or analytical purposes. In industrial applications the ultimate goal is the isolation of a product of given purity, whereas in analysis…

The science that embraces the properties, composition, and structure of matter, the changes in structure and composition that matter undergoes, and the accompanying energy changes. It is important to distinguish chemical change, implicit in this…

A chemical element, Cl, atomic number 17 and atomic weight 35.453. Chlorine exists as a greenish-yellow gas at ordinary temperatures and pressures (see illustration). It is second in reactivity only to fluorine among the halogen elements, and hence…

A physical method of separation in which the components of a mixture of substances are separated from each other by differences in their distribution between two phases, one stationary and the other mobile. The mobile phase moves through the…

A hydroxytricarboxylic acid, general formula C6H8O7. Citric acid (see illustration) and its salts are widely used because they are nontoxic, safe to handle, and easily biodegraded. They are universally accepted, without restriction on level, as food…

It has been a long-held belief that, as a result of hunting habits, early humans were partly or even primarily responsible for the decline and eventual extinction of most of the largest mammalian species that inhabited ancient Africa. These large…

The principal alkaloid of coca leaves, a topical anesthetic and stimulant, and popular recreational drug. The molecular formula is C17H21NO4 and the structure is shown below. Cocaine was first isolated by A. Niemann in 1860. In 1884 C. Koller…

A class of the phylum Porifera, including sponges with a skeleton of one- to four-rayed siliceous spicules or of spongin fibers, or both. Four taxonomic classes comprise the phylum Porifera—Calcarea, Demospongiae, Hexactinellida, and…