272. Change of Valence.

Experiment 122.--Dissolve 2 g. of iron filings in diluted HCl.
Filter or pour off the clear liquid, divide it into two parts,
and add NH4OH to one part till a ppt. occurs. Notice the greenish
color of Fe(OH)2. Oxidize the other part by adding a few drops of
HNO3 and boiling a minute. Now add NH4OH, and observe the reddish
color of the ppt., Fe2(OH)6.

Solutions of ferrous salts will gradually change to ferric, if
allowed to stand, thus showing the greater stability of the
latter. In changing from FeCl2 to Fe2Cl6 oxidation does not
consist in adding O, but in increasing the negative element or
radical. This is possible only by changing the valence of Fe from
2 to 4. Hence oxidation, in its larger sense, means increasing
the valence of the positive element. To oxidize FeSO4 is to make
it Fe2(SO4)3, changing the valence of Fe as before. Reduction or
deoxidation diminishes the valence of the positive element.
Illustrate this by the same iron salts. Illustrate it by PbO and
Pb02; AuCl and AuCl3; Sb2S3 and Sb2S5. In this sense define an
oxidizing agent. A reducing agent.

273. Ferrous Sulphate.

Experiment 123.--Dissolve a few iron filings in dilute H2SO4, and
slowly evaporate for a few minutes. Write the equation.

Ferrous sulphate, green vitriol, or copperas, FeSO4 + 7 H2O, is
the source of what acid?  See page 66. It is also one of the
ingredients in many writing inks. On being heated, or exposed to
the air, it loses its water of crystallization and becomes a
white powder. It is prepared as above, or by oxidizing moistened
FeS2 by exposure to the air.

Ferrous sulphide, protosulphide of iron, FeS, is how prepared?
See Experiment 6. State its use. See Experiment 108. It also
occurs native.

Ferric sulphide, pyrite, FeS2, occurs native in large quantities.
What is its use? See page 65.

CHAPTER XLIX.

LEAD AND TIN.

LEAD.

Examine galena, lead protoxide and dioxide, red-lead, lead
carbonate, acetate, and nitrate. Note especially the colors of
the oxides, the cubical crystallization and cleavage of galena,
the specific gravity of the compounds, the softness of Pb, and
the tarnish, Pb2O, which covers it,if long exposed.

274. Distribution of Pb.--Pb is widely distributed, occurring as
PbS and PbCO3. PbS, galenite or galena, is its main source. By
heating it in air, SO2 is formed, and Pb liberated and drawn off.

Pb is but little acted on by cold H2SO4, unless concentrated.
Describe its use in making that acid. See page 65. To show that a
little Pb has been dissolved, as PbSO4, in the manufacture of
that acid, perform this experiment.

Experiment 124.--To 5cc. of water in a clean t.t. add the same
volume of H2SO4, not C.P.; shake, and notice any fine powder
suspended. PbSO4, being insoluble in water, is precipitated. What
is the test for Pb?  See Experiment 109.

275. Poisonous Properties.--Ph is very flexible and soft, and is
much used for water pipes. In moist air it is soon coated with
suboxide, Pb20, as may be seen by exposing a fresh surface. Some
portion of this is liable to dissolve in water, and, as all
soluble salts of Pb are poisonous, water that has stood in pipes
should not be used fordrinking. Lead is employed as an alloy of
tin for covering sheet-iron in "terne plate." T his plate is
rarely used except for roofing. The "bright plate," used for tin
cans and other purposes, scarcely ever contains any lead except
the small portion in solder. In soldering, ZnCl2 is employed for
a flux. Sn, Pb, and Zn are somewhat soluble in vegetable acids.
If citric acid be present, as it usually is, citrates of these
metals are formed, and all of them are poisonous. The action is
far more rapid after opening the can, since oxidation is
hastened. Hence the contents should be taken out directly after
opening.

Lead poisons seem to have an affinity for the tissues of the
body, and accumulate little by little. Painter's colic results
from lead poisoning. Epsom salt, or other soluble sulphate, is an
antidote, since with Pb it makes insoluble PbSO4.

276. Some Lead Compounds.--Lead salts form the basis of many
paints. White paint is a mixture of PbCO3 and Pb(OH)2 suspended
in linseed oil. It is often adulterated with BaSO4, ZnO, CaCO3.
Other lead compounds are used for colored paints. The two chief
soluble salts are Pb(NO3)2 and lead acetate, Pb(C2H302)2.

Red-lead, Pb3O4, and, to some extent, litharge, PbO, are employed
in glass manufacture. Name the kind of glass in which it is used,
describe its manufacture, and write a symbol for lead silicate.
What is the characteristic of lead glass? See page 132.

Experiment 125.--Put a small fragment of Pb on a piece of
charcoal, and blow the oxidizing flame against it for some time
with a mouth blow-pipe. Note the color of the coating on the
coal. PbO has formed.

Experiment 126.--Dissolve a small piece of lead in dilute HNO3.
Pour off the solution into a t.t. and add HCl or other soluble
chloride. Pb(NO3)2 + 2 HCl = ? What is the insoluble product?

Experiment 127.--Add to a solution of Pb(C2H3O2)2 some H2SO4.
Give the reaction and the explanation. TIN.

Examine cassiterite, tin foil, "terne plate," "bright plate."

277. Sn occurs as the mineral cassiterite, tin stone, Sn02, and
is found in only a few localities, as Banca, Malacca, and
England. It does not readily tarnish, and is used to cover thin
plates of copper and iron. Tin foil is generally an alloy of Pb
and Sn.

Sn is sometimes a dyad, at others a tetrad. Write symbols for its
two chlorides, stannous and stannic, also for its sulphides and
oxides.

CHAPTER L.

COPPER, MERCURY, AND SILVER.

COPPER.

Examine native copper, chalcopyrite, malachite, azurite, copper
acetate, copper nitrate, copper sulphate.

278. Occurrence.--Copper occurs both native and in many
compounds, being diffused in rocks and, in minute quantities, in
soils, waters, plants, and animals. Spain, Chili, and the United
States are the chief Cu producing countries. The extensive mines
of Michigan yield the native ore. The Calumet and Heela mine
alone produces 4,000,000 pounds per month. The most abundant
compound of Cu is chalcopyrite, or copper pyrites, CuFeS2.
Malachite, which is green, and azurite, which is blue, are
carbonates, the former being used for ornamental purposes.

Cu is, next to Ag, the best conductor of electricity and heat
among the elements; it is very ductile, malleable, and tenacious.

Cu has two valences, 1 and 2. Symbolize and name its chlorides,
iodides, sulphides, and oxides. Cupric compounds, as a rule, are
more stable than cuprous.

279. Uses.--Thousands of tons of Cu find use in domestic
utensils, ocean vessels, electric wires, batteries, and plating.
Name the chief alloys of Cu and their uses. See page 136. How may
CuS be obtained?  See Experiment 7. Cu2O, cuprous oxide, is used
to color glass red. CUSO4 is employed in calico-printing,
electric batteries, etc. It is called blue vitriol.

Paris green, used for killing potato-beetles, is composed chiefly
of copper arsenite. Write the symbol for this compound. All
soluble salts of Cu are poisonous; hence care should be taken not
to bring any acid in contact with copper vessels of domestic use.
With acetic acid, what would be formed?

MERCURY AND ITS COMPOUNDS.

Examine cinnabar, vermilion, mercury, red oxide, mercurous and
mercuric chloride.

280. Cinnabar, HgS, is practically the only source of mercury--
quicksilver. Austria, Spain, and California contain nearly all
the mines. In these mines the metal also occurs native to a small
extent. It is the only commonly occurring metal that is liquid at
ordinary temperatures; it solidifies at about -40 degrees. What
other common liquid element? See page 12. Hg is reduced from the
ore by Fe, Hg being distilled over and collected in water. Heat
regularly expands the metal.

281. Uses.--For uses see Reduction of Ag and Au, pages 165 and
170; amalgams, page 137; laboratory work, page 68. It is also
employed for thermometers and barometers, and as the source of
the red pigment vermilion, which is artificial HgS.

Compare the vapor density and the atomic weight of Hg, and
explain. See page 12. Hg is either a monad or a dyad. Symbolize
its ous and ic oxides and chlorides. Which of the following are
is salts, and which are ous, and why? HgNO3, Hg(NO3)2, HgCl,
HgCl2? Calomel, HgCl or Hg2Cl2, used in medicine, and corrosive
sublimate, HgCl2, are illustrations of the ous and ic salts. The
former is insoluble, the latter soluble. All soluble compounds of
Hg are virulent poisons, for which the antidote is the white of
egg, albumen. With it they coagulate or form an insoluble mass.

SILVER AND ITS COMPOUNDS.

282. Occurrence and Reduction.--Silver is found uncombined, and
combined, as Ag2S, argenite, and AgCl, horn silver. It occurs
usually with galena, PbS. It is abundant in the Western States,
Mexico, and Peru. Silver is separated from galena by melting the
two metals. As they slowly cool, Pb crystallizes, and is removed
by asieve, while Ag is left in the liquid mass. The principle is
much like crystallizing NaCl from solution and leaving behind the
salts of Mg, etc., in the mother liquor. When, by repeating the
process, most of the Pb is eliminated, the rest is oxidized by
heating in the air. Pb + O = PbO. Ag does not oxidize, and is
left in the metallic state.

Another mode of reduction is to change the silver salt to its
chloride, and then remove the Cl with Fe. Roasting with NaCl
makes the first change, 2 NaCl + Ag2S = Na2S + 2 AgCl, and with
Fe the second, 2 AgCl + Fe = FeCl2 + 2 Ag. Ag is separated from
the other products by adding Hg, with which it forms an amalgam.
By distilling this, Hg passes over and Ag remains. This is the
amalgamating process.

283. Salts of Silver are much employed in organic chemistry, and
AgCl, AgBr, and AgNO3 are used in photography. AgNO3 is a
soluble, colorless crystal, and is the basis of the silver salts.
It blackens when in contact with organic matter. Stains on a
photographer's hands are due to this substance, and the use of

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