SILICON, TITANIUM, BORON
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| | | | |
| 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}
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~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.
~Properties.~ As would be expected from its place in the periodic table,
silicon resembles carbon in many respects. It can be obtained in several
allotropic forms, corresponding to those of carbon. The crystallized
form is very hard, and is inactive toward reagents. The amorphous
variety has, in general, properties more similar to charcoal.
~Compounds of silicon with hydrogen and the halogens.~ Silicon hydride
(SiH_{4}) corresponds in formula to methane (CH_{4}), but its properties
are more like those of phosphine (PH_{3}). It is a very inflammable gas
of disagreeable odor, and, as ordinarily prepared, takes fire
spontaneously on account of the presence of impurities.
Silicon combines with the elements of the chlorine family to form such
compounds as SiCl_{4} and SiF_{4}. Of these silicon fluoride is the most
familiar and interesting. As stated in the discussion of fluorine, it is
formed when hydrofluoric acid acts upon silicon dioxide or a silicate.
With silica the reaction is thus expressed:
SiO_{2} + 4HF = SiF_{4} + 2H_{2}O.
It is a very volatile, invisible, poisonous gas. In contact with water
it is partially decomposed, as shown in the equation
SiF_{4} + 4H_{2}O = 4HF + Si(OH)_{4}.
The hydrofluoric acid so formed combines with an additional amount of
silicon fluoride, forming the complex fluosilicic acid (H_{2}SiF_{6}),
thus:
2HF + SiF_{4} = H_{2}SiF_{6}.
~Silicides.~ As the name indicates, silicides are binary compounds
consisting of silicon and some other element. They are very stable at
high temperatures, and are usually made by heating the appropriate
substances in an electric furnace. The most important one is
_carborundum_, which is a silicide of carbon of the formula CSi. It is
made by heating coke and sand, which is a form of silicon dioxide, in an
electric furnace, the process being extensively carried on at Niagara
Falls. The following equation represents the reaction
SiO_{2} + 3C = CSi + 2CO.
The substance so prepared consists of beautiful purplish-black crystals,
which are very hard. Carborundum is used as an abrasive, that is, as a
material for grinding and polishing very hard substances. Ferrosilicon
is a silicide of iron alloyed with an excess of iron, which finds
extensive use in the manufacture of certain kinds of steel.
~Manufacture of carborundum.~ The mixture of materials is heated in a
large resistance furnace for about thirty-six hours. After the reaction
is completed there is left a core of graphite G. Surrounding this core
is a layer of crystallized carborundum C, about 16 in. thick. Outside
this is a shell of amorphous carborundum A. The remaining materials
M are unchanged and are used for a new charge.
[Illustration: Fig. 73]
~Silicon dioxide~ (_silica_) (SiO_{2}). This substance is found in a great
variety of forms in nature, both in the amorphous and in the crystalline
condition. In the form of quartz it is found in beautifully formed
six-sided prisms, sometimes of great size. When pure it is perfectly
transparent and colorless. Some colored varieties are given special
names, as amethyst (violet), rose quartz (pale pink), smoky or milky
quartz (colored and opaque). Other varieties of silicon dioxide, some of
which also contain water, are chalcedony, onyx, jasper, opal, agate, and
flint. Sand and sandstone are largely silicon dioxide.
~Properties.~ As obtained by chemical processes silicon dioxide is an
amorphous white powder. In the crystallized state it is very hard and
has a density of 2.6. It is insoluble in water and in most chemical
reagents, and requires the hottest oxyhydrogen flame for fusion. Acids,
excepting hydrofluoric acid, have little action on it, and it requires
the most energetic reducing agents to deprive it of oxygen. It is the
anhydride of an acid, and consequently it dissolves in fused alkalis to
form silicates. Being nonvolatile, it will drive out most other
anhydrides when heated to a high temperature with their salts,
especially when the silicates so formed are fusible. The following
equations illustrate this property:
Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},
Na_{2}SO_{4} + SiO_{2} = Na_{2}SiO_{3} + SO_{3}.
~Silicic acids.~ Silicon forms two simple acids, orthosilicic acid
(H_{4}SiO_{4}) and metasilicic acid (H_{2}SiO_{3}). Orthosilicic acid is
formed as a jelly-like mass when orthosilicates are treated with strong
acids such as hydrochloric. On attempting to dry this acid it loses
water, passing into metasilicic or common silicic acid:
H_{4}SiO_{4} = H_{2}SiO_{3} + H_{2}O.
Metasilicic acid when heated breaks up into silica and water, thus:
H_{2}SiO_{3} = H_{2}O + SiO_{2}.
~Salts of silicic acids,--silicates.~ A number of salts of the
orthosilicic and metasilicic acids occur in nature. Thus mica
(KAlSiO_{4}) is a salt of orthosilicic acid.
~Polysilicic acids.~ Silicon has the power to form a great many complex
acids which may be regarded as derived from the union of several
molecules of the orthosilicic acid, with the loss of water. Thus we have
3H_{4}SiO_{4} = H_{4}Si_{3}O_{8} + 4H_{2}O.
These acids cannot be prepared in the pure state, but their salts form
many of the crystalline rocks in nature. Feldspar, for example, has the
formula KAlSi_{3}O_{8}, and is a mixed salt of the acid
H_{4}Si_{3}O_{8}, whose formation is represented in the equation above.
Kaolin has the formula Al_{2}Si_{2}O_{7}·2H_{2}O. Many other examples
will be met in the study of the metals.
~Glass.~ When sodium and calcium silicates, together with silicon dioxide,
are heated to a very high temperature, the mixture slowly fuses to a
transparent liquid, which on cooling passes into the solid called glass.
Instead of starting with sodium and calcium silicates it is more
convenient and economical to heat sodium carbonate (or sulphate) and
lime with an excess of clean sand, the silicates being formed during the
heating:
Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},
CaO + SiO_{2} = CaSiO_{3}.
[Illustration: Fig. 74]
The mixture is heated below the fusing point for some time, so that the
escaping carbon dioxide may not spatter the hot liquid; the heat is then
increased and the mixture kept in a state of fusion until all gases
formed in the reaction have escaped.
_Molding and blowing of glass._ The way in which the melted mixture is
handled in the glass factory depends upon the character of the article
to be made. Many articles, such as bottles, are made by blowing the
plastic glass into hollow molds of the desired shape. The mold is first
opened, as shown in Fig. 74. A lump of plastic glass A on the hollow
rod B is lowered into the mold, which is then closed by the handles
C. By blowing into the tube the glass is blown into the shape of the
mold. The mold is then opened and the bottle lifted out. The neck of the
bottle must be cut off at the proper place and the sharp edges rounded
off in a flame.
Other objects, such as lamp chimneys, are made by getting a lump of
plastic glass on the end of a hollow iron rod and blowing it into the
desired shape without the help of a mold, great skill being required in
the manipulation of the glass. Window glass is made by blowing large
hollow cylinders about 6 ft. long and 1-1/2 ft. in diameter. These are
cut longitudinally, and are then placed in an oven and heated until they
soften, when they are flattened out into plates (Fig. 75). Plate glass
is cast into flat slabs, which are then ground and polished to perfectly
plane surfaces.
_Varieties of glass._ The ingredients mentioned above make a soft,
easily fusible glass. If potassium carbonate is substituted for the
sodium carbonate, the glass is much harder and less easily fused;
increasing the amount of sand has somewhat the same effect. Potassium
glass is largely used in making chemical glassware, since it resists the
action of reagents better than the softer sodium glass. If lead oxide is
substituted for the whole or a part of the lime, the glass is very soft,
but has a high index of refraction and is valuable for making optical
instruments and artificial jewels.
[Illustration: Fig. 75]
_Coloring of glass._ Various substances fused along with the glass
mixture give characteristic colors. The amber color of common bottles is
due to iron compounds in the glass; in other cases iron colors the glass
green. Cobalt compounds color it deep blue; those of manganese give it
an amethyst tint and uranium compounds impart a peculiar yellowish green
color. Since iron is nearly always present in the ingredients, glass is
usually slightly yellow. This color can be removed by adding the proper
amount of manganese dioxide, for the amethyst color of manganese and the
yellow of iron together produce white light.
_Nature of glass._ Glass is not a definite chemical compound and its
composition varies between wide limits. Fused glass is really a solution
of various silicates, such as those of calcium and lead, in fused sodium
or potassium silicate. A certain amount of silicon dioxide is also
present. This solution is then allowed to solidify under such conditions
of cooling that the dissolved substances do not separate from the
solvent. The compounds which are used to color the glass are sometimes
converted into silicates, which then dissolve in the glass, giving it a
uniform color. In other cases, as in the milky glasses which resemble
porcelain in appearance, the color or opaqueness is due to the finely
divided color material evenly distributed throughout the glass, but not
dissolved in it. Milky glass is made by mixing calcium fluoride, tin
oxide, or some other insoluble substance in the melted glass. Copper or
gold in metallic form scattered through glass gives it shades of red.
TITANIUM
Titanium is a very widely distributed element in nature, being
found in almost all soils, in many rocks, and even in plant and
animal tissues. It is not very abundant in any one locality,
and it possesses little commercial value save in connection
with the iron industry. Its most common ore is rutile
(TiO_{2}), which resembles silica in many respects.
In both physical and chemical properties titanium resembles
silicon, though it is somewhat more metallic in character. This
resemblance is most marked in the acids of titanium. It not
only forms metatitanic and orthotitanic acids but a great
variety of polytitanic acids as well.
BORON
~Occurrence.~ Boron is never found free in nature. It occurs as boric acid
(H_{3}BO_{3}), and in salts of polyboric acids, which usually have very
complicated formulas.
~Preparation and properties.~ Boron can be prepared from its oxide by
reduction with magnesium, exactly as in the case of silicon. It
resembles silicon very strikingly in its properties. It occurs in
several allotropic forms, is very hard when crystallized, and is rather
inactive toward reagents. It forms a hydride, BH_{3}, and combines
directly with the elements of the chlorine family. Boron fluoride
(BF_{3}) is very similar to silicon fluoride in its mode of formation
and chemical properties.
~Boric oxide~ (B_{2}O_{3}). Boron forms one well-known oxide, B_{2}O_{3},
called boric anhydride. It is formed as a glassy mass by heating boric
acid to a high temperature. It absorbs water very readily, uniting with
it to form boric acid again:
B_{2}O_{3} + 3H_{2}O = 2H_{3}BO_{3}.
In this respect it differs from silicon dioxide, which will not combine
directly with water.
~Boric acid~ (H_{3}BO_{3}). This is found in nature in considerable
quantities and forms one of the chief sources of boron compounds. It is
found dissolved in the water of hot springs in some localities,
particularly in Italy. Being volatile with steam, the vapor which
escapes from these springs has some boric acid in it. It is easily
obtained from these sources by condensation and evaporation, the
necessary heat being supplied by other hot springs.
Boric acid crystallizes in pearly flakes, which are greasy to the touch.
In the laboratory it is easily prepared by treating a strong, hot
solution of borax with sulphuric acid. Boric acid being sparingly
soluble in water crystallizes out on cooling:
Na_{2}B_{4}O_{7} + 5H_{2}O + H_{2}SO_{4} = Na_{2}SO_{4} + 4H_{3}BO_{3}.
The substance is a mild antiseptic, and on this account is often used in
medicine and as a preservative for canned foods and milk.
~Metaboric and polyboric acids.~ When boric acid is gently heated it is
converted into metaboric acid (HBO_{2}):
H_{3}BO_{3} = HBO_{2} + H_{2}O.
On heating metaboric acid to a somewhat higher temperature tetraboric
acid (H_{2}B_{4}O_{7}) is formed:
4HBO_{2} = H_{2}B_{4}O_{7} + H_{2}O.
Many other complex acids of boron are known.
~Borax.~ Borax is the sodium salt of tetraboric acid, having the formula
Na_{2}B_{4}O_{7}·10 H_{2}O. It is found in some arid countries, as
southern California and Tibet, but is now made commercially from the
mineral colemanite, which is the calcium salt of a complex boric acid.
When this is treated with a solution of sodium carbonate, calcium
carbonate is precipitated and borax crystallizes from the solution.
When heated borax at first swells up greatly, owing to the expulsion of
the water of crystallization, and then melts to a clear glass. This
glass has the property of easily dissolving many metallic oxides, and on
this account borax is used as a flux in soldering, for the purpose of
removing from the metallic surfaces to be soldered the film of oxide
with which they are likely to be covered. These oxides often give a
characteristic color to the clear borax glass, and borax beads are
therefore often used in testing for the presence of metals, instead of
the metaphosphoric acid bead already described.
The reason that metallic oxides dissolve in borax is that borax
contains an excess of acid anhydride, as can be more easily
seen if its formula is written 2NaBO_{2} + B_{2}O_{3}. The
metallic oxide combines with this excess of acid anhydride,
forming a mixed salt of metaboric acid.
Borax is extensively used as a constituent of enamels and glazes for
both metal ware and pottery. It is also used as a flux in soldering and
brazing, and in domestic ways it serves as a mild alkali, as a
preservative for meats, and in a great variety of less important
applications.
EXERCISES
1. Account for the fact that a solution of borax in water is alkaline.
2. What weight of water of crystallization does 1 kg. of borax contain?
3. When a concentrated solution of borax acts on silver nitrate a borate
of silver is formed. If the solution of borax is dilute, however, an
hydroxide of silver forms. Account for this difference in behavior.