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Documents for "Chemistry: General":
  • 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.
  • activation energy in chemistry, minimum energy needed to cause a chemical reaction. A chemical reaction between two substances occurs only when an atom, ion, or molecule of one collides with an atom, ion, or molecule of the other. Only a fraction of the total collisions result in...
  • aerogel any of a group of extremely light and porous solid materials; the lightest is less than four times as dense as dry air. Aerogels are produced from certain gels (see colloid ) by heating the gel under pressure, which causes the liquid in the gel to become supercritical (in a state between a liquid and a gas) and lose its surface tension. In this state, the liquid may be...
  • allotropy [Gr.,=other form]. A chemical element is said to exhibit allotropy when it occurs in two or more forms in the same physical state; the forms are called allotropes. Allotropes generally differ in...
  • amphoterism in chemistry, the property of certain substances of acting either as acids or as bases depending on the reaction in which they are involved. Many hydroxide compounds are amphoteric. For example,...
  • anion atom or group of atoms carrying a negative charge. The charge results because there are more electrons than protons in the anion. Anions can be formed from nonmetals by reduction (see oxidation and...
  • atomic mass the mass of a single atom , usually expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus. Each proton or neutron weighs about 1 amu, and thus the atomic mass is always very close to...
  • atomic mass unit or amu, in chemistry and physics, unit defined as exactly 1/12 the mass of an atom of carbon-12, the isotope of carbon with six protons and six neutrons in its nucleus. One amu is equal to...
  • atomic number often represented by the symbol Z, the number of protons in the nucleus of an atom , as well as the number of electrons in the neutral atom. Atoms with the same atomic number make up a chemical element. Atomic numbers were first assigned to the elements c.1913 by H. G. J. Moseley; he arranged the elements in an order based on certain characteristics of their X-ray spectra and then numbered them...
  • atomic weight mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element , as contrasted with atomic mass , which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent., it was not until the discovery of isotopes by F. Soddy (c.1913)...
  • Avogadro's number ävōgä´drō [for Amedeo Avogadro ], number of particles contained in one mole of any substance; it is equal to 602,252,000,000,000,000,000,000, or in scientific notation, 6.02252×10 23. For example, 12.011 grams of carbon (one mole of carbon) contains 6.02252×10 23 carbon atoms, and 180.16 grams of glucose, C 6 H 12 O 6 , contains 6.02252×10 23 molecules of glucose. Avogadro's number is determined by calculating the spacing of the atoms in a crystalline solid through X-ray methods and combining this data with the measured volume of one...
  • bead test test used in the identification of certain metals. Some metallic ions that cannot be identified by a flame test are identified by a bead test. The test can also be used to confirm the results of a flame test. The borax bead test is the most common. A small loop is formed at the end of a platinum wire. The...
  • Beer's law [for August Beer], physical law stating that the quantity of light absorbed by a substance dissolved in a nonabsorbing solvent is directly proportional to the concentration of the substance and the...
  • bleaching process of whitening by chemicals or by exposure to sun and air, commonly applied to textiles, paper pulp, wheat flour, petroleum products, oils and fats, straw, hair, feathers, and wood. Chemical...
  • buffer solution that can keep its relative acidity or alkalinity constant, i.e., keep its p H constant, despite the addition of strong acids or strong bases. Buffer solutions are frequently solutions that contain either a weak acid and one of its salts or a weak base and one of its salts...
  • catalyst substance that can cause a change in the rate of a chemical reaction without itself being consumed in the reaction; the changing of the reaction rate by use of a catalyst is called catalysis. Substances that increase the rate of reaction are called positive...
  • cation atom or group of atoms carrying a positive charge. The charge results because there are more protons than electrons in the cation. Cations can be formed from a metal by oxidation (see oxidation and...
  • 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...
  • 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...
  • chemical equilibrium state of balance in which two opposing reversible chemical reactions proceed at constant equal rates with no net change in the system. For example, when hydrogen gas, H 2 , and iodine gas, I 2 , are mixed, and gaseous hydrogen iodide, HI, is formed according to the equation H 2  + I 2  → 2HI, no matter how long the reaction is allowed to proceed some quantity of hydrogen and iodine will remain unreacted. The reason reactants in a reversible reaction are never completely converted...
  • 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...
  • chemistry branch of science concerned with the properties, composition, and structure of substances and the changes they undergo when they combine or react under specified conditions.
  • chemurgy branch of applied chemistry concerned with preparing industrial products from agricultural raw materials. Among such products are plastics manufactured from casein and soybean; soaps derived from...
  • chromatography resolution of a chemical mixture into its component compounds by passing it through a system that retards each compound to a varying degree; a system capable of accomplishing this is called a...
  • coagulation the collecting into a mass of minute particles of a solid dispersed throughout a liquid (a sol), usually followed by the precipitation or separation of the solid mass from the liquid. The casein...
  • colligative properties properties of a solution that depend on the number of solute particles present but not on the chemical properties of the solute. Colligative properties of a solution include freezing point (see freezing ), boiling point , osmotic pressure (see osmosis ), and solvent vapor pressure. By measuring these properties and comparing them with the corresponding properties of the pure solvent, it is possible to determine the number of particles of solute present in the solution. If...
  • colloid [Gr.,=gluelike], a mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance. The mixture is also called a colloidal...
  • combining weight the proportion (by weight) in which a chemical element combines with other elements to form compounds. The determination of combining weights was a very important part of early chemical endeavor...
  • combustion rapid chemical reaction of two or more substances with a characteristic liberation of heat and light; it is commonly called burning. The burning of a fuel (e.g., wood, coal, oil, or natural gas) in...
  • common-ion effect decrease in solubility of an ionic salt , i.e., one that dissociates in solution into its ions , caused by the presence in solution of another solute that contains one of the same ions as the salt. The common-ion effect is an example of chemical equilibrium. For example, silver chloride, AgCl, is a slightly soluble salt that in solution dissociates into the ions Ag + and Cl - , the equilibrium state being represented by the equation AgCl solid [double arrow] Ag + +Cl -. According to Le Châtelier's principle , when a stress is placed on a system in equilibrium, the system responds by tending to reduce that stress. In the system taken as an example, if another solute containing one of those ions is...
  • complex ion charged molecular aggregate (see ion ), consisting of a metallic atom or ion to which is attached one or more electron-donating molecules. In some complex ions, such as sulfate, SO 4-2 , the atoms are so tightly bound together that they act as a single unit. Many complex ions, however, such as tetrammine zinc (II), Zn(NH 3 ) 4+2 , are only loosely aggregated and tend to dissociate in a water solution until an equilibrium is established between the complex ion and its components (see chemical equilibrium ). Such complex ions, or coordinated complexes as they are also called, generally consist of a positively charged central metal atom or ion, like the zinc in tetramine zinc, surrounded by...
  • compound in chemistry, a substance composed of atoms of two or more elements in chemical combination, occurring in a fixed, definite proportion and arranged in a fixed, definite structure. A compound is often...
  • concentration in chemistry, measure of the relative proportions of two or more quantities in a mixture. The concentration of a solute is very important in studying chemical reactions because it determines how often molecules collide in solution and thus indirectly determines the rates of reactions...
  • corrosion atmospheric oxidation of metals (see oxidation and reduction ). By far the most important form of corrosion is the rusting of iron. Rusting is essentially a process of oxidation in which iron combines with water and oxygen to form rust, the reddish-brown crust that forms on the surface of the iron. Rust, a chemical compound,...
  • deliquescence conversion of a solid substance into a liquid as a result of absorption of water vapor from the air. Since impurities in a solid lower its melting point, the absorbed water causes a decrease in...
  • dialysis in chemistry, transfer of solute (dissolved solids) across a semipermeable membrane. Strictly speaking, dialysis refers only to the transfer of the solute; transfer of the solvent is called osmosis. Dialysis is frequently used to separate different components of a solution. For example, a solution of starch and sodium chloride in water can be separated by placing the mixture in a vessel on...
  • diffusion in chemistry, the spontaneous migration of substances from regions where their concentration is high to regions where their concentration is low. Diffusion is important in many life processes. It...
  • dipole see pole , in electricity and magnetism.