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.

Zinc

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Zinc, symbol Zn, bluish-white metallic element that has many industrial applications. Zinc is one of the transition elements of the periodic table. The atomic number of zinc is 30. The ores of zinc have long been known, but zinc was not recognized as a separate element until 1746, when the German chemist Andreas Sigismund Marggraf isolated the pure metal by heating calamine and charcoal.

PROPERTIES AND OCCURRENCE

Pure zinc is a crystalline metal, insoluble in hot and cold water and soluble in alcohol, acids, and alkalies. It is extremely brittle at ordinary temperatures, but becomes malleable between 120° and 150° C (248° and 302° F) and may be rolled into sheets between heated rollers. Zinc is unaffected by dry air; in moist air it is oxidized and becomes coated with a carbonate film that protects it from further corrosion. Zinc melts at about 420° C (about 788° F), boils at about 907° C (about 1665° F), and has a specific gravity of 7.14. The atomic weight of zinc is 65.38.

Zinc ranks 24th in abundance among the elements in the crust of the earth. It never occurs free in nature, but is found as zinc oxide in the mineral zincite; as zinc silicate in the mineral hemimorphite; as zinc carbonate in the mineral smithsonite; as a mixed oxide of zinc and iron in the mineral franklinite; and as zinc sulfide in the mineral sphalerite, or zinc blende. The ores most commonly used as a source of zinc are smithsonite and sphalerite.

The first step in the metallurgy process is to transform the ores into oxides by subjecting them to high temperatures. The oxides are then reduced by carbon in an electric furnace, the zinc boiling and distilling in the retort in which the reduction takes place. The zinc obtained by distillation contains small amounts of iron, arsenic, cadmium, and lead and is known in metallurgy as spelter. In another method of refining zinc, the roasted ores are leached with sulfuric acid. After the impurities have been removed, the solution is electrolyzed. Electrolytic zinc is pure and has superior qualities, such as high resistance to corrosion.

USES

The metal is used principally as a protective coating, or galvanizer, for iron and steel; as an ingredient of various alloys, especially brass; as plates for dry electric cells; and for die castings. Zinc oxide, known as zinc white or Chinese white, is used as a paint pigment. It is also used as a filler in rubber tires and is employed in medicine as an antiseptic ointment. Zinc chloride is used as a wood preservative and as a soldering fluid. Zinc sulfide is useful in applications involving electroluminescence, photoconductivity, and semiconductivity and has other electronic uses. It is employed as a phosphor for the screens of television tubes and in fluorescent coatings.