Kids Research Express

. Aliphatic Compounds, large class of organic molecules consisting essentially of straight or branched chains of carbon atoms. Aliphatic compounds include the alkanes, alkenes, and alkynes and their derivatives. The other major class of organic molecules, aromatic compounds , forms closed ring structures.

. Aromatic Compounds, large group of organic chemical compounds that usually contain closed rings of carbon atoms. Some aromatic rings, however, may also contain an oxygen or a nitrogen atom. The term was originally restricted to the coal tar product benzene and its derivatives, but it now includes about half of all organic compounds, the remaining half being classified as aliphatic compounds . Important aromatic compounds include all hormones and vitamins except vitamin C; virtually all flavorings, perfumes, and organic dyes, synthetic and natural; those alkaloids that are not alicyclic (aliphatic bases such as putrescine are sometimes incorrectly classified as alkaloids); some substances of military importance, such as TNT and tear gas ; many drugs; and a wide variety of other substances. Certain derivatives of aromatic compounds have special properties and special names. For example, the addition of the hydroxyl group to an aromatic hydrocarbon yields a phenol . See also Hydrocar

. Boiling Point, temperature at which the vapor pressure of a liquid slightly exceeds the pressure of the atmosphere above the liquid. At temperatures below the boiling point (b.p.), evaporation takes place only from the surface of the liquid; during boiling, vapor forms within the body of the liquid; and as the vapor bubbles rise through the liquid, they cause the turbulence and seething associated with boiling. If the liquid is a single substance or an azeotropic solution (a mixture that has a constant b.p.), it will continue to boil as heat is added without any rise in temperature; that is, boiling occurs at constant temperature regardless of the amount of heat applied to the liquid. When the pressure on a liquid is increased, the b.p. goes up. Water at 1 atmosphere pressure (760 torr, or about 14.7 lb/sq in) boils at 100° C (212° F), but when the pressure is 218 atmospheres (165,000 torr, or 3200 lb/sq in), the b.p. reaches its maximum, 374° C (705° F). Above this temperature (the

. Evaporation, gradual change of state from liquid to gas that occurs at a liquid’s surface. Examples of evaporation include rainwater evaporating from warm pavement after a thunderstorm and wet paint drying as solvents in the paint evaporate. Fingernail polish also hardens as acetone evaporates from the liquid polish. A liquid is made up of atoms or molecules (bound groups of atoms) that are in constant motion, traveling at different speeds. The average speed of these particles depends only on the liquid’s temperature. If the particles have enough energy, fast-moving particles striking other particles near the liquid’s surface may impart enough speed, and therefore enough kinetic energy (energy of motion) to cause the surface particle to leave the liquid and become gas atoms or molecules. The particle’s kinetic energy is directly related to its speed. As particles with the most kinetic energy evaporate, the average kinetic energy of the remaining liquid decreases. In a similar way,

Distillation, process of heating a liquid until its more volatile constituents pass into the vapor phase, and then cooling the vapor to recover such constituents in liquid form by condensation. The main purpose of distillation is to separate a mixture of several components by taking advantage of their different volatilities, or the separation of volatile materials from nonvolatile materials. In evaporation and in drying, the purpose usually is to obtain the less volatile constituent; the more volatile constituent, in most cases water, is discarded. In distillation, on the other hand, the principal object of the operation is to obtain the more volatile constituent in pure form. The removal of water from glycerin by vaporizing the water, for example, is called evaporation, but the removal of water from alcohol by vaporizing the alcohol is called distillation, although similar apparatus is used in both cases. If the difference in volatility (and hence in boiling point) between the two co

. Toluene, colorless hydrocarbon, occurring in coal tar , specific gravity 0.86, boiling point 110.6° C (231.1° F). Toluene is sometimes called toluol or methylbenzene. The name toluene reflects the way in which it was once prepared, through destructive distillation of balsam of tolu. A great deal of industrial toluene is prepared in oil refineries by the alkylation of benzene . It is used as a solvent and as a source of synthetic compounds, for example, in the production of trinitrotoluene. Prolonged breathing of toluene vapor is harmful and should be avoided.

. Coal Tar, viscous black liquid produced in the destructive distillation of coal to make coke and gas. Coal tar is a complex mixture of organic compounds, mostly hydrocarbons . Its composition varies with the coal, the temperature at which it is formed, and the equipment used. When formed at temperatures below 600° C (1112° F), coal tar consists of hydrocarbons of the paraffin and olefin types, together with alcohols , phenols , and water. Coke is usually produced at about 1000° C (about 1832° F), and coal tar formed in that temperature range consists mostly of aromatic hydrocarbons plus phenols and some compounds containing nitrogen, sulfur, and oxygen. The variation in composition means that most of the compounds in coal tar are formed during the coking process and are not present as such in the original coal. Some 300 distinct compounds have been identified in coal tar, of which about 50 are separated and used commercially. Some of the constituents are carcinogens, that is, they i