Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Coal Tar

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 incite cancerous growths.

Separation of coal tar into its constituent compounds or groups of compounds is achieved by extraction and distillation. The phenols, which are acidic, can be extracted by alkali solutions, from which they are released later by acidification. The nitrogen compounds react with and dissolve in acidic solutions and are freed later by alkalizing. Distillation produces benzene, toluene, naphthalene, xylene, anthracene, phenanthrene, and other valuable products. The processing may be varied to give different proportions. Left after distillation are residues of pitch, used in making roads, in roofing mixtures, and in electrodes for the production of aluminum.

Coal tar was once regarded as a useless nuisance. Since then, however, it has led to a whole new field of chemistry , and its compounds are indispensable to a vast number of products, including dyes, drugs, explosives, food flavorings, perfumes, artificial sweeteners, paints, preservatives, stains, insecticides, and resins. Coal tar is also the chief source of cresols, a group of chemicals used in antiseptics, creosote oil, paint removers, and plastics.