Interstellar Matter

Interstellar Matter, gas and dust between the stars in a galaxy. In our own galaxy, the Milky Way, we can see glowing gas and dark, obscuring dust between the galaxy’s many visible stars. This gas and dust makes up interstellar matter. Galaxies differ in the density of interstellar matter that they contain. Spiral galaxies, such as the Milky Way, have much more interstellar matter than elliptical galaxies, which have almost none. About 3 percent of the mass of the Milky Way Galaxy is interstellar gas, and 1 percent is interstellar dust. Stars make up the rest of the ordinary matter in the galaxy. Dark matter—a material that does not reflect or emit light or other forms of electromagnetic radiation—also makes up some of the mass of the galaxy. Astronomers consider interstellar matter separately from intergalactic matter, or matter between galaxies.

Hydrogen gas makes up most of the interstellar matter, but essentially all of the chemical elements occur in interstellar matter. About 90 percent of the atoms in space are hydrogen, about 9 percent helium, and less than 1 percent consists of all the other chemical elements. The interstellar matter is so spread out that the space it occupies would be considered a vacuum in laboratories on Earth.

Ultraviolet Radiation

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Ultraviolet Radiation, electromagnetic radiation that has wavelengths in the range between 4000 angstrom units (Å), the wavelength of violet light, and 150 Å, the length of X rays. Natural ultraviolet radiation is produced principally by the sun. Ultraviolet radiation is produced artificially by electric-arc lamps.

Ultraviolet radiation is often divided into three categories based on wavelength, UV-A, UV-B, and UV-C. In general shorter wavelengths of ultraviolet radiation are more dangerous to living organisms. UV-A has a wavelength from 4000 Å to about 3150 Å. UV-B occurs at wavelengths from about 3150 Å to about 2800 Å and causes sunburn; prolonged exposure to UV-B over many years can cause skin cancer. UV-C has wavelengths of about 2800 Å to 150 Å and is used to sterilize surfaces because it kills bacteria and viruses.

The earth's atmosphere protects living organisms from the sun's ultraviolet radiation. If all the ultraviolet radiation produced by the sun were allowed to reach the surface of the earth, most life on earth would probably be destroyed. Fortunately, the ozone layer of the atmosphere absorbs almost all of the short-wavelength ultraviolet radiation, and much of the long-wavelength ultraviolet radiation. However, ultraviolet radiation is not entirely harmful; a large portion of the vitamin D that humans and animals need for good health is produced when the human's or animal's skin is irradiated by ultraviolet rays.

When exposed to ultraviolet light, many substances behave differently than when exposed to visible light. For example, when exposed to ultraviolet radiation, certain minerals, dyes, vitamins, natural oils, and other products become fluorescent—that is, they appear to glow. Molecules in the substances absorb the invisible ultraviolet light, become energetic, then shed their excess energy by emitting visible light. As another example, ordinary window glass, transparent to visible light, is opaque to a large portion of ultraviolet rays, particularly ultraviolet rays with short wavelengths. Special-formula glass is transparent to the longer ultraviolet wavelengths, and quartz is transparent to the entire naturally occurring range.

In astronomy, ultraviolet-radiation detectors have been used since the early 1960s on artificial satellites, providing data on stellar objects that cannot be obtained from the earth's surface. An example of such a satellite is the International Ultraviolet Explorer, launched in 1978.

Air

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Air, mixture of gases that composes the atmosphere surrounding Earth. These gases consist primarily of the elements nitrogen, oxygen, argon, and smaller amounts of hydrogen, carbon dioxide, water vapor, helium, neon, krypton, xenon, and others. The most important attribute of air is its life-sustaining property. Human and animal life would not be possible without oxygen in the atmosphere. In addition to providing life-sustaining properties, the various atmospheric gases can be isolated from air and used in industrial and scientific applications, ranging from steelmaking to the manufacture of semiconductors. This article discusses how atmospheric gases are isolated and used for industrial and scientific purposes.

GASES IN THE ATMOSPHERE

The atmosphere begins at sea level, and its first layer, the troposphere, extends from 8 to 16 km (5 and 10 mi) from Earth’s surface. The air in the troposphere consists of the following proportions of gases: 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, 0.03 percent carbon dioxide, and the remaining 0.07 percent is a mixture of hydrogen, water, ozone, neon, helium, krypton, xenon, and other trace components. Companies that isolate gases from air use air from the troposphere, so they produce gases in these same proportions.


PURIFYING AIR

Most larger air-separation plants continue to use cryogenic distillation to separate air gases. Before pure gases can be isolated from air, unwanted components such as water vapor, dust, and carbon dioxide must be removed. First, the air is filtered to remove dust and other particles. Next, the air is compressed as the first step in liquefying the air. However, as the air is compressed, the molecules begin striking each other more frequently, raising the air’s temperature. To offset the higher temperatures, water heat exchangers cool the air both during and after compression. As the air cools, most of its water vapor content condenses into liquid and is removed.

After being compressed, the air passes through beds of adsorption beads that remove carbon dioxide, the remaining water vapor, and molecules of heavy hydrocarbons, such as acetylene, butane, and propylene. These compounds all freeze at a higher temperature than do the other air gases. They must be removed before the air is liquefied or they will freeze in the column where distillation occurs.

COMPRESSED AIR

Not all industrial uses of air require it to be separated into its component gases. Compressed air—plain air that has been pressurized by squeezing it into a smaller-than-normal volume—is used in many industrial applications. When air is compressed, the gas molecules collide with each other more frequently and with more force, producing higher kinetic energy. The kinetic energy in compressed air can be converted into mechanical energy or it can be used to produce a powerful air flow or an air cushion. Compressed air is easily transmitted through pipes and hoses with little loss of energy, so it can be utilized at a considerable distance from the compressor or pressure tank.

Alabaster

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Alabaster, varietal name applied to two different minerals. One, Oriental alabaster, was extensively used by the ancient Egyptians. It is a variety of calcite, with a hardness of 3; it is usually white and translucent, but is often banded with dark or colored streaks. The other mineral, true alabaster, is a variety of gypsum, usually snow-white in color with a uniform, fine grain. True alabaster is softer than Oriental alabaster; it has a hardness of 1.5 and is easily carved into intricate shapes. Deposits of fine gypsum alabaster are found in Italy, England, Iran, and Pakistan.

Alkalies

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Alkalies or Alkalis (Arabic al-qili, “ashes of the saltwort plant”), originally the hydroxides and carbonates of potassium and sodium, leached from plant ashes. The term now applies to the corresponding compounds of ammonium, NH4, and the other alkali metals and to the hydroxides of calcium, strontium, and barium. All of these substances produce hydroxide ions, OH-, when dissolved in water. The carbonates and ammonium hydroxide give only moderate concentrations of hydroxide ions and are termed mild alkalis. The hydroxides of sodium and potassium, however, produce hydroxide ions in high enough concentration to destroy flesh; for this reason they are called caustic alkalis. Solutions of alkalis turn red litmus blue, react with and neutralize acids, feel slippery, and are electrical conductors.

Caustic soda, or sodium hydroxide, NaOH, is an important commercial product, used in making soap, rayon, and cellophane; in processing paper pulp; in petroleum refining; and in the manufacture of many other chemical products. Caustic soda is manufactured principally by electrolysis of a common salt solution, with chlorine and hydrogen as important by-products.

Sodium carbonate, Na2CO3, one of the mild alkalis, is manufactured from natural deposits or made from common salt brines by the Solvay process. It is used in the manufacture of glass and as a cleaning agent and water softener.

Alum

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Alum, any of a group of chemical compounds, made up of water molecules and two kinds of salts, one of which is usually aluminum sulfate combined in definite proportions. Potassium alum, also known as common alum, is the most important type of alum.

Potassium alum is a colorless substance that forms large octahedral or cubic crystals when potassium sulfate and aluminum sulfate are dissolved together and the solution is cooled. The solutions of potassium alum are acidic.

Potassium alum is soluble in seven times its weight of water at room temperature and is very soluble in hot water. When crystalline potassium alum is heated, some of the water of hydration becomes chemically separated, and the partly dehydrated salt dissolves in this water, so that the alum appears to melt at about 90°C (about 194°F). When heated to about 200°C (about 392°F), potassium alum swells up, loses all the water and some sulfur trioxide, and becomes a basic salt called burnt alum. Potassium alum has a density of 1.725.

Other types of alums made with aluminum sulfate include sodium alum, ammonium alum, and silver alum. Alums are used for flameproofing textiles and in baking powders, mordants for delicate dyeing operations, and medicines. Potassium alum is a powerful

Alumina

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Alumina or Aluminum Oxide, an oxide found in nature as the minerals corundum, diaspore, gibbsite, and most commonly, bauxite, an impure form of gibbsite. It is the only oxide formed by the metal aluminum. The precious stones ruby and sapphire are composed of corundum colored by small amounts of impurities.

Fused alumina, alumina that has been melted and recrystallized, is identical in chemical and physical properties with natural corundum. It is exceeded in hardness only by diamond and by a few synthetic substances, notably carborundum, or silicon carbide. Both impure natural corundum (emery) and pure synthetic corundum (Alundum) are used as abrasives. At room temperature alumina is insoluble in all ordinary chemical reagents. Its melting point is high, slightly above 2000°C (3632°F), and so alumina is useful as a refractory, for example, for the linings of special furnaces.

Alumina can be purified by fusing it with sodium carbonate. The resulting sodium aluminate is dissolved in water, leaving impurities, such as iron, as an insoluble residue. Hydrated alumina is reprecipitated from the solution by carbon dioxide. Because the alumina contained in bauxite is soluble in sodium hydroxide solution, a less expensive method may be used. By alternately concentrating and diluting the solution, hydrated alumina is precipitated, and the sodium hydroxide may be reused without neutralization. Hydrated alumina, also called aluminum hydroxide or aluminum hydrate, is readily soluble in acids or alkalies and is used as a raw material in the manufacturing process of all aluminum compounds.