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withdrawn from solution by a second, this second by a third, the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