Silica aerogels are transparent, highly porous substances consisting of tiny spheres of bonded silicon and oxygen atoms joined into long strands. The strands are separated by innumerable air spaces. Although they are solids, aerogels have remarkably low densities, appearing almost like frozen wisps of smoke. They are one of the lightest known solids. They also have the highest thermal resistance of any solid ever tested and thus could have important applications as high-efficiency insulators and radiation detectors.
Symbol Si
Atomic Number 14
Atomic Weight 28.0855
Group in Periodic Table IVa
Specific Gravity at 77o F (25o C) 2.33
Boiling Point 4,271o F (2,355o C)
Melting Point 2,570o F (1,410o C)
coating to protect steel. Many of the billions of cans in use every year are made of tin-plated steel because tin is resistant to corrosion. In many food containers, however, plastic coatings have replaced tin. And with the increased use of aluminum in making cans, the use of tin has dropped sharply.
The protective tin coating may be as thin as 15/1,000,000 inch (0.000038 centimeter). Steel for cans is plated by dipping sheets of it in molten tin or by passing continuous strips on high-speed rubber rollers through an electrolytic tinning bath. Kitchen utensils are electroplated with a thicker coating of tin (see Electrochemistry). Terneplate for roofs is made by immersing sheets of steel in a molten mixture of lead and tin.
Tin is a constituent of alloys having low melting points, such as bronze, babbitt metal, type metal and solder metal (see Alloy). Tin alloys are used in collapsible tubes, though toothpaste is now sold in plastic containers. Tin chloride is used in dyeing and weighting silk and as a stabilizer for perfumes. Tin can also form a bond with carbon, as in the more than 500 organotin compounds. Some of these compounds are used in making wood, paper, paint, textile products, agricultural sprays, and hospital disinfectants.
Tin is found mainly in the ore cassiterite, or tinstone, a dioxide of tin. Before smelting, the ore is crushed to a powder and roasted to remove arsenic and sulfur. At the smelter it is heated with carbon to separate the tin from the zinc, copper, bismuth, and iron that it contains. Tin is also refined by electrolysis. The refined product is called block tin.
In the early 1990s China led the world in the production of tin ore, followed by Brazil, Indonesia, Malaysia, and Bolivia. In October 1985 the world tin market collapsed, and trading in the metal was temporarily suspended for fear that falling tin prices would start a downward spiral in the prices of other metals. For years the International Tin Council (ITC), a cartel, had supported artificially high prices for tin while world use dropped.
The main tin-producing nations had maintained control of their exports, but certain other nations not belonging to the cartel kept producing and marketing tin. User nations, such as the United States, had huge stockpiles. As the use of tin decreased, in part replaced by other metals and plastics, it seemed unlikely that the council could reestablish its control of the market. The ITC suspended export controls in 1985, but in 1987 a complementary organization, the Association of Tin Producing Countries, reinstituted quotas, which such nonmember countries as Brazil and China agreed to observe.
Although the United States is the world's largest user of tin, it has no major deposits. Small quantities of tin concentrates were produced from placer deposits in Alaska. A major secondary source is tin scrap, from which tin is recovered at detinning plants. Most imported tin comes from Brazil, Bolivia, China, and Indonesia.
The use of tin alloyed with copper to make bronze ushered in the Bronze Age of civilization (see Bronze). To get tin the Phoenicians sailed from their Mediterranean homes as far as the British Isles.
Symbol Sn
Atomic Number 50
Atomic Weight 118.69
Group in Periodic Table IVa
Specific Gravity at 68? F (20? C) 5.75-7.31
Boiling Point 4,118? F (2,270? C)
Melting Point 450? F (232? C)
Germanium, brittle silver element predicted in 1871 by Mendeleev but not discovered until 1886 by Clemens Winkler. It is used as superconductor in electronics; window and lens component in equipment to measure infrared radiation; component of camera lenses and microscopes; and in transistors and in phosphors for fluorescent lamps. It is found as a part of the minerals argyrodite, germanite, and renierite and in coal. It can kill certain harmful bacteria without causing toxicity to humans and is being studied as a therapeutic agent.
Properties of Germanium
Symbol Ge Atomic number 32 Atomic weight 72.59 Group in periodic table IVa Boiling point 4,892o F (2,700o C) Melting point 1,719o F (937.2o C) Specific gravity 5.323
![The Effect of Concentration on the Rate of Reaction between Magnesium [Mg] and Hydrochloric acid [Hcl]](https://mbt-essays-prod-public.s3.eu-west-1.amazonaws.com/934664/listing/934664_1.jpg)
