Kids Research Express

Diffusion, in physical science, the flow of energy or matter from a higher concentration to a lower concentration, resulting in a homogeneous distribution. If one end of a rod is heated or electrically charged, the heat or electricity will diffuse from the hot or charged portion to the cool or uncharged portion. If the bar is made of metal, this diffusion will be rapid for heat and almost instantaneous for electricity; if the bar is made of asbestos, the diffusion will be slow for heat and extremely slow for electricity (see Insulation ). Diffusion of matter occurs most rapidly in gases, more slowly in liquids, and most slowly in solids. The spreading of odoriferous molecules (a smell) throughout a room is a common example of gaseous diffusion. A solid may dissolve and diffuse through a liquid, as when a lump of sugar is placed in a cup of water. This process is much slower than the diffusion of a gas; if the water is not stirred, it may take weeks for the solution to become homogeneou

Pressure, in mechanics, the force per unit area exerted by a liquid or gas on a body or surface, with the force acting at right angles to the surface uniformly in all directions. In the British system, pressure is usually measured in pounds per square inch (PSI); in international usage, in kilograms per square centimeters, or in atmospheres; and in the international metric system (SI), in newtons per square meter. The unit atmosphere (atm) is defined as a pressure of 1.03323 kg/sq cm (14.696 lb/sq in), which, in terms of the conventional mercury barometer, corresponds to 760 mm (29.921 in) of mercury. The unit kilopascal (kPa) is defined as a pressure of 0.0102 kg/sq cm (0.145 lb/sq in). PRESSURE GAUGES Most gauges record the difference between the fluid pressure and local atmospheric pressure. For small pressure differences, a U-tube manometer is used. It consists of a U-shaped tube with one end connected to the container and the other open to the atmosphere. Filled with a liquid, suc

Cellular respiration is a complex series of chemical reactions that harvests the energy trapped in the bonds of glucose sugar molecules and converts it into a chemical form that can be stored for later use. Humans and other animals obtain glucose sugar molecules from food, such as the fruits and grains shown here, while plants create glucose sugars in the process of photosynthesis. Essential to the survival of most organisms on Earth, cellular respiration yields the energy that makes our bodies function properly.

Urea, colorless, crystalline compound with melting point 132.7° C (270.9° F), also known as carbamide. It is found abundantly in the urine of humans and other mammals (see Urinary System ). In lesser quantities, it is present in the blood, liver, lymph, and serous fluids and is found in the excrement of fish and many other lower animals. Urea is produced mostly in the liver as the end product of protein metabolism . The nitrogen in urea, which constitutes most of the nitrogen in the urine, is produced mainly from food protein, but part comes from the breakdown of body cells. Urea is also present in various fungus molds as well as in the leaves and seeds of numerous legumes and cereals . The compound is soluble in both water and alcohol and is slightly soluble in ether. Urea is prepared synthetically by the Wöhler synthesis, which was devised in 1828 by the German chemist Friedrich Wöhler. Because of its high nitrogen content, commercially prepared urea is used in the manufacture of agr

Methyl (wood alcohol, methanol) Manufacture: By destructive distillation of wood. Also by synthesis from hydrogen and carbon monoxide under high pressure. Uses: Solvent for fats, oils, resins, nitrocellulose. Manufacture of dyes, formaldehyde, antifreeze solutions, special fuels, plastics. Ethyl (grain alcohol, ethanol) Manufacture: By fermentation of sugar, starch, or waste sulfite liquor. Synthesis from ethylene or acetylene. Direct hydration of ethylene. Uses: Solvent for products such as lacquers, paints, varnishes, glues, pharmaceuticals, explosives. Also as "building block" in making high-molecular-weight chemicals. Isopropyl (isopropanol) Manufacture: By hydration of propylene from cracked gases. Also as by-product of certain fermentation processes. Uses: Solvent for oils, gums, alkaloids, resins. Making acetone, soap, antiseptic solutions. Normal propyl Manufacture: As a coproduct of air oxidation of propane and butane mixtures. Uses: Solvent for lacquers, resins,

Higher alcohols, those of greater molecular weight than ethyl alcohol, have many specific and general uses. Isopropyl alcohol is used extensively as a rubbing alcohol, butyl alcohol is a base for perfumes and fixatives, and others are important flavoring agents and perfumes. Polyhydric alcohols, those containing more than one 8OH group, are also important—as, for example, the trihydric alcohol known as glycerol .

Ethyl alcohol, or ethanol, C2 H5OH, is a clear, colorless liquid, with a burning taste and characteristic, agreeable odor. Ethanol is the alcohol in such beverages as beer, wine , and brandy. Because of its low freezing point, it has been used as the fluid in thermometers for temperatures below -40° C (-40° F), the freezing point of mercury, and for other special low-temperature purpose, such as for antifreeze in automobile radiators. Ethanol is normally concentrated by distillation of dilute solutions. Commercial ethanol contains 95 percent by volume of ethanol and 5 percent of water. Dehydrating agents remove the remaining water and produce absolute ethanol. Ethanol melts at -114.1° C (-173.4° F), boils at 78.5° C (173.3° F), and has a specific gravity of 0.789 at 20° C (68° F). Ethanol has been made since ancient times by the fermentation of sugars . All beverage ethanol and more than half of industrial ethanol is still made by this process. Starch from potatoes, corn, or other ce