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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…

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…

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 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 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 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…

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 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 method for separating liquid mixtures by vaporization and condensation. As a separation technique, distillation exploits the fact that when a liquid is partially vaporized the compositions of the two phases are different. By separating the phases…

A colored substance that imparts more or less permanent color to other materials. Dyes are generally water-soluble, although some are soluble only during application, after which they become insoluble. Colored water-insoluble substances are called…

The migration of electrically charged particles in solution or suspension in the presence of an applied electric field. Each particle moves toward the electrode of opposite electrical polarity. For a given set of solution conditions, the velocity…