SILICON DIOXIDE. 359 390. Silicon Carbide. Silicon carbide (SiC) or carborundum is among the three hardest substances known, the others being boron carbide and the diamond. It is made by heating a mixture of powdered quartz, coke, saw-dust, and common salt in the electric furnace. Carborundum is not attacked by acids nor by solutions of alkalies. It burns only with great difficulty. Because of its hardness, powdered carborundum is used as a polishing and cutting agent. 391. Silicon Dioxide. - Silicon dioxide is found as sand, quartz, etc. (cf. § 387). The pure substance may be prepared by heating silicic acids, H4SiO4, H2SiO3, etc. Silicon dioxide is thus silicic anhydride. When any form of silicon dioxide is fused with sodium or potassium hydroxide or carbonate, sodium or potassium silicate results. 2 NaOH+SiO2 →→→→ Na2SiO3+H2O. Calcium carbonate acts in the same way. When silica is fused with the oxide of a metal, a silicate is also formed (cf. § 162). Acids do not act upon silica (except hydrofluoric acid as in § 389). 392. Silicic Acid. When a soluble silicate is treated with hydrochloric acid, a bulky mass, like gelatine, is precipitated. This probably consists of orthosilicic acid, H4SiO4. When the gelatinous mass is dried at the ordinary temperature, it loses water and becomes a non-crystalline powder. This is probably ordinary silicic acid, H2SiO3. When the powder is heated to a high temperature it loses water, forming silica, SiO2. The equations are given in § 391. Polysilicic Acids. Besides the ortho and ordinary forms of silicic acid, many other forms are possible; these are known as polysilicic acids. They are derived from orthosilicic acid by the loss of different proportions of water. A general formula for them all would be, x Si(OH)4-y H2O. Thus, if x=2 and y = 1, we have, 2 H4SiO4-H2O = H¿Si2O7. Among the varieties of amorphous silica found in nature are agate, chalcedony, opal, carnelian, flint, amethyst, etc. These all contain water, and may, therefore, be looked upon as forms of the polysilicic acids. The colors of these substances are usually due to traces of impurities. 393. Silicates. The silicates are salts of silicic acid. The mineral chrysolite is magnesium silicate, MgSiO4. Serpentine is MgSiO7. These are salts of the acids H4SiO4 and H¿Si2O7, respectively. Potassium, sodium, and calcium silicates are derived from the acid H2SiO3. GLASS. 361 Potassium and sodium silicates are known as water glass "; they are used to make cements and artificial stone. Kaolin is practically pure aluminum silicate, H2Al2(SiO4)2.H2O. It fuses at a very high temperature. It is used for making china and crockery ware, fire-bricks, etc. Clay, which is impure aluminum silicate, meits lower than kaolin; it is used for making pottery, bricks, etc. The red color of baked clay is due to ferric silicate, Fe2(SiO3)3. 394. Glass.Glass is a mixture of certain silicates, generally of sodium or potassium silicate with calcium or lead silicate. Silicon dioxide is also present. The silicates of calcium, lead, etc., crystallize when they solidify; a glass made from them would break into fragments on cooling. The silicates of sodium and potassium, however, not only do not crystallize themselves, but even prevent the other silicates from crystallizing. Ordinary, soft glass, such as is used for window panes and bottles, is essentially a mixture of the silicates of calcium and sodium. It is made by melting together silica, calcium carbonate, and sodium carbonate, in fire-clay pots about 4 feet high and 4 feet in diameter. Carbon dioxide escapes. Hard glass is a mixture of the silicates of calcium and potassium. It is used for making chemical apparatus, lamp globes, etc. Crown glass is a variety of it. Jena glass is a hard glass containing boron trioxide. Flint glass, such as is used in making optical instruments, etc., is a mixture of potassium and lead silicates. Enameled or "milky" glass is made by adding cryolite (cf. § 316), tin oxide, etc., to ordinary glass. "Granite ironware " or "porcelain-lined ware consists of iron covered with an easily fusible glass, called enamel. Color is imparted to glass by the addition of small amounts of other substances. Thus, a cobalt compound colors glass blue; a cuprous compound, red; a chromic compound, green. The etching of designs on glass is done with hydrofluoric acid, as described in § 318, or with a sand blast. Window glass is made by blowing the viscous material from the melting pots into hollow cylinders several feet long, and about 1 feet in diameter. These are cut open lengthwise, and flattened out in ovens. Plate glass is made by pouring melted glass upon iron plates, and rolling it out with heated rollers. The plates are then ground and polished. Bottles, etc., are made by attaching lumps of the viscous glass to hollow tubes, and blowing the glass out in molds. Pressed glassware is made in molds; in cut glassware the designs are ground or polished by means of emery, carborundum, or sandstone wheels. All articles of glass, to be permanent, must be annealed. Annealing consists in allowing the hot object to cool regularly, so that its molecules may assume permanent positions with reference to one another. Unannealed glass may fly to pieces at the slightest jar. B. Boron. 395. Occurrence of Boron. -The element boron is the first member of the aluminum group of elements (cf. § 383); yet in its free state it closely resembles silicon and carbon. It occurs in nature chiefly as boric acid (H3BO3) and as borax (Na2B4O7). Boric acid is found chiefly in Tuscany, where it issues, mixed with steam, from the earth. Borax is found in Nevada and California in dry borax lakes. Boracite, (Mg3B8O15) 2.MgCl2, occurs at Stassfurt, in Germany. BORIC ACID. -- 363 396. Preparation. — Boron exists in two allotropic forms, the amorphous and the crystalline; it is difficult to prepare either form in a pure state. Crystalline boron is made by heating boron trioxide, B2O3, with aluminum. The aluminum reduces the oxide; and the liberated boron dissolves in the excess of aluminum. When the cooled mass is treated with hydrochloric acid, the aluminum dissolves, leaving crystals of boron. 397. Properties. Crystalline boron has a specific gravity of 2.6, and resembles the diamond. The amorphous form is brown. It burns when heated in the air, giving boron trioxide, B2O3. Nitric acid converts it into boric acid. The crystalline form is more difficult to ignite than the amorphous. Boron dissolves in melted sodium hydroxide, giving sodium borate. When it is heated in nitrogen or ammonia it gives boron nitride, BN. This is a white powder which is decomposed by steam, giving boric acid and ammonia. BN+3 H2O →→→→→→ B(OH)3+NH3. With chlorine, boron forms boron trichloride, BCl3, a colorless liquid. This is decomposed by water, giving boric acid and hydrochloric acid (cf. §§ 354 and 389). BC13+3 H2O - → B(OH)3+3 HCl. 398. Boric Acid. - Boric acid is made by adding concentrated hydrochloric or nitric acid to a hot solution of borax. It is a white, crystalline solid. Its aqueous solution has a faintly acid reaction with litmus, but colors turmeric paper brown, just as |