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List Of Contents | Contents of An Introduction to Chemical Science
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oxides of tin, SnO and SnO2, or Sn=O and O=Sn=O. Write symbols
for the two chlorides of tin; two oxides of P; two oxides of
arsenic.

The chlorides of iron are FeCl2 and Fe2Cl6. In the latter, it
might be supposed that the quantivalence of Fe is 3, but the
graphic symbol shows it to be 4. It is called a pseudo-triad, or
false triad. Cr and Al are also pseudo-triads.

Cl  Cl |   | Cl--Fe--Fe--Cl |   | Cl  Cl

Write formulae for two oxides of iron; the oxide of Al.

57. A Radical is a Group of Elements which has no separate
existence, but enters into combination like a single atom; e.g.
(NO3) in the compounds HNO3 or KNO3; (SO4) in H2SO4. In HNO3 the
radical has a valence of 1, to balance that of H, H-NO3). In
H2SO4, what is the valence of (SO4)? Give it in each of these
radicals, noting first that of the first element: K(NO3),
Na2(SO4), Na2(CO3), K(ClO3), H3(PO4), Ca3(PO4)2, Na4(SiO4).

Suppose we wish to know the symbol for calcium phosphate. Ca and
PO4 are the two parts. In H3(PO4) the radical is a triad, to
balance H3. Ca is a dyad, Ca==(P04). The least common multiple of
the bonds (2 and 3) is 6, which, divided by 2 (no. Ca bonds),
gives 3 (no. Ca atoms to be taken). 6 / 3 (no. (PO4) bonds) gives
2 (no. PO4 radicals to be taken). Hence the symbol Ca3(P04)2.
Verify this by writing graphically.

Write symbols for the union of Mg and (SO4), Na and (PO4), Zn and
(NO3), K and (NO3), K and (SO4), Mg and (PO4), Fe and (SO4) (both
valences of Fe), Fe and (NO3), taking the valences of the
radicals from HNO3, H2SO4, H3PO4.

Chapter XII.

ELECTRO-CHEMICAL RELATION OF ELEMENTS.

58. Examine untarnished pieces of iron, silver, nickel, lead,
etc.; also quartz, resin, silk, wood, paper. Notice that from the
first four light is reflected in a different way from that of the
others. This property of reflecting light is known as luster.
Metals have a metallic luster which is peculiar to themselves;
and this, for the present, may be regarded as their chief
characteristic. Are they at the positive or negative end of the
list? See page 43. How is it with the non-metals? This
arrangement has a significance in chemistry which we must now
examine. The three appended experiments show how one metal can be
withdrawn from solution by a second, this second by a third, the
third by a fourth, and so on. For expedition, three pupils can
work together for the three following experiments, each doing
one, and examining the results of the others.

59. Deposition of Silver.

Experiment 35.--Put a ten-cent Ag coin into an evaporating-dish,
and pour over it a mixture of 5 cc. HNO3 and 10 cc. H2O. Warm
till all, or nearly all, the Ag dissolves. Remove the lamp. 3 Ag
+ 4 HNO3 = 3 AgNO3 + 2 H2O + NO. Then add 10 cc. H2O, and at once
put in a short piece of Cu wire, or a cent. Leave till quite a
deposit appears, then pour off the liquid, wash the deposit
thoroughly, and remove it from the coin. See whether the metal
resembles Ag. 2 AgNO3 + Cu =?60. Deposition of Copper.

Experiment 36.--Dissolve a cent or some Cu turnings in dilute
HNO3, as in Experiment 35, and dilute the solution. 3 Cu + 8 HN09
- 3 Cu (NOA+4 H2O+2 NO.)

Then put in a clean strip of Pb, and set aside as before,
examining the deposit finally. Cu(NO3), + Pb - ?

61. Deposition of Lead.

Experiment 37.--Perform this experiment in the same manner as the
two previous ones, dissolving a small piece of Pb, and using a
strip of Zn to precipitate the Pb. 3 Pb + 8 HNO3 - 3 Pb (NO4)2 +
4 Ha0 + 2 NO. Pb (NO3) 2 + Zn = ? h.

62. Explanation. -These experiments show that Cu will replace Ag
in a solution of AgNO3, that Pb will replace and deposit Cu from
a similar compound, and that Zn will deposit Pb in the same way.
They show that the affinity of Zn for (NO3) is stronger than
either Ag, Cu, or Pb. We. express this affinity by saying that Zn
is the most positive of the four metals, while Ag is the most
nega- tive. Cu is positive to Ag, but negative to Pb and Zn.
Which of the four elements are positive to Pb, and which
negative? Mg would withdraw Zn from a similar solution, and be in
its turn withdrawn by Na. The table on page 43 is founded on this
relation. A given element is positive to every element above it
in the list, and negative to all below it.

Metals are usually classed as positive, non-metals as negative.
Each in union with O and 1=I gives rise to a very important class
of compounds,=--the negative to acids, the positive to bases.

In the following, note whether the positive or the negative
element is written first:--HCl, Na20,-As2S3, -MgBr2, Ag2S. Na2SO4
is made up of two parts, Na2 being positive, the radical SO4
negative. Like elements, radicals are either positive or
negative. In the following, separate the positive element from
the negative radical by a vertical line: Na2CO3, NaNO3, ZnSO4,
KClO3.

The most common positive radical is NH4, ammonium, as in NH4Cl.
It always deports itself as a metal. The commonest radical is the
negative OH, called hydroxyl, from hydrogen- oxygen. Take away H
from the symbol of water, H-O-H, and hydroxyl --(OH) with one
free bond is left. If an element takes the place of H, i.e.
unites with OH, the compound is called a hydrate. KOH is
potassium hydrate. Name NaOH, Ca(OH)2, NH4OH, Zn(OH)2, Al2(OH)6.
Is the first part of each symbol above positive or negative?

H has an intermediate place in the list. It is a constituent of
both acids and bases, and of the neutral substance, water.

ORDER.

--


Negative or Non-Metallic Elements.
Acid-forming with H(usually OH).

Oxygen
Sulphur
Nitrogen
Fluorine
Chlorine
Bromine
Iodine
Phosphorus
Arsenic
Carbon
Silicon
Hydrogen

Positive or Metallic Elements.
Base-forming with OH.

Gold
Platinum
Mercury
Silver
Copper
Tin
Lead
Iron
Zinc
Aluminium
Magnesium
Calcium
Sodium
Potassium

CHAPTER XIII.

ELECTROLYSIS.

The following experiment is to be performed only by the teacher,
but pupils should make drawings and explain.

63. Decomposition of Water.

Experiment 38.--Arrange "in series" two or more cells of a Bunsen
battery (Physics, page 164), [References are made in this book to
Gage's Introduction to Physical Science.] and attach the terminal
wires to an electrolytic apparatus (Fig. 19) filled with water
made slightly acid with H2SO4. Construct a diagram of the
apparatus, marking the Zn in the liquid +, since it is positive,
and the C, or other element, -. Mark the electrode attached to
the Zn -, and that attached to the C +; positive electricity at
one end of a body commonly implies negative at the other.
Opposites attract, while like electricities repel each other.
These analogies will aid the memory. At the + electrode is the -
element of H2O, and at the - electrode the + element. Note, page
43, whether H or O is positive with reference to the other, and
write the symbol for each at the proper electrode. Compare the
diagram with the apparatus, to verify your conclusion. Why does
gas collect twice as fast at one electrode as at the other? What
does this prove of the composition of water? When filled, test
the gases in each tube, for O and H, with a burning stick.
Electrical analysis is called electrolysis.

If a solution of NaCl be electrolyzed, which element will go to
the + pole? Which, if the salt were K2SO4? Explain these
reactions in the electrolysis of that salt. K2SO4 = K2 + S03 + O.
SO4 is unstable, and breaks up into SO3 and O. Both K and SO3
have great affinity for water. K2 + 2 H2O = 2 KOH + H2. S03 + H2O
= H2SO4.

The base KOH would be found at the - electrode, and the acid
H2SO4 at the + electrode.

The positive portion, K, uniting with H2O forms a base; the
negative part, S03, with H2O forms an acid. Of what does this
show a salt to be composed?

64. Conclusions.--These experiments show (1) that at the +
electrode there always appears the negative element, or radical,
of the compound, and at the - electrode the positive element; (2)
that these elements unite with those of water, to make, in the
former case, acids, in the latter, bases; (3) that acids and
bases differ as negative and positive elements differ, each being
united with O and H, and yet producing compounds of a directly
opposite character; (4) that salts are really compounded of acids
and bases. This explains why salts are usually inactive and
neutral in character, while acids and bases are active agents.
Thus we see why the most positive or the most negative elements
in general have the strongest affinities, while those
intermediate in the list are inactive, and have weak affinities;
why alloys of the metals are weak compounds; why a neutral
substance, like water, has such a weak affinity for the salts
which it holds in solution; and why an aqueous solution is
regarded as a mechanical mixture rather than a chemical compound.
In this view, the division line between chemistry and physics is
not a distinct one. These will be better understood after
studying the chapters on acids, bases and salts.

Chapter XIV.

UNION BY VOLUME.

66. Avogadro's Law of Gases.--Equal volumes of all gases, the
temperature and pressure being the same, have the same number of
molecules. This law is the foundation of modern chemistry. A
cubic centimeter of O has as many molecules as a cubic centimeter
of H, a liter of N the same number as a liter of steam, under
similar conditions. Compare the number of molecules in 5 l. of
N2O with that in 10 l. Cl. 7 cc. vapor of I to 6 cc. vapor of S.
The half-molecules of two gases have, of course, the same
relation to each other, and in elements the half-molecule is
usually the atom.

The molecular volumes--molecules and the surrounding space--of
all gases must therefore be equal, as must the half-volumes.
Notice that this law applies only to gases, not to liquids or
solids. Let us apply it to the experiment for the electrolysis of
water. In this we found twice as much H by volume as O.
Evidently, then, steam has twice as many molecules of H as of O,
and twice as many half-molecules, or atoms. If the molecule has
one atom of O, it must have two of H, and the formula will be

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