MAGNESIUM, ALUMINIUM, AND ZINC.
MAGNESIUM AND ITS COMPOUNDS.
Examine magnesite, dolomite, talc, serpentine, hornblende,
meerschaum, magnesium ribbon, magnesia alba, Epsom salt.
258. Occurrence and Preparation.--Mg is very widely distributed,
but does not occur uncombined. Its salts are found in rocks and
soils, in sea water and in the water of some springs, to which
they impart a brackish taste.
The most common minerals containing Mg are magnesite, MgCO3,
dolomite, MgCO3 + CaCO3, and talc, serpentine, hornblende, and
meerschaum. The last four are silicates, and often are unctious
to the touch. What proportion of the earth's crust is composed of
Mg? See page 173.
259. Metallic Mg is prepared by fusing MgCl2 with Na. Why is the
process expensive? Write the reaction.
Experiment 120.--With forceps hold a short strip of Mg ribbon in
a flame. Note the brilliancy of the light, and give the reaction.
Examine and name the product.
Photographs of the interior of caverns, where sunlight does not
penetrate, are taken by Mg light. Gun-cotton sprinkled with
powdered Mg has recently been employed for that purpose. Mg
tarnishes slightly in moist air. Compounds of Mg.--MgO, magnesia,
like CaO, is very infusible, and is used for crucibles. Magnesia
alba, a variable mixture of MgCO2 and Mg(OH)2, is employed in
medicine, as is also Epsom salt, MgSO4 + 7 H2O.
ALUMINIUM AND ITS COMPOUNDS.
Examine aluminium, aluminium bronze, corundum, emery, feldspar,
argillite, clay. Note especially the color, luster, specific
gravity and flexibility of Al.
What elements are more common in the earth than Al? What metals?
Compare the abundance of Al with that of Fe.
260. Compounds of Al.--Al occurs only in combination with other
elements. Feldspar, mica, slate, and clay are silicates of it. It
occurs in all rocks except CaCO3 and SiO2, and in nearly 200
minerals. Though found in all soils, its compounds are not taken
up by plants, except by a few cryptogams. Corundum, Al2O3, is the
richest of its ores. Compute its percent of Al. Compounds of Al
are very infusible and difficult of reduction.
261. Reduction.--Like most other metals not easily reducible by C
or H, it was originally obtained by electrolysis, but more
recently from its chloride, by the reducing action of strongly
heated K or Na. Al2Cl6 + 6 Na = 6 NaCl + 2 Al.
What is the chief use of Na? As it takes three pounds of Na to
make one pound of Al, the cost of the latter has been fifteen
dollars or more per pound. Its use has thus been restricted to
light apparatus and aluminium bronze, an alloy of Cu 90, Al 10,
which is not unlike gold in appearance.
Al2O3 has lately been reduced by C. Higher temperatures than have
heretofore been known are obtained by means of the electric arc
and large dynamo machines. Afurnace made of graphite, because
fire-clay melts like wax at such a high temperature, is filled
with Al2O3--corundum, --C, and Cu. In the midst of this are
embedded large carbon terminals, connected with dynamos. The
reduction takes several hours.
The following reaction takes place: Al2O3 + 3 C = 2 Al + 3 CO. Cu
is also added, and an alloy of Al and Cu is thus formed. This
alloy is not easily separable into its elements. Explain the
action of the C. CO escapes through perforations in the top of
the furnace, burning there to CO2. Only alloys of Al have yet
been obtained by this process. This method has not been employed
before, simply because the highest temperatures of combustion,
2000 degrees or 2500 degrees, would not effect a reduction. In
the same way Si, B, K, Na, Ca, Mg, Cr, have recently been reduced
from their oxides; but a process has yet to be found for
separating them easily from their alloys.
262. Properties and Uses.--Al is a silvery white metal, lighter
than glass, and only one-third the weight of iron. It does not
readily rust or oxidize, it fuses at 1000 degrees (compare with
Fe), is unaffected by acids, except by HCl and, slightly, by
H2SO4, is a good conductor of electricity, can be cast and
hammered, and alloys with most metals, forming thus many valuable
compounds. Every clay-bank is a mine of this metal, which has so
many of the useful properties of metals and has so few defects
that, if it could be obtained in sufficient quantities, it might,
for many purposes, take the place of iron, steel, tin, and other
metals. From its properties state any advantages which it would
have over iron in ocean vessels, railroads, and bridges. Why is
it better than Sn or Cu for culinary utensils? An alloy of Al,
Cu, and Si is used for telephone wires in Europe, and the
Bennett-Mackay cable is of the same material. Washington
monument, the tallest shaft in the world, is capped with a
pyramid of Al,ten inches high.
For the uses of alumina, Al2O3, and its silicates, see page 133.
ZINC AND ITS COMPOUNDS.
Examine zincite, sphalerite, Smithsonite, sheet zinc, galvanized
iron, granulated zinc, zinc dust.
263. Compounds.--The compounds of zinc are abundant. Its chief
ores are zincite, ZnO, sphalerite or blende, ZnS, Smithsonite,
ZnCO3. For their reduction these ores are first roasted, i.e.
heated in presence of air. With ZnS this reaction takes place:
ZnS + 3 O = Zn0 + S02. The oxide is reduced with C, and then Zn
is distilled. State the reaction. Zinc is sublimed-in the form of
zinc dust-like flowers of S. Granulated Zn is made by pouring a
stream of the molten metal into water.
Experiment 121.--Burn a strip of Zn foil, and note the color of
the flame and of the product. State the reaction. The red color
of zincite is supposed to be imparted by Mn present in the
compound.
264. Uses.--Name any use of Zn in the chemical laboratory. It is
employed for coating wire and sheet iron --galvanized iron. This
is done by plunging the wire or the sheets of iron into melted
Zn. Describe the use of Zn as an alloy. See page 136.
ZnO forms the basis of a white paint called zinc white. White
vitriol, ZnSO4 + 7 H2O, is employed in medicine. Name two other
vitriols.