11. Write the equations for all the reactions involved in Marsh's test for arsenic. 12. Write the names and formulas for the acids of antimony. 13. Write the equations showing the hydrolysis of antimony trichloride; of bismuth nitrate. 14. In what respects does nitrogen resemble the members of the phosphorus family? CHAPTER XXI SILICON, TITANIUM, BORON ================================================================= | | | | | | SYMBOL | ATOMIC | DENSITY | CHLORIDES | OXIDES | | WEIGHT | | | ____________|________|________|_________|___________|____________ | | | | | Silicon | Si | 28.4 | 2.35 | SiCl_{4} | SiO_{2} Titanium | Ti | 48.1 | 3.5 | TiCl_{4} | TiO_{2} Boron | B | 11.0 | 2.45 | BCl_{3} | B_{2}O_{3} ================================================================= ~General.~ Each of the three elements, silicon, titanium, and boron, belongs to a separate periodic family, but they occur near together in the periodic grouping and are very similar in both physical and chemical properties. Since the other elements in their families are either so rare that they cannot be studied in detail, or are best understood in connection with other elements, it is convenient to consider these three together at this point. The three elements are very difficult to obtain in the free state, owing to their strong attraction for other elements. They can be prepared by the action of aluminium or magnesium on their oxides and in impure state by reduction with carbon in an electric furnace. They are very hard and melt only at the highest temperatures. At ordinary temperatures they are not attacked by oxygen, but when strongly heated they burn with great brilliancy. Silicon and boron are not attacked by acids under ordinary conditions; titanium is easily dissolved by them. SILICON ~Occurrence.~ Next to oxygen silicon is the most abundant element. It does not occur free in nature, but its compounds are very abundant and of the greatest importance. It occurs almost entirely in combination with oxygen as silicon dioxide (SiO_{2}), often called silica, or with oxygen and various metals in the form of salts of silicic acids, or silicates. These compounds form a large fraction of the earth's crust. Most plants absorb small amounts of silica from the soil, and it is also found in minute quantities in animal organisms. ~Preparation.~ The element is most easily prepared by reducing pure powdered quartz with magnesium powder: SiO_{2} + 2Mg = 2MgO + Si.